Last update : SWIG-3.0.10 (12 Jun 2016)
SWIG (Simplified Wrapper and Interface Generator) is a software development tool for building scripting language interfaces to C and C++ programs. Originally developed in 1995, SWIG was first used by scientists in the Theoretical Physics Division at Los Alamos National Laboratory for building user interfaces to simulation codes running on the Connection Machine 5 supercomputer. In this environment, scientists needed to work with huge amounts of simulation data, complex hardware, and a constantly changing code base. The use of a scripting language interface provided a simple yet highly flexible foundation for solving these types of problems. SWIG simplifies development by largely automating the task of scripting language integration--allowing developers and users to focus on more important problems.
Although SWIG was originally developed for scientific applications, it has since evolved into a general purpose tool that is used in a wide variety of applications--in fact almost anything where C/C++ programming is involved.
In the late 1990's, the most stable version of SWIG was release 1.1p5. Versions 1.3.x were officially development versions and these were released over a period of 10 years starting from the year 2000. The final version in the 1.3.x series was 1.3.40, but in truth the 1.3.x series had been stable for many years. An official stable version was released along with the decision to make SWIG license changes and this gave rise to version 2.0.0 in 2010.
The LICENSE file shipped with SWIG in the top level directory contains the SWIG license. For further insight into the license including the license of SWIG's output code, please visit the SWIG legal page - http://www.swig.org/legal.html.
The license was clarified in version 2.0.0 so that the code that SWIG generated could be distributed under license terms of the user's choice/requirements and at the same time the SWIG source was placed under the GNU General Public License version 3.
The official location of SWIG related material is
This site contains the latest version of the software, users guide, and information regarding bugs, installation problems, and implementation tricks.
You can also subscribe to the swig-user mailing list by visiting the page
The mailing list often discusses some of the more technical aspects of SWIG along with information about beta releases and future work.
Git and Subversion access to the latest version of SWIG is also available. More information about this can be obtained at:
This manual assumes that you know how to write C/C++ programs and that you have at least heard of scripting languages such as Tcl, Python, and Perl. A detailed knowledge of these scripting languages is not required although some familiarity won't hurt. No prior experience with building C extensions to these languages is required---after all, this is what SWIG does automatically. However, you should be reasonably familiar with the use of compilers, linkers, and makefiles since making scripting language extensions is somewhat more complicated than writing a normal C program.
Over time SWIG releases have become significantly more capable in their C++ handling--especially support for advanced features like namespaces, overloaded operators, and templates. Whenever possible, this manual tries to cover the technicalities of this interface. However, this isn't meant to be a tutorial on C++ programming. For many of the gory details, you will almost certainly want to consult a good C++ reference. If you don't program in C++, you may just want to skip those parts of the manual.
The first few chapters of this manual describe SWIG in general and provide an overview of its capabilities. The remaining chapters are devoted to specific SWIG language modules and are self contained. Thus, if you are using SWIG to build Python interfaces, you can probably skip to that chapter and find almost everything you need to know.
If you hate reading manuals, glance at the "Introduction" which contains a few simple examples. These examples contain about 95% of everything you need to know to use SWIG. After that, simply use the language-specific chapters as a reference. The SWIG distribution also comes with a large directory of examples that illustrate different topics.
If you are a previous user of SWIG, don't expect SWIG to provide complete backwards compatibility. Although the developers strive to the utmost to keep backwards compatibility, this isn't always possible as the primary goal over time is to make SWIG better---a process that would simply be impossible if the developers are constantly bogged down with backwards compatibility issues. Potential incompatibilities are clearly marked in the detailed release notes.
If you need to work with different versions of SWIG and backwards compatibility is an issue, you can use the SWIG_VERSION preprocessor symbol which holds the version of SWIG being executed. SWIG_VERSION is a hexadecimal integer such as 0x010311 (corresponding to SWIG-1.3.11). This can be used in an interface file to define different typemaps, take advantage of different features etc:
#if SWIG_VERSION >= 0x010311 /* Use some fancy new feature */ #endif
Note: The version symbol is not defined in the generated SWIG wrapper file. The SWIG preprocessor has defined SWIG_VERSION since SWIG-1.3.11.
The CHANGES.current, CHANGES and RELEASENOTES files shipped with SWIG in the top level directory contain, respectively, detailed release notes for the current version, detailed release notes for previous releases and summary release notes from SWIG-1.3.22 onwards.
SWIG is an unfunded project that would not be possible without the contributions of many people working in their spare time. If you have benefitted from using SWIG, please consider Donating to SWIG to keep development going. There have been a large varied number of people who have made contributions at all levels over time. Contributors are mentioned either in the COPYRIGHT file or CHANGES files shipped with SWIG or in submitted bugs.
Although every attempt has been made to make SWIG bug-free, we are also trying to make feature improvements that may introduce bugs. To report a bug, either send mail to the SWIG developer list at the swig-devel mailing list or report a bug at the SWIG bug tracker. In your report, be as specific as possible, including (if applicable), error messages, tracebacks (if a core dump occurred), corresponding portions of the SWIG interface file used, and any important pieces of the SWIG generated wrapper code. We can only fix bugs if we know about them.
Please see the dedicated Windows chapter for instructions on installing SWIG on Windows and running the examples. The Windows distribution is called swigwin and includes a prebuilt SWIG executable, swig.exe, included in the top level directory. Otherwise it is exactly the same as the main SWIG distribution. There is no need to download anything else.
These installation instructions are for using the distributed tarball, for example, swig-3.0.8.tar.gz. If you wish to build and install from source on Github, extra steps are required. Please see the Bleeding Edge page on the SWIG website.
You must use GNU make to build and install SWIG.
PCRE needs to be installed on your system to build SWIG, in particular pcre-config must be available. If you have PCRE headers and libraries but not pcre-config itself or, alternatively, wish to override the compiler or linker flags returned by pcre-config, you may set PCRE_LIBS and PCRE_CFLAGS variables to be used instead. And if you don't have PCRE at all, the configure script will provide instructions for obtaining it.
To build and install SWIG, simply type the following:
$ ./configure $ make $ make install
By default SWIG installs itself in /usr/local. If you need to install SWIG in a different location or in your home directory, use the --prefix option to ./configure. For example:
$ ./configure --prefix=/home/yourname/projects $ make $ make install
Note: the directory given to --prefix must be an absolute pathname. Do not use the ~ shell-escape to refer to your home directory. SWIG won't work properly if you do this.
The INSTALL file shipped in the top level directory details more about using configure. Also try
$ ./configure --help.
The configure script will attempt to locate various packages on your machine including Tcl, Perl5, Python and all the other target languages that SWIG supports. Don't panic if you get 'not found' messages -- SWIG does not need these packages to compile or run. The configure script is actually looking for these packages so that you can try out the SWIG examples contained in the 'Examples' directory without having to hack Makefiles. Note that the --without-xxx options, where xxx is a target language, have minimal effect. All they do is reduce the amount of testing done with 'make check'. The SWIG executable and library files installed cannot currently be configured with a subset of target languages.
SWIG used to include a set of runtime libraries for some languages for working with multiple modules. These are no longer built during the installation stage. However, users can build them just like any wrapper module as described in the Modules chapter. The CHANGES file shipped with SWIG in the top level directory also lists some examples which build the runtime library.
Note:
SWIG is known to work on various flavors of OS X. Follow the Unix installation instructions above. However, as of this writing, there is still great deal of inconsistency with how shared libaries are handled by various scripting languages on OS X.
Users of OS X should be aware that Darwin handles shared libraries and linking in a radically different way than most Unix systems. In order to test SWIG and run the examples, SWIG configures itself to use flat namespaces and to allow undefined symbols (-flat_namespace -undefined suppress). This mostly closely follows the Unix model and makes it more likely that the SWIG examples will work with whatever installation of software you might have. However, this is generally not the recommended technique for building larger extension modules. Instead, you should utilize Darwin's two-level namespaces. Some details about this can be found here Understanding Two-Level Namespaces.
Needless to say, you might have to experiment a bit to get things working at first.
If you want to test SWIG after building it, a check can be performed on Unix operating systems. Type the following:
$ make -k check
This step can be performed either before or after installation. The check requires at least one of the target languages to be installed. If it fails, it may mean that you have an uninstalled language module or that the file 'Examples/Makefile' has been incorrectly configured. It may also fail due to compiler issues such as a broken C++ compiler. Even if the check fails, there is a pretty good chance SWIG still works correctly --- you will just have to mess around with one of the examples and some makefiles to get it to work. Some tests may also fail due to missing dependency packages, eg PCRE or Boost, but this will require careful analysis of the configure output done during configuration.
The test suite executed by the check is designed to stress-test many parts of the implementation including obscure corner cases. If some of these tests fail or generate warning messages, there is no reason for alarm --- the test may be related to some new SWIG feature or a difficult bug that we're trying to resolve. Chances are that SWIG will work just fine for you. Note that if you have more than one CPU/core, then you can use parallel make to speed up the check as it does take quite some time to run, for example:
$ make -j2 -k check
Also, SWIG's support for C++ is sufficiently advanced that certain tests may fail on older C++ compilers (for instance if your compiler does not support member templates). These errors are harmless if you don't intend to use these features in your own programs.
Note: The test-suite currently contains over 500 tests. If you have many different target languages installed and a slow machine, it might take more than an hour to run the test-suite.
The Examples directory contains a variety of examples of using SWIG and it has some browsable documentation. Simply point your browser to the file "Example/index.html".
The Examples directory also includes Visual C++ project 6 (.dsp) files for building some of the examples on Windows. Later versions of Visual Studio will convert these old style project files into a current solution file.
SWIG is a software development tool that simplifies the task of interfacing different languages to C and C++ programs. In a nutshell, SWIG is a compiler that takes C/C++ declarations and creates the wrappers needed to access those declarations from other languages including Perl, Python, Tcl, Ruby, Guile, and Java. SWIG normally requires no modifications to existing code and can often be used to build a usable interface in only a few minutes. Possible applications of SWIG include:
SWIG was originally designed to make it extremely easy for scientists and engineers to build extensible scientific software without having to get a degree in software engineering. Because of this, the use of SWIG tends to be somewhat informal and ad-hoc (e.g., SWIG does not require users to provide formal interface specifications as you would find in a dedicated IDL compiler). Although this style of development isn't appropriate for every project, it is particularly well suited to software development in the small; especially the research and development work that is commonly found in scientific and engineering projects. However, nowadays SWIG is known to be used in many large open source and commercial projects.
As stated in the previous section, the primary purpose of SWIG is to simplify the task of integrating C/C++ with other programming languages. However, why would anyone want to do that? To answer that question, it is useful to list a few strengths of C/C++ programming:
Next, let's list a few problems with C/C++ programming
To address these limitations, many programmers have arrived at the conclusion that it is much easier to use different programming languages for different tasks. For instance, writing a graphical user interface may be significantly easier in a scripting language like Python or Tcl (consider the reasons why millions of programmers have used languages like Visual Basic if you need more proof). An interactive interpreter might also serve as a useful debugging and testing tool. Other languages like Java might greatly simplify the task of writing distributed computing software. The key point is that different programming languages offer different strengths and weaknesses. Moreover, it is extremely unlikely that any programming is ever going to be perfect. Therefore, by combining languages together, you can utilize the best features of each language and greatly simplify certain aspects of software development.
From the standpoint of C/C++, a lot of people use SWIG because they want to break out of the traditional monolithic C programming model which usually results in programs that resemble this:
Instead of going down that route, incorporating C/C++ into a higher level language often results in a more modular design, less code, better flexibility, and increased programmer productivity.
SWIG tries to make the problem of C/C++ integration as painless as possible. This allows you to focus on the underlying C program and using the high-level language interface, but not the tedious and complex chore of making the two languages talk to each other. At the same time, SWIG recognizes that all applications are different. Therefore, it provides a wide variety of customization features that let you change almost every aspect of the language bindings. This is the main reason why SWIG has such a large user manual ;-).
The best way to illustrate SWIG is with a simple example. Consider the following C code:
/* File : example.c */
double My_variable = 3.0;
/* Compute factorial of n */
int fact(int n) {
if (n <= 1)
return 1;
else
return n*fact(n-1);
}
/* Compute n mod m */
int my_mod(int n, int m) {
return(n % m);
}
Suppose that you wanted to access these functions and the global variable My_variable from Tcl. You start by making a SWIG interface file as shown below (by convention, these files carry a .i suffix) :
/* File : example.i */
%module example
%{
/* Put headers and other declarations here */
extern double My_variable;
extern int fact(int);
extern int my_mod(int n, int m);
%}
extern double My_variable;
extern int fact(int);
extern int my_mod(int n, int m);
The interface file contains ANSI C function prototypes and variable declarations. The %module directive defines the name of the module that will be created by SWIG. The %{ %} block provides a location for inserting additional code, such as C header files or additional C declarations, into the generated C wrapper code.
SWIG is invoked using the swig command. We can use this to build a Tcl module (under Linux) as follows :
unix > swig -tcl example.i unix > gcc -c -fpic example.c example_wrap.c -I/usr/local/include unix > gcc -shared example.o example_wrap.o -o example.so unix > tclsh % load ./example.so % fact 4 24 % my_mod 23 7 2 % expr $My_variable + 4.5 7.5 %
The swig command produced a new file called example_wrap.c that should be compiled along with the example.c file. Most operating systems and scripting languages now support dynamic loading of modules. In our example, our Tcl module has been compiled into a shared library that can be loaded into Tcl. When loaded, Tcl can now access the functions and variables declared in the SWIG interface. A look at the file example_wrap.c reveals a hideous mess. However, you almost never need to worry about it.
Now, let's turn these functions into a Perl5 module. Without making any changes type the following (shown for Solaris):
unix > swig -perl5 example.i
unix > gcc -c example.c example_wrap.c \
-I/usr/local/lib/perl5/sun4-solaris/5.003/CORE
unix > ld -G example.o example_wrap.o -o example.so # This is for Solaris
unix > perl5.003
use example;
print example::fact(4), "\n";
print example::my_mod(23,7), "\n";
print $example::My_variable + 4.5, "\n";
<ctrl-d>
24
2
7.5
unix >
Finally, let's build a module for Python (shown for Irix).
unix > swig -python example.i unix > gcc -c -fpic example.c example_wrap.c -I/usr/local/include/python2.0 unix > gcc -shared example.o example_wrap.o -o _example.so unix > python Python 2.0 (#6, Feb 21 2001, 13:29:45) [GCC egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)] on linux2 Type "copyright", "credits" or "license" for more information. >>> import example >>> example.fact(4) 24 >>> example.my_mod(23,7) 2 >>> example.cvar.My_variable + 4.5 7.5
To the truly lazy programmer, one may wonder why we needed the extra interface file at all. As it turns out, you can often do without it. For example, you could also build a Perl5 module by just running SWIG on the C header file and specifying a module name as follows
unix > swig -perl5 -module example example.h
unix > gcc -c example.c example_wrap.c \
-I/usr/local/lib/perl5/sun4-solaris/5.003/CORE
unix > ld -G example.o example_wrap.o -o example.so
unix > perl5.003
use example;
print example::fact(4), "\n";
print example::my_mod(23,7), "\n";
print $example::My_variable + 4.5, "\n";
<ctrl-d>
24
2
7.5
A primary goal of the SWIG project is to make the language binding process extremely easy. Although a few simple examples have been shown, SWIG is quite capable in supporting most of C++. Some of the major features include:
Most of C++11 is also supported. Details are in the C++11 section.
It is important to stress that SWIG is not a simplistic C++ lexing tool like several apparently similar wrapper generation tools. SWIG not only parses C++, it implements the full C++ type system and it is able to understand C++ semantics. SWIG generates its wrappers with full knowledge of this information. As a result, you will find SWIG to be just as capable of dealing with nasty corner cases as it is in wrapping simple C++ code. In fact, SWIG is able to handle C++ code that stresses the very limits of many C++ compilers.
When used as intended, SWIG requires minimal (if any) modification to existing C or C++ code. This makes SWIG extremely easy to use with existing packages and promotes software reuse and modularity. By making the C/C++ code independent of the high level interface, you can change the interface and reuse the code in other applications. It is also possible to support different types of interfaces depending on the application.
SWIG is a command line tool and as such can be incorporated into any build system that supports invoking external tools/compilers. SWIG is most commonly invoked from within a Makefile, but is also known to be invoked from popular IDEs such as Microsoft Visual Studio.
If you are using the GNU Autotools ( Autoconf/ Automake / Libtool) to configure SWIG use in your project, the SWIG Autoconf macros can be used. The primary macro is ax_pkg_swig, see http://www.gnu.org/software/autoconf-archive/ax_pkg_swig.html#ax_pkg_swig . The ax_python_devel macro is also helpful for generating Python extensions. See the Autoconf Archive for further information on this and other Autoconf macros.
There is growing support for SWIG in some build tools, for example CMake is a cross-platform, open-source build manager with built in support for SWIG. CMake can detect the SWIG executable and many of the target language libraries for linking against. CMake knows how to build shared libraries and loadable modules on many different operating systems. This allows easy cross platform SWIG development. It can also generate the custom commands necessary for driving SWIG from IDEs and makefiles. All of this can be done from a single cross platform input file. The following example is a CMake input file for creating a python wrapper for the SWIG interface file, example.i:
# This is a CMake example for Python
FIND_PACKAGE(SWIG REQUIRED)
INCLUDE(${SWIG_USE_FILE})
FIND_PACKAGE(PythonLibs)
INCLUDE_DIRECTORIES(${PYTHON_INCLUDE_PATH})
INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR})
SET(CMAKE_SWIG_FLAGS "")
SET_SOURCE_FILES_PROPERTIES(example.i PROPERTIES CPLUSPLUS ON)
SET_SOURCE_FILES_PROPERTIES(example.i PROPERTIES SWIG_FLAGS "-includeall")
SWIG_ADD_MODULE(example python example.i example.cxx)
SWIG_LINK_LIBRARIES(example ${PYTHON_LIBRARIES})
The above example will generate native build files such as makefiles, nmake files and Visual Studio projects which will invoke SWIG and compile the generated C++ files into _example.so (UNIX) or _example.pyd (Windows). For other target languages on Windows a dll, instead of a .pyd file, is usually generated.
SWIG is designed to produce working code that needs no hand-modification (in fact, if you look at the output, you probably won't want to modify it). You should think of your target language interface being defined entirely by the input to SWIG, not the resulting output file. While this approach may limit flexibility for hard-core hackers, it allows others to forget about the low-level implementation details.
No, this isn't a special section on the sorry state of world politics. However, it may be useful to know that SWIG was written with a certain "philosophy" about programming---namely that programmers are smart and that tools should just stay out of their way. Because of that, you will find that SWIG is extremely permissive in what it lets you get away with. In fact, you can use SWIG to go well beyond "shooting yourself in the foot" if dangerous programming is your goal. On the other hand, this kind of freedom may be exactly what is needed to work with complicated and unusual C/C++ applications.
Ironically, the freedom that SWIG provides is countered by an extremely conservative approach to code generation. At its core, SWIG tries to distill even the most advanced C++ code down to a small well-defined set of interface building techniques based on ANSI C programming. Because of this, you will find that SWIG interfaces can be easily compiled by virtually every C/C++ compiler and that they can be used on any platform. Again, this is an important part of staying out of the programmer's way----the last thing any developer wants to do is to spend their time debugging the output of a tool that relies on non-portable or unreliable programming features. Dependencies are often a source of incompatibilities and problems and so additional third party libraries are not used in the generated code. SWIG will also generally avoid generating code that introduces a dependency on the C++ Standard Template Library (STL). SWIG will generate code that depends on the C libraries though.
This chapter describes SWIG usage on Microsoft Windows. Installing SWIG and running the examples is covered as well as building the SWIG executable. Usage within the Unix like environments MinGW and Cygwin is also detailed.
SWIG does not come with the usual Windows type installation program, however it is quite easy to get started. The main steps are:
The swigwin distribution contains the SWIG Windows executable, swig.exe, which will run on 32 bit versions of Windows, ie Windows 95 and later. If you want to build your own swig.exe have a look at Building swig.exe on Windows.
Using Microsoft Visual C++ is the most common approach to compiling and linking SWIG's output. The Examples directory has a few Visual C++ project files (.dsp files). These were produced by Visual C++ 6. Newer versions of Visual Studio should be able to open and convert these project files. Each C# example comes with a Visual Studio 2005 solution and associated project files instead of Visual C++ 6 project files. The project files have been set up to execute SWIG in a custom build rule for the SWIG interface (.i) file. Alternatively run the examples using Cygwin.
More information on each of the examples is available with the examples distributed with SWIG (Examples/index.html).
Ensure the SWIG executable is as supplied in the SWIG root directory in order for the examples to work. Most languages require some environment variables to be set before running Visual C++. Note that Visual C++ must be re-started to pick up any changes in environment variables. Open up an example .dsp file, Visual C++ will create a workspace for you (.dsw file). Ensure the Release build is selected then do a Rebuild All from the Build menu. The required environment variables are displayed with their current values.
The list of required environment variables for each module language is also listed below. They are usually set from the Control Panel and System properties, but this depends on which flavour of Windows you are running. If you don't want to use environment variables then change all occurrences of the environment variables in the .dsp files with hard coded values. If you are interested in how the project files are set up there is explanatory information in some of the language module's documentation.
The C# examples do not require any environment variables to be set as a C# project file is included. Just open up the .sln solution file in Visual Studio .NET 2003 or later, select Release Build, and do a Rebuild All from the Build menu. The accompanying C# and C++ project files are automatically used by the solution file.
JAVA_INCLUDE : Set this to the directory containing
jni.h
JAVA_BIN : Set this to the bin directory containing
javac.exe
Example using JDK1.3:
JAVA_INCLUDE: D:\jdk1.3\include
JAVA_BIN: D:\jdk1.3\bin
PERL5_INCLUDE : Set this to the directory containing
perl.h
PERL5_LIB : Set this to the Perl library including
path for linking
Example using nsPerl 5.004_04:
PERL5_INCLUDE: D:\nsPerl5.004_04\lib\CORE
PERL5_LIB: D:\nsPerl5.004_04\lib\CORE\perl.lib
PYTHON_INCLUDE : Set this to the directory that
contains Python.h
PYTHON_LIB : Set this to the python library
including path for linking
Example using Python 2.1.1:
PYTHON_INCLUDE: D:\python21\include
PYTHON_LIB: D:\python21\libs\python21.lib
TCL_INCLUDE : Set this to the directory containing
tcl.h
TCL_LIB : Set this to the TCL library including
path for linking
Example using ActiveTcl 8.3.3.3
TCL_INCLUDE: D:\tcl\include
TCL_LIB: D:\tcl\lib\tcl83.lib
R_INCLUDE : Set this to the directory containing R.h
R_LIB : Set this to the R library (Rdll.lib)
including path for linking. The library needs to be built as described
in the R README.packages file (the pexports.exe approach is the
easiest).
Example using R 2.5.1:
R_INCLUDE: C:\Program Files\R\R-2.5.1\include
R_LIB: C:\Program Files\R\R-2.5.1\bin\Rdll.lib
RUBY_INCLUDE : Set this to the directory containing
ruby.h
RUBY_LIB : Set this to the ruby library including
path for linking
Example using Ruby 1.6.4:
RUBY_INCLUDE: D:\ruby\lib\ruby\1.6\i586-mswin32
RUBY_LIB: D:\ruby\lib\mswin32-ruby16.lib
If you do not have access to Visual C++ you will have to set up project files / Makefiles for your chosen compiler. There is a section in each of the language modules detailing what needs setting up using Visual C++ which may be of some guidance. Alternatively you may want to use Cygwin as described in the following section.
SWIG can also be compiled and run using Cygwin or MinGW which provides a Unix like front end to Windows and comes free with gcc, an ANSI C/C++ compiler. However, this is not a recommended approach as the prebuilt executable is supplied.
If you want to replicate the build of swig.exe that comes with the download, follow the MinGW instructions below. This is not necessary to use the supplied swig.exe. This information is provided for those that want to modify the SWIG source code in a Windows environment. Normally this is not needed, so most people will want to ignore this section.
The short abbreviated instructions follow...
The step by step instructions to download and install MinGW and MSYS, then download and build the latest version of SWIG from Github follow... Note that the instructions for obtaining SWIG from Github are also online at SWIG Bleeding Edge.
Pitfall note: Execute the steps in the order shown and don't use spaces in path names. In fact it is best to use the default installation directories.
cd / tar -jxf msys-automake-1.8.2.tar.bz2 tar -jxf msys-autoconf-2.59.tar.bz2 tar -zxf bison-2.0-MSYS.tar.gz
mkdir /usr/src cd /usr/src git clone https://github.com/swig/swig.git
cd /usr/src/swig Tools/pcre-build.sh
cd /usr/src/swig ./autogen.sh ./configure make
Note that SWIG can also be built using Cygwin. However, SWIG will then require the Cygwin DLL when executing. Follow the Unix instructions in the README file in the SWIG root directory. Note that the Cygwin environment will also allow one to regenerate the autotool generated files which are supplied with the release distribution. These files are generated using the autogen.sh script and will only need regenerating in circumstances such as changing the build system.
If you don't want to install Cygwin or MinGW, use a different compiler to build SWIG. For example, all the source code files can be added to a Visual C++ project file in order to build swig.exe from the Visual C++ IDE.
The examples and test-suite work as successfully on Cygwin as on any other Unix operating system. The modules which are known to work are Python, Tcl, Perl, Ruby, Java and C#. Follow the Unix instructions in the README file in the SWIG root directory to build the examples.
A common problem when using SWIG on Windows are the Microsoft function calling conventions which are not in the C++ standard. SWIG parses ISO C/C++ so cannot deal with proprietary conventions such as __declspec(dllimport), __stdcall etc. There is a Windows interface file, windows.i, to deal with these calling conventions though. The file also contains typemaps for handling commonly used Windows specific types such as __int64, BOOL , DWORD etc. Include it like you would any other interface file, for example:
%include <windows.i> __declspec(dllexport) ULONG __stdcall foo(DWORD, __int32);
Note that if you follow Microsoft's recommendation of wrapping the __declspec calls in a preprocessor definition, you will need to make sure that the definition is included by SWIG as well, by either defining it manually or via a header. For example, if you have specified the preprocessor definition in a header named export_lib.h and include other headers which depend on it, you should use the %include directive to include the definition explicitly. For example, if you had a header file, bar.h, which depended on export_lib.h, your SWIG definition file might look like:
// bar.i %module bar %include <windows.i> %include "export_lib.h" %include "bar.h"
where export_lib.h may contain:
// export_lib.h #define BAR_API __declspec(dllexport)
and bar.h may look like:
// bar.h #include "export_lib.h" BAR_API void bar_function(int, double);
Using the preprocessor to remove BAR_API is a popular simpler solution:
// bar.i %module bar #define BAR_API %include "bar.h"
This chapter provides a brief overview of scripting language extension programming and the mechanisms by which scripting language interpreters access C and C++ code.
When a scripting language is used to control a C program, the resulting system tends to look as follows:
In this programming model, the scripting language interpreter is used for high level control whereas the underlying functionality of the C/C++ program is accessed through special scripting language "commands." If you have ever tried to write your own simple command interpreter, you might view the scripting language approach to be a highly advanced implementation of that. Likewise, If you have ever used a package such as MATLAB or IDL, it is a very similar model--the interpreter executes user commands and scripts. However, most of the underlying functionality is written in a low-level language like C or Fortran.
The two-language model of computing is extremely powerful because it exploits the strengths of each language. C/C++ can be used for maximal performance and complicated systems programming tasks. Scripting languages can be used for rapid prototyping, interactive debugging, scripting, and access to high-level data structures such associative arrays.
Scripting languages are built around a parser that knows how to execute commands and scripts. Within this parser, there is a mechanism for executing commands and accessing variables. Normally, this is used to implement the builtin features of the language. However, by extending the interpreter, it is usually possible to add new commands and variables. To do this, most languages define a special API for adding new commands. Furthermore, a special foreign function interface defines how these new commands are supposed to hook into the interpreter.
Typically, when you add a new command to a scripting interpreter you need to do two things; first you need to write a special "wrapper" function that serves as the glue between the interpreter and the underlying C function. Then you need to give the interpreter information about the wrapper by providing details about the name of the function, arguments, and so forth. The next few sections illustrate the process.
Suppose you have an ordinary C function like this :
int fact(int n) {
if (n <= 1)
return 1;
else
return n*fact(n-1);
}
In order to access this function from a scripting language, it is necessary to write a special "wrapper" function that serves as the glue between the scripting language and the underlying C function. A wrapper function must do three things :
As an example, the Tcl wrapper function for the fact() function above example might look like the following :
int wrap_fact(ClientData clientData, Tcl_Interp *interp, int argc, char *argv[]) {
int result;
int arg0;
if (argc != 2) {
interp->result = "wrong # args";
return TCL_ERROR;
}
arg0 = atoi(argv[1]);
result = fact(arg0);
sprintf(interp->result,"%d", result);
return TCL_OK;
}
Once you have created a wrapper function, the final step is to tell the scripting language about the new function. This is usually done in an initialization function called by the language when the module is loaded. For example, adding the above function to the Tcl interpreter requires code like the following :
int Wrap_Init(Tcl_Interp *interp) {
Tcl_CreateCommand(interp, "fact", wrap_fact, (ClientData) NULL,
(Tcl_CmdDeleteProc *) NULL);
return TCL_OK;
}
When executed, Tcl will now have a new command called "fact " that you can use like any other Tcl command.
Although the process of adding a new function to Tcl has been illustrated, the procedure is almost identical for Perl and Python. Both require special wrappers to be written and both need additional initialization code. Only the specific details are different.
Variable linking refers to the problem of mapping a C/C++ global variable to a variable in the scripting language interpreter. For example, suppose you had the following variable:
double Foo = 3.5;
It might be nice to access it from a script as follows (shown for Perl):
$a = $Foo * 2.3; # Evaluation $Foo = $a + 2.0; # Assignment
To provide such access, variables are commonly manipulated using a pair of get/set functions. For example, whenever the value of a variable is read, a "get" function is invoked. Similarly, whenever the value of a variable is changed, a "set" function is called.
In many languages, calls to the get/set functions can be attached to evaluation and assignment operators. Therefore, evaluating a variable such as $Foo might implicitly call the get function. Similarly, typing $Foo = 4 would call the underlying set function to change the value.
In many cases, a C program or library may define a large collection of constants. For example:
#define RED 0xff0000 #define BLUE 0x0000ff #define GREEN 0x00ff00
To make constants available, their values can be stored in scripting language variables such as $RED, $BLUE, and $GREEN. Virtually all scripting languages provide C functions for creating variables so installing constants is usually a trivial exercise.
Although scripting languages have no trouble accessing simple functions and variables, accessing C/C++ structures and classes present a different problem. This is because the implementation of structures is largely related to the problem of data representation and layout. Furthermore, certain language features are difficult to map to an interpreter. For instance, what does C++ inheritance mean in a Perl interface?
The most straightforward technique for handling structures is to implement a collection of accessor functions that hide the underlying representation of a structure. For example,
struct Vector {
Vector();
~Vector();
double x,y,z;
};
can be transformed into the following set of functions :
Vector *new_Vector(); void delete_Vector(Vector *v); double Vector_x_get(Vector *v); double Vector_y_get(Vector *v); double Vector_z_get(Vector *v); void Vector_x_set(Vector *v, double x); void Vector_y_set(Vector *v, double y); void Vector_z_set(Vector *v, double z);
Now, from an interpreter these function might be used as follows:
% set v [new_Vector] % Vector_x_set $v 3.5 % Vector_y_get $v % delete_Vector $v % ...
Since accessor functions provide a mechanism for accessing the internals of an object, the interpreter does not need to know anything about the actual representation of a Vector.
In certain cases, it is possible to use the low-level accessor functions to create a proxy class, also known as a shadow class. A proxy class is a special kind of object that gets created in a scripting language to access a C/C++ class (or struct) in a way that looks like the original structure (that is, it proxies the real C++ class). For example, if you have the following C++ definition :
class Vector {
public:
Vector();
~Vector();
double x,y,z;
};
A proxy classing mechanism would allow you to access the structure in a more natural manner from the interpreter. For example, in Python, you might want to do this:
>>> v = Vector() >>> v.x = 3 >>> v.y = 4 >>> v.z = -13 >>> ... >>> del v
Similarly, in Perl5 you may want the interface to work like this:
$v = new Vector;
$v->{x} = 3;
$v->{y} = 4;
$v->{z} = -13;
Finally, in Tcl :
Vector v v configure -x 3 -y 4 -z -13
When proxy classes are used, two objects are really at work--one in the scripting language, and an underlying C/C++ object. Operations affect both objects equally and for all practical purposes, it appears as if you are simply manipulating a C/C++ object.
The final step in using a scripting language with your C/C++ application is adding your extensions to the scripting language itself. There are two primary approaches for doing this. The preferred technique is to build a dynamically loadable extension in the form of a shared library. Alternatively, you can recompile the scripting language interpreter with your extensions added to it.
To create a shared library or DLL, you often need to look at the manual pages for your compiler and linker. However, the procedure for a few common platforms is shown below:
# Build a shared library for Solaris gcc -fpic -c example.c example_wrap.c -I/usr/local/include ld -G example.o example_wrap.o -o example.so # Build a shared library for Linux gcc -fpic -c example.c example_wrap.c -I/usr/local/include gcc -shared example.o example_wrap.o -o example.so
To use your shared library, you simply use the corresponding command in the scripting language (load, import, use, etc...). This will import your module and allow you to start using it. For example:
% load ./example.so % fact 4 24 %
When working with C++ codes, the process of building shared libraries may be more complicated--primarily due to the fact that C++ modules may need additional code in order to operate correctly. On many machines, you can build a shared C++ module by following the above procedures, but changing the link line to the following :
c++ -shared example.o example_wrap.o -o example.so
When building extensions as shared libraries, it is not uncommon for your extension to rely upon other shared libraries on your machine. In order for the extension to work, it needs to be able to find all of these libraries at run-time. Otherwise, you may get an error such as the following :
>>> import graph
Traceback (innermost last):
File "<stdin>", line 1, in ?
File "/home/sci/data1/beazley/graph/graph.py", line 2, in ?
import graphc
ImportError: 1101:/home/sci/data1/beazley/bin/python: rld: Fatal Error: cannot
successfully map soname 'libgraph.so' under any of the filenames /usr/lib/libgraph.so:/
lib/libgraph.so:/lib/cmplrs/cc/libgraph.so:/usr/lib/cmplrs/cc/libgraph.so:
>>>
What this error means is that the extension module created by SWIG depends upon a shared library called "libgraph.so" that the system was unable to locate. To fix this problem, there are a few approaches you can take.
With static linking, you rebuild the scripting language interpreter with extensions. The process usually involves compiling a short main program that adds your customized commands to the language and starts the interpreter. You then link your program with a library to produce a new scripting language executable.
Although static linking is supported on all platforms, this is not the preferred technique for building scripting language extensions. In fact, there are very few practical reasons for doing this--consider using shared libraries instead.
This chapter describes the basic operation of SWIG, the structure of its input files, and how it handles standard ANSI C declarations. C++ support is described in the next chapter. However, C++ programmers should still read this chapter to understand the basics. Specific details about each target language are described in later chapters.
To run SWIG, use the swig command with options and a filename like this:
swig [ options ] filename
where filename is a SWIG interface file or a C/C++ header file. Below is a subset of options that can be used. Additional options are also defined for each target language. A full list can be obtained by typing swig -help or swig -<lang> -help for language <lang> specific options.
-allegrocl Generate ALLEGROCL wrappers
-chicken Generate CHICKEN wrappers
-clisp Generate CLISP wrappers
-cffi Generate CFFI wrappers
-csharp Generate C# wrappers
-d Generate D wrappers
-go Generate Go wrappers
-guile Generate Guile wrappers
-java Generate Java wrappers
-javascript Generate Javascript wrappers
-lua Generate Lua wrappers
-modula3 Generate Modula 3 wrappers
-mzscheme Generate Mzscheme wrappers
-ocaml Generate Ocaml wrappers
-octave Generate Octave wrappers
-perl Generate Perl wrappers
-php Generate PHP wrappers
-pike Generate Pike wrappers
-python Generate Python wrappers
-r Generate R (aka GNU S) wrappers
-ruby Generate Ruby wrappers
-scilab Generate Scilab wrappers
-sexp Generate Lisp S-Expressions wrappers
-tcl Generate Tcl wrappers
-uffi Generate Common Lisp / UFFI wrappers
-xml Generate XML wrappers
-c++ Enable C++ parsing
-cppext ext Change file extension of C++ generated files to ext
(default is cxx, except for PHP which uses cpp)
-Dsymbol Define a preprocessor symbol
-Fstandard Display error/warning messages in commonly used format
-Fmicrosoft Display error/warning messages in Microsoft format
-help Display all options
-Idir Add a directory to the file include path
-lfile Include a SWIG library file.
-module name Set the name of the SWIG module
-o outfile Set name of C/C++ output file to <outfile>
-oh headfile Set name of C++ output header file for directors to <headfile>
-outcurrentdir Set default output dir to current dir instead of input file's path
-outdir dir Set language specific files output directory
-pcreversion Display PCRE version information
-swiglib Show location of SWIG library
-version Show SWIG version number
As input, SWIG expects a file containing ANSI C/C++ declarations and special SWIG directives. More often than not, this is a special SWIG interface file which is usually denoted with a special .i or .swg suffix. In certain cases, SWIG can be used directly on raw header files or source files. However, this is not the most typical case and there are several reasons why you might not want to do this (described later).
The most common format of a SWIG interface is as follows:
%module mymodule
%{
#include "myheader.h"
%}
// Now list ANSI C/C++ declarations
int foo;
int bar(int x);
...
The module name is supplied using the special %module directive. Modules are described further in the Modules Introduction section.
Everything in the %{ ... %} block is simply copied verbatim to the resulting wrapper file created by SWIG. This section is almost always used to include header files and other declarations that are required to make the generated wrapper code compile. It is important to emphasize that just because you include a declaration in a SWIG input file, that declaration does not automatically appear in the generated wrapper code---therefore you need to make sure you include the proper header files in the %{ ... %} section. It should be noted that the text enclosed in %{ ... %} is not parsed or interpreted by SWIG. The %{...%} syntax and semantics in SWIG is analogous to that of the declarations section used in input files to parser generation tools such as yacc or bison.
The output of SWIG is a C/C++ file that contains all of the wrapper code needed to build an extension module. SWIG may generate some additional files depending on the target language. By default, an input file with the name file.i is transformed into a file file_wrap.c or file_wrap.cxx (depending on whether or not the -c++ option has been used). The name of the output C/C++ file can be changed using the -o option. In certain cases, file suffixes are used by the compiler to determine the source language (C, C++, etc.). Therefore, you have to use the -o option to change the suffix of the SWIG-generated wrapper file if you want something different than the default. For example:
$ swig -c++ -python -o example_wrap.cpp example.i
The C/C++ output file created by SWIG often contains everything that is needed to construct an extension module for the target scripting language. SWIG is not a stub compiler nor is it usually necessary to edit the output file (and if you look at the output, you probably won't want to). To build the final extension module, the SWIG output file is compiled and linked with the rest of your C/C++ program to create a shared library.
For many target languages SWIG will also generate proxy class files in the target language. The default output directory for these language specific files is the same directory as the generated C/C++ file. This can be modified using the -outdir option. For example:
$ swig -c++ -python -outdir pyfiles -o cppfiles/example_wrap.cpp example.i
If the directories cppfiles and pyfiles exist, the following will be generated:
cppfiles/example_wrap.cpp pyfiles/example.py
If the -outcurrentdir option is used (without -o) then SWIG behaves like a typical C/C++ compiler and the default output directory is then the current directory. Without this option the default output directory is the path to the input file. If -o and -outcurrentdir are used together, -outcurrentdir is effectively ignored as the output directory for the language files is the same directory as the generated C/C++ file if not overridden with -outdir.
C and C++ style comments may appear anywhere in interface files. In previous versions of SWIG, comments were used to generate documentation files. However, this feature is currently under repair and will reappear in a later SWIG release.
Like C, SWIG preprocesses all input files through an enhanced version of the C preprocessor. All standard preprocessor features are supported including file inclusion, conditional compilation and macros. However, #include statements are ignored unless the -includeall command line option has been supplied. The reason for disabling includes is that SWIG is sometimes used to process raw C header files. In this case, you usually only want the extension module to include functions in the supplied header file rather than everything that might be included by that header file (i.e., system headers, C library functions, etc.).
It should also be noted that the SWIG preprocessor skips all text enclosed inside a %{...%} block. In addition, the preprocessor includes a number of macro handling enhancements that make it more powerful than the normal C preprocessor. These extensions are described in the "Preprocessor" chapter.
Most of SWIG's operation is controlled by special directives that are always preceded by a "%" to distinguish them from normal C declarations. These directives are used to give SWIG hints or to alter SWIG's parsing behavior in some manner.
Since SWIG directives are not legal C syntax, it is generally not possible to include them in header files. However, SWIG directives can be included in C header files using conditional compilation like this:
/* header.h --- Some header file */ /* SWIG directives -- only seen if SWIG is running */ #ifdef SWIG %module foo #endif
SWIG is a special preprocessing symbol defined by SWIG when it is parsing an input file.
Although SWIG can parse most C/C++ declarations, it does not provide a complete C/C++ parser implementation. Most of these limitations pertain to very complicated type declarations and certain advanced C++ features. Specifically, the following features are not currently supported:
Non-conventional type declarations. For example, SWIG does not support declarations such as the following (even though this is legal C):
/* Non-conventional placement of storage specifier (extern) */ const int extern Number; /* Extra declarator grouping */ Matrix (foo); // A global variable /* Extra declarator grouping in parameters */ void bar(Spam (Grok)(Doh));
In practice, few (if any) C programmers actually write code like this since this style is never featured in programming books. However, if you're feeling particularly obfuscated, you can certainly break SWIG (although why would you want to?).
Running SWIG on C++ source files (the code in a .C, .cpp or .cxx file) is not recommended. The usual approach is to feed SWIG header files for parsing C++ definitions and declarations. The main reason is if SWIG parses a scoped definition or declaration (as is normal for C++ source files), it is ignored, unless a declaration for the symbol was parsed earlier. For example
/* bar not wrapped unless foo has been defined and
the declaration of bar within foo has already been parsed */
int foo::bar(int) {
... whatever ...
}
Certain advanced features of C++ such as nested classes are not yet fully supported. Please see the C++ Nested classes section for more information.
In the event of a parsing error, conditional compilation can be used to skip offending code. For example:
#ifndef SWIG ... some bad declarations ... #endif
Alternatively, you can just delete the offending code from the interface file.
One of the reasons why SWIG does not provide a full C++ parser implementation is that it has been designed to work with incomplete specifications and to be very permissive in its handling of C/C++ datatypes (e.g., SWIG can generate interfaces even when there are missing class declarations or opaque datatypes). Unfortunately, this approach makes it extremely difficult to implement certain parts of a C/C++ parser as most compilers use type information to assist in the parsing of more complex declarations (for the truly curious, the primary complication in the implementation is that the SWIG parser does not utilize a separate typedef-name terminal symbol as described on p. 234 of K&R).
SWIG wraps simple C declarations by creating an interface that closely matches the way in which the declarations would be used in a C program. For example, consider the following interface file:
%module example
%inline %{
extern double sin(double x);
extern int strcmp(const char *, const char *);
extern int Foo;
%}
#define STATUS 50
#define VERSION "1.1"
In this file, there are two functions sin() and strcmp(), a global variable Foo, and two constants STATUS and VERSION. When SWIG creates an extension module, these declarations are accessible as scripting language functions, variables, and constants respectively. For example, in Tcl:
% sin 3 5.2335956 % strcmp Dave Mike -1 % puts $Foo 42 % puts $STATUS 50 % puts $VERSION 1.1
Or in Python:
>>> example.sin(3)
5.2335956
>>> example.strcmp('Dave','Mike')
-1
>>> print example.cvar.Foo
42
>>> print example.STATUS
50
>>> print example.VERSION
1.1
Whenever possible, SWIG creates an interface that closely matches the underlying C/C++ code. However, due to subtle differences between languages, run-time environments, and semantics, it is not always possible to do so. The next few sections describe various aspects of this mapping.
In order to build an interface, SWIG has to convert C/C++ datatypes to equivalent types in the target language. Generally, scripting languages provide a more limited set of primitive types than C. Therefore, this conversion process involves a certain amount of type coercion.
Most scripting languages provide a single integer type that is implemented using the int or long datatype in C. The following list shows all of the C datatypes that SWIG will convert to and from integers in the target language:
int short long unsigned signed unsigned short unsigned long unsigned char signed char bool
When an integral value is converted from C, a cast is used to convert it to the representation in the target language. Thus, a 16 bit short in C may be promoted to a 32 bit integer. When integers are converted in the other direction, the value is cast back into the original C type. If the value is too large to fit, it is silently truncated.
unsigned char and signed char are special cases that are handled as small 8-bit integers. Normally, the char datatype is mapped as a one-character ASCII string.
The bool datatype is cast to and from an integer value of 0 and 1 unless the target language provides a special boolean type.
Some care is required when working with large integer values. Most scripting languages use 32-bit integers so mapping a 64-bit long integer may lead to truncation errors. Similar problems may arise with 32 bit unsigned integers (which may appear as large negative numbers). As a rule of thumb, the int datatype and all variations of char and short datatypes are safe to use. For unsigned int and long datatypes, you will need to carefully check the correct operation of your program after it has been wrapped with SWIG.
Although the SWIG parser supports the long long datatype, not all language modules support it. This is because long long usually exceeds the integer precision available in the target language. In certain modules such as Tcl and Perl5, long long integers are encoded as strings. This allows the full range of these numbers to be represented. However, it does not allow long long values to be used in arithmetic expressions. It should also be noted that although long long is part of the ISO C99 standard, it is not universally supported by all C compilers. Make sure you are using a compiler that supports long long before trying to use this type with SWIG.
SWIG recognizes the following floating point types :
float double
Floating point numbers are mapped to and from the natural representation of floats in the target language. This is almost always a C double. The rarely used datatype of long double is not supported by SWIG.
The char datatype is mapped into a NULL terminated ASCII string with a single character. When used in a scripting language it shows up as a tiny string containing the character value. When converting the value back into C, SWIG takes a character string from the scripting language and strips off the first character as the char value. Thus if the value "foo" is assigned to a char datatype, it gets the value `f'.
The char * datatype is handled as a NULL-terminated ASCII string. SWIG maps this into a 8-bit character string in the target scripting language. SWIG converts character strings in the target language to NULL terminated strings before passing them into C/C++. The default handling of these strings does not allow them to have embedded NULL bytes. Therefore, the char * datatype is not generally suitable for passing binary data. However, it is possible to change this behavior by defining a SWIG typemap. See the chapter on Typemaps for details about this.
At this time, SWIG provides limited support for Unicode and wide-character strings (the C wchar_t type). Some languages provide typemaps for wchar_t, but bear in mind these might not be portable across different operating systems. This is a delicate topic that is poorly understood by many programmers and not implemented in a consistent manner across languages. For those scripting languages that provide Unicode support, Unicode strings are often available in an 8-bit representation such as UTF-8 that can be mapped to the char * type (in which case the SWIG interface will probably work). If the program you are wrapping uses Unicode, there is no guarantee that Unicode characters in the target language will use the same internal representation (e.g., UCS-2 vs. UCS-4). You may need to write some special conversion functions.
Whenever possible, SWIG maps C/C++ global variables into scripting language variables. For example,
%module example double foo;
results in a scripting language variable like this:
# Tcl set foo [3.5] ;# Set foo to 3.5 puts $foo ;# Print the value of foo # Python cvar.foo = 3.5 # Set foo to 3.5 print cvar.foo # Print value of foo # Perl $foo = 3.5; # Set foo to 3.5 print $foo,"\n"; # Print value of foo # Ruby Module.foo = 3.5 # Set foo to 3.5 print Module.foo, "\n" # Print value of foo
Whenever the scripting language variable is used, the underlying C global variable is accessed. Although SWIG makes every attempt to make global variables work like scripting language variables, it is not always possible to do so. For instance, in Python, all global variables must be accessed through a special variable object known as cvar (shown above). In Ruby, variables are accessed as attributes of the module. Other languages may convert variables to a pair of accessor functions. For example, the Java module generates a pair of functions double get_foo() and set_foo(double val) that are used to manipulate the value.
Finally, if a global variable has been declared as const, it only supports read-only access. Note: this behavior is new to SWIG-1.3. Earlier versions of SWIG incorrectly handled const and created constants instead.
Constants can be created using #define, enumerations, or a special %constant directive. The following interface file shows a few valid constant declarations :
#define I_CONST 5 // An integer constant
#define PI 3.14159 // A Floating point constant
#define S_CONST "hello world" // A string constant
#define NEWLINE '\n' // Character constant
enum boolean {NO=0, YES=1};
enum months {JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG,
SEP, OCT, NOV, DEC};
%constant double BLAH = 42.37;
#define PI_4 PI/4
#define FLAGS 0x04 | 0x08 | 0x40
In #define declarations, the type of a constant is inferred by syntax. For example, a number with a decimal point is assumed to be floating point. In addition, SWIG must be able to fully resolve all of the symbols used in a #define in order for a constant to actually be created. This restriction is necessary because #define is also used to define preprocessor macros that are definitely not meant to be part of the scripting language interface. For example:
#define EXTERN extern EXTERN void foo();
In this case, you probably don't want to create a constant called EXTERN (what would the value be?). In general, SWIG will not create constants for macros unless the value can be completely determined by the preprocessor. For instance, in the above example, the declaration
#define PI_4 PI/4
defines a constant because PI was already defined as a constant and the value is known. However, for the same conservative reasons even a constant with a simple cast will be ignored, such as
#define F_CONST (double) 5 // A floating point constant with cast
The use of constant expressions is allowed, but SWIG does not evaluate them. Rather, it passes them through to the output file and lets the C compiler perform the final evaluation (SWIG does perform a limited form of type-checking however).
For enumerations, it is critical that the original enum definition be included somewhere in the interface file (either in a header file or in the %{,%} block). SWIG only translates the enumeration into code needed to add the constants to a scripting language. It needs the original enumeration declaration in order to get the correct enum values as assigned by the C compiler.
The %constant directive is used to more precisely create constants corresponding to different C datatypes. Although it is not usually needed for simple values, it is more useful when working with pointers and other more complex datatypes. Typically, %constant is only used when you want to add constants to the scripting language interface that are not defined in the original header file.
A common confusion with C programming is the semantic meaning of the const qualifier in declarations--especially when it is mixed with pointers and other type modifiers. In fact, previous versions of SWIG handled const incorrectly--a situation that SWIG-1.3.7 and newer releases have fixed.
Starting with SWIG-1.3, all variable declarations, regardless of any use of const, are wrapped as global variables. If a declaration happens to be declared as const, it is wrapped as a read-only variable. To tell if a variable is const or not, you need to look at the right-most occurrence of the const qualifier (that appears before the variable name). If the right-most const occurs after all other type modifiers (such as pointers), then the variable is const. Otherwise, it is not.
Here are some examples of const declarations.
const char a; // A constant character char const b; // A constant character (the same) char *const c; // A constant pointer to a character const char *const d; // A constant pointer to a constant character
Here is an example of a declaration that is not const:
const char *e; // A pointer to a constant character. The pointer
// may be modified.
In this case, the pointer e can change---it's only the value being pointed to that is read-only.
Please note that for const parameters or return types used in a function, SWIG pretty much ignores the fact that these are const, see the section on const-correctness for more information.
Compatibility Note: One reason for changing SWIG to handle const declarations as read-only variables is that there are many situations where the value of a const variable might change. For example, a library might export a symbol as const in its public API to discourage modification, but still allow the value to change through some other kind of internal mechanism. Furthermore, programmers often overlook the fact that with a constant declaration like char *const, the underlying data being pointed to can be modified--it's only the pointer itself that is constant. In an embedded system, a const declaration might refer to a read-only memory address such as the location of a memory-mapped I/O device port (where the value changes, but writing to the port is not supported by the hardware). Rather than trying to build a bunch of special cases into the const qualifier, the new interpretation of const as "read-only" is simple and exactly matches the actual semantics of const in C/C++. If you really want to create a constant as in older versions of SWIG, use the %constant directive instead. For example:
%constant double PI = 3.14159;
or
#ifdef SWIG #define const %constant #endif const double foo = 3.4; const double bar = 23.4; const int spam = 42; #ifdef SWIG #undef const #endif ...
Before going any further, there is one bit of caution involving char * that must now be mentioned. When strings are passed from a scripting language to a C char *, the pointer usually points to string data stored inside the interpreter. It is almost always a really bad idea to modify this data. Furthermore, some languages may explicitly disallow it. For instance, in Python, strings are supposed to be immutable. If you violate this, you will probably receive a vast amount of wrath when you unleash your module on the world.
The primary source of problems are functions that might modify string data in place. A classic example would be a function like this:
char *strcat(char *s, const char *t)
Although SWIG will certainly generate a wrapper for this, its behavior will be undefined. In fact, it will probably cause your application to crash with a segmentation fault or other memory related problem. This is because s refers to some internal data in the target language---data that you shouldn't be touching.
The bottom line: don't rely on char * for anything other than read-only input values. However, it must be noted that you could change the behavior of SWIG using typemaps.
Most C programs manipulate arrays, structures, and other types of objects. This section discusses the handling of these datatypes.
Pointers to primitive C datatypes such as
int * double *** char **
are fully supported by SWIG. Rather than trying to convert the data being pointed to into a scripting representation, SWIG simply encodes the pointer itself into a representation that contains the actual value of the pointer and a type-tag. Thus, the SWIG representation of the above pointers (in Tcl), might look like this:
_10081012_p_int _1008e124_ppp_double _f8ac_pp_char
A NULL pointer is represented by the string "NULL" or the value 0 encoded with type information.
All pointers are treated as opaque objects by SWIG. Thus, a pointer may be returned by a function and passed around to other C functions as needed. For all practical purposes, the scripting language interface works in exactly the same way as you would use the pointer in a C program. The only difference is that there is no mechanism for dereferencing the pointer since this would require the target language to understand the memory layout of the underlying object.
The scripting language representation of a pointer value should never be manipulated directly. Even though the values shown look like hexadecimal addresses, the numbers used may differ from the actual machine address (e.g., on little-endian machines, the digits may appear in reverse order). Furthermore, SWIG does not normally map pointers into high-level objects such as associative arrays or lists (for example, converting an int * into an list of integers). There are several reasons why SWIG does not do this:
By allowing pointers to be manipulated from a scripting language, extension modules effectively bypass compile-time type checking in the C/C++ compiler. To prevent errors, a type signature is encoded into all pointer values and is used to perform run-time type checking. This type-checking process is an integral part of SWIG and can not be disabled or modified without using typemaps (described in later chapters).
Like C, void * matches any kind of pointer. Furthermore, NULL pointers can be passed to any function that expects to receive a pointer. Although this has the potential to cause a crash, NULL pointers are also sometimes used as sentinel values or to denote a missing/empty value. Therefore, SWIG leaves NULL pointer checking up to the application.
For everything else (structs, classes, arrays, etc...) SWIG applies a very simple rule :
In other words, SWIG manipulates everything else by reference. This model makes sense because most C/C++ programs make heavy use of pointers and SWIG can use the type-checked pointer mechanism already present for handling pointers to basic datatypes.
Although this probably sounds complicated, it's really quite simple. Suppose you have an interface file like this :
%module fileio FILE *fopen(char *, char *); int fclose(FILE *); unsigned fread(void *ptr, unsigned size, unsigned nobj, FILE *); unsigned fwrite(void *ptr, unsigned size, unsigned nobj, FILE *); void *malloc(int nbytes); void free(void *);
In this file, SWIG doesn't know what a FILE is, but since it's used as a pointer, so it doesn't really matter what it is. If you wrapped this module into Python, you can use the functions just like you expect :
# Copy a file
def filecopy(source,target):
f1 = fopen(source,"r")
f2 = fopen(target,"w")
buffer = malloc(8192)
nbytes = fread(buffer,8192,1,f1)
while (nbytes > 0):
fwrite(buffer,8192,1,f2)
nbytes = fread(buffer,8192,1,f1)
free(buffer)
In this case f1, f2, and buffer are all opaque objects containing C pointers. It doesn't matter what value they contain--our program works just fine without this knowledge.
When SWIG encounters an undeclared datatype, it automatically assumes that it is a structure or class. For example, suppose the following function appeared in a SWIG input file:
void matrix_multiply(Matrix *a, Matrix *b, Matrix *c);
SWIG has no idea what a "Matrix" is. However, it is obviously a pointer to something so SWIG generates a wrapper using its generic pointer handling code.
Unlike C or C++, SWIG does not actually care whether Matrix has been previously defined in the interface file or not. This allows SWIG to generate interfaces from only partial or limited information. In some cases, you may not care what a Matrix really is as long as you can pass an opaque reference to one around in the scripting language interface.
An important detail to mention is that SWIG will gladly generate wrappers for an interface when there are unspecified type names. However, all unspecified types are internally handled as pointers to structures or classes! For example, consider the following declaration:
void foo(size_t num);
If size_t is undeclared, SWIG generates wrappers that expect to receive a type of size_t * (this mapping is described shortly). As a result, the scripting interface might behave strangely. For example:
foo(40); TypeError: expected a _p_size_t.
The only way to fix this problem is to make sure you properly declare type names using typedef.
Like C, typedef can be used to define new type names in SWIG. For example:
typedef unsigned int size_t;
typedef definitions appearing in a SWIG interface are not propagated to the generated wrapper code. Therefore, they either need to be defined in an included header file or placed in the declarations section like this:
%{
/* Include in the generated wrapper file */
typedef unsigned int size_t;
%}
/* Tell SWIG about it */
typedef unsigned int size_t;
or
%inline %{
typedef unsigned int size_t;
%}
In certain cases, you might be able to include other header files to collect type information. For example:
%module example %import "sys/types.h"
In this case, you might run SWIG as follows:
$ swig -I/usr/include -includeall example.i
It should be noted that your mileage will vary greatly here. System headers are notoriously complicated and may rely upon a variety of non-standard C coding extensions (e.g., such as special directives to GCC). Unless you exactly specify the right include directories and preprocessor symbols, this may not work correctly (you will have to experiment).
SWIG tracks typedef declarations and uses this information for run-time type checking. For instance, if you use the above typedef and had the following function declaration:
void foo(unsigned int *ptr);
The corresponding wrapper function will accept arguments of type unsigned int * or size_t *.
So far, this chapter has presented almost everything you need to know to use SWIG for simple interfaces. However, some C programs use idioms that are somewhat more difficult to map to a scripting language interface. This section describes some of these issues.
Sometimes a C function takes structure parameters that are passed by value. For example, consider the following function:
double dot_product(Vector a, Vector b);
To deal with this, SWIG transforms the function to use pointers by creating a wrapper equivalent to the following:
double wrap_dot_product(Vector *a, Vector *b) {
Vector x = *a;
Vector y = *b;
return dot_product(x,y);
}
In the target language, the dot_product() function now accepts pointers to Vectors instead of Vectors. For the most part, this transformation is transparent so you might not notice.
C functions that return structures or classes datatypes by value are more difficult to handle. Consider the following function:
Vector cross_product(Vector v1, Vector v2);
This function wants to return Vector, but SWIG only really supports pointers. As a result, SWIG creates a wrapper like this:
Vector *wrap_cross_product(Vector *v1, Vector *v2) {
Vector x = *v1;
Vector y = *v2;
Vector *result;
result = (Vector *) malloc(sizeof(Vector));
*(result) = cross(x,y);
return result;
}
or if SWIG was run with the -c++ option:
Vector *wrap_cross(Vector *v1, Vector *v2) {
Vector x = *v1;
Vector y = *v2;
Vector *result = new Vector(cross(x,y)); // Uses default copy constructor
return result;
}
In both cases, SWIG allocates a new object and returns a reference to it. It is up to the user to delete the returned object when it is no longer in use. Clearly, this will leak memory if you are unaware of the implicit memory allocation and don't take steps to free the result. That said, it should be noted that some language modules can now automatically track newly created objects and reclaim memory for you. Consult the documentation for each language module for more details.
It should also be noted that the handling of pass/return by value in C++ has some special cases. For example, the above code fragments don't work correctly if Vector doesn't define a default constructor. The section on SWIG and C++ has more information about this case.
When global variables or class members involving structures are encountered, SWIG handles them as pointers. For example, a global variable like this
Vector unit_i;
gets mapped to an underlying pair of set/get functions like this :
Vector *unit_i_get() {
return &unit_i;
}
void unit_i_set(Vector *value) {
unit_i = *value;
}
Again some caution is in order. A global variable created in this manner will show up as a pointer in the target scripting language. It would be an extremely bad idea to free or destroy such a pointer. Also, C++ classes must supply a properly defined copy constructor in order for assignment to work correctly.
When a global variable of type char * appears, SWIG uses malloc() or new to allocate memory for the new value. Specifically, if you have a variable like this
char *foo;
SWIG generates the following code:
/* C mode */
void foo_set(char *value) {
if (foo) free(foo);
foo = (char *) malloc(strlen(value)+1);
strcpy(foo,value);
}
/* C++ mode. When -c++ option is used */
void foo_set(char *value) {
if (foo) delete [] foo;
foo = new char[strlen(value)+1];
strcpy(foo,value);
}
If this is not the behavior that you want, consider making the variable read-only using the %immutable directive. Alternatively, you might write a short assist-function to set the value exactly like you want. For example:
%inline %{
void set_foo(char *value) {
strncpy(foo,value, 50);
}
%}
Note: If you write an assist function like this, you will have to call it as a function from the target scripting language (it does not work like a variable). For example, in Python you will have to write:
>>> set_foo("Hello World")
A common mistake with char * variables is to link to a variable declared like this:
char *VERSION = "1.0";
In this case, the variable will be readable, but any attempt to change the value results in a segmentation or general protection fault. This is due to the fact that SWIG is trying to release the old value using free or delete when the string literal value currently assigned to the variable wasn't allocated using malloc() or new. To fix this behavior, you can either mark the variable as read-only, write a typemap (as described in Chapter 6), or write a special set function as shown. Another alternative is to declare the variable as an array:
char VERSION[64] = "1.0";
When variables of type const char * are declared, SWIG still generates functions for setting and getting the value. However, the default behavior does not release the previous contents (resulting in a possible memory leak). In fact, you may get a warning message such as this when wrapping such a variable:
example.i:20. Typemap warning. Setting const char * variable may leak memory
The reason for this behavior is that const char * variables are often used to point to string literals. For example:
const char *foo = "Hello World\n";
Therefore, it's a really bad idea to call free() on such a pointer. On the other hand, it is legal to change the pointer to point to some other value. When setting a variable of this type, SWIG allocates a new string (using malloc or new) and changes the pointer to point to the new value. However, repeated modifications of the value will result in a memory leak since the old value is not released.
Arrays are fully supported by SWIG, but they are always handled as pointers instead of mapping them to a special array object or list in the target language. Thus, the following declarations :
int foobar(int a[40]); void grok(char *argv[]); void transpose(double a[20][20]);
are processed as if they were really declared like this:
int foobar(int *a); void grok(char **argv); void transpose(double (*a)[20]);
Like C, SWIG does not perform array bounds checking. It is up to the user to make sure the pointer points to a suitably allocated region of memory.
Multi-dimensional arrays are transformed into a pointer to an array of one less dimension. For example:
int [10]; // Maps to int * int [10][20]; // Maps to int (*)[20] int [10][20][30]; // Maps to int (*)[20][30]
It is important to note that in the C type system, a multidimensional array a[][] is NOT equivalent to a single pointer *a or a double pointer such as **a. Instead, a pointer to an array is used (as shown above) where the actual value of the pointer is the starting memory location of the array. The reader is strongly advised to dust off their C book and re-read the section on arrays before using them with SWIG.
Array variables are supported, but are read-only by default. For example:
int a[100][200];
In this case, reading the variable 'a' returns a pointer of type int (*)[200] that points to the first element of the array &a[0][0]. Trying to modify 'a' results in an error. This is because SWIG does not know how to copy data from the target language into the array. To work around this limitation, you may want to write a few simple assist functions like this:
%inline %{
void a_set(int i, int j, int val) {
a[i][j] = val;
}
int a_get(int i, int j) {
return a[i][j];
}
%}
To dynamically create arrays of various sizes and shapes, it may be useful to write some helper functions in your interface. For example:
// Some array helpers
%inline %{
/* Create any sort of [size] array */
int *int_array(int size) {
return (int *) malloc(size*sizeof(int));
}
/* Create a two-dimension array [size][10] */
int (*int_array_10(int size))[10] {
return (int (*)[10]) malloc(size*10*sizeof(int));
}
%}
Arrays of char are handled as a special case by SWIG. In this case, strings in the target language can be stored in the array. For example, if you have a declaration like this,
char pathname[256];
SWIG generates functions for both getting and setting the value that are equivalent to the following code:
char *pathname_get() {
return pathname;
}
void pathname_set(char *value) {
strncpy(pathname,value,256);
}
In the target language, the value can be set like a normal variable.
A read-only variable can be created by using the %immutable directive as shown :
// File : interface.i int a; // Can read/write %immutable; int b,c,d; // Read only variables %mutable; double x,y; // read/write
The %immutable directive enables read-only mode until it is explicitly disabled using the %mutable directive. As an alternative to turning read-only mode off and on like this, individual declarations can also be tagged as immutable. For example:
%immutable x; // Make x read-only ... double x; // Read-only (from earlier %immutable directive) double y; // Read-write ...
The %mutable and %immutable directives are actually %feature directives defined like this:
#define %immutable %feature("immutable")
#define %mutable %feature("immutable","")
If you wanted to make all wrapped variables read-only, barring one or two, it might be easier to take this approach:
%immutable; // Make all variables read-only
%feature("immutable","0") x; // except, make x read/write
...
double x;
double y;
double z;
...
Read-only variables are also created when declarations are declared as const. For example:
const int foo; /* Read only variable */ char * const version="1.0"; /* Read only variable */
Compatibility note: Read-only access used to be controlled by a pair of directives %readonly and %readwrite. Although these directives still work, they generate a warning message. Simply change the directives to %immutable; and %mutable; to silence the warning. Don't forget the extra semicolon!
Normally, the name of a C declaration is used when that declaration is wrapped into the target language. However, this may generate a conflict with a keyword or already existing function in the scripting language. To resolve a name conflict, you can use the %rename directive as shown :
// interface.i %rename(my_print) print; extern void print(const char *); %rename(foo) a_really_long_and_annoying_name; extern int a_really_long_and_annoying_name;
SWIG still calls the correct C function, but in this case the function print() will really be called "my_print()" in the target language.
The placement of the %rename directive is arbitrary as long as it appears before the declarations to be renamed. A common technique is to write code for wrapping a header file like this:
// interface.i %rename(my_print) print; %rename(foo) a_really_long_and_annoying_name; %include "header.h"
%rename applies a renaming operation to all future occurrences of a name. The renaming applies to functions, variables, class and structure names, member functions, and member data. For example, if you had two-dozen C++ classes, all with a member function named `print' (which is a keyword in Python), you could rename them all to `output' by specifying :
%rename(output) print; // Rename all `print' functions to `output'
SWIG does not normally perform any checks to see if the functions it wraps are already defined in the target scripting language. However, if you are careful about namespaces and your use of modules, you can usually avoid these problems.
Closely related to %rename is the %ignore directive. %ignore instructs SWIG to ignore declarations that match a given identifier. For example:
%ignore print; // Ignore all declarations named print %ignore MYMACRO; // Ignore a macro ... #define MYMACRO 123 void print(const char *); ...
Any function, variable etc which matches %ignore will not be wrapped and therefore will not be available from the target language. A common usage of %ignore is to selectively remove certain declarations from a header file without having to add conditional compilation to the header. However, it should be stressed that this only works for simple declarations. If you need to remove a whole section of problematic code, the SWIG preprocessor should be used instead.
Compatibility note: Older versions of SWIG provided a special %name directive for renaming declarations. For example:
%name(output) extern void print(const char *);
This directive is still supported, but it is deprecated and should probably be avoided. The %rename directive is more powerful and better supports wrapping of raw header file information.
While writing %rename for specific declarations is simple enough, sometimes the same renaming rule needs to be applied to many, maybe all, identifiers in the SWIG input. For example, it may be necessary to apply some transformation to all the names in the target language to better follow its naming conventions, like adding a specific prefix to all wrapped functions. Doing it individually for each function is impractical so SWIG supports applying a renaming rule to all declarations if the name of the identifier to be renamed is not specified:
%rename("myprefix_%s") ""; // print -> myprefix_print
This also shows that the argument of %rename doesn't have to be a literal string but can be a printf()-like format string. In the simplest form, "%s" is replaced with the name of the original declaration, as shown above. However this is not always enough and SWIG provides extensions to the usual format string syntax to allow applying a (SWIG-defined) function to the argument. For example, to wrap all C functions do_something_long() as more Java-like doSomethingLong() you can use the "lowercamelcase" extended format specifier like this:
%rename("%(lowercamelcase)s") ""; // foo_bar -> fooBar; FooBar -> fooBar
Some functions can be parametrized, for example the "strip" one strips the provided prefix from its argument. The prefix is specified as part of the format string, following a colon after the function name:
%rename("%(strip:[wx])s") ""; // wxHello -> Hello; FooBar -> FooBar
Below is the table summarizing all currently defined functions with an example of applying each one. Note that some of them have two names, a shorter one and a more descriptive one, but the two functions are otherwise equivalent:
| Function | Returns | Example (in/out) | |
|---|---|---|---|
| uppercase or upper | Upper case version of the string. | ||
| lowercase or lower | Lower case version of the string. | ||
| title | String with first letter capitalized and the rest in lower case. | ||
| firstuppercase | String with the first letter capitalized and the rest unchanged. | printIt | PrintIt |
| firstlowercase | String with the first letter in lower case and the rest unchanged. | PrintIt | printIt |
| camelcase or ctitle | String with capitalized first letter and any letter following an underscore (which are removed in the process) and rest in lower case. | print_it | PrintIt |
| lowercamelcase or lctitle | String with every letter following an underscore (which is removed in the process) capitalized and rest, including the first letter, in lower case. | print_it | printIt |
| undercase or utitle | Lower case string with underscores inserted before every upper case letter in the original string and any number not at the end of string. Logically, this is the reverse of camelcase. | PrintIt | print_it |
| schemify | String with all underscores replaced with dashes, resulting in more Lispers/Schemers-pleasing name. | print_it | print-it |
| strip:[prefix] | String without the given prefix or the original string if it doesn't start with this prefix. Note that square brackets should be used literally, e.g. %rename("strip:[wx]") | wxPrint | |
| rstrip:[suffix] | String without the given suffix or the original string if it doesn't end with this suffix. Note that square brackets should be used literally, e.g. %rename("rstrip:[Cls]") | PrintCls | |
| regex:/pattern/subst/ | String after (Perl-like) regex substitution operation. This function allows to apply arbitrary regular expressions to the identifier names. The pattern part is a regular expression in Perl syntax (as supported by the Perl Compatible Regular Expressions (PCRE)) library and the subst string can contain back-references of the form \N where N is a digit from 0 to 9, or one of the following escape sequences: \l, \L, \u, \U or \E. The back-references are replaced with the contents of the corresponding capture group while the escape sequences perform the case conversion in the substitution string: \l and \L convert to the lower case, while \u and \U convert to the upper case. The difference between the elements of each pair is that \l and \u change the case of the next character only, while \L and \U do it for all the remaining characters or until \E is encountered. Finally please notice that backslashes need to be escaped in C strings, so in practice "\\" must be used in all these escape sequences. For example, to remove any alphabetic prefix before an underscore and capitalize the remaining part you could use the following directive: %rename("regex:/(\\w+)_(.*)/\\u\\2/") | prefix_print | |
| command:cmd | Output of an external command cmd with the string passed to it as input. Notice that this function is extremely slow compared to all the other ones as it involves spawning a separate process and using it for many declarations is not recommended. The cmd is not enclosed in square brackets but must be terminated with a triple '<' sign, e.g. %rename("command:tr -d aeiou <<<") (nonsensical example removing all vowels) | Prnt | |
The most general function of all of the above ones (not counting command which is even more powerful in principle but which should generally be avoided because of performance considerations) is the regex one. Here are some more examples of its use:
// Strip the wx prefix from all identifiers except those starting with wxEVT
%rename("%(regex:/wx(?!EVT)(.*)/\\1/)s") ""; // wxSomeWidget -> SomeWidget
// wxEVT_PAINT -> wxEVT_PAINT
// Apply a rule for renaming the enum elements to avoid the common prefixes
// which are redundant in C#/Java
%rename("%(regex:/^([A-Z][a-z]+)+_(.*)/\\2/)s", %$isenumitem) ""; // Colour_Red -> Red
// Remove all "Set/Get" prefixes.
%rename("%(regex:/^(Set|Get)(.*)/\\2/)s") ""; // SetValue -> Value
// GetValue -> Value
As before, everything that was said above about %rename also applies to %ignore. In fact, the latter is just a special case of the former and ignoring an identifier is the same as renaming it to the special "$ignore" value. So the following snippets
%ignore print;
and
%rename("$ignore") print;
are exactly equivalent and %rename can be used to selectively ignore multiple declarations using the previously described matching possibilities.
As explained in the previous sections, it is possible to either rename individual declarations or apply a rename rule to all of them at once. In practice, the latter is however rarely appropriate as there are always some exceptions to the general rules. To deal with them, the scope of an unnamed %rename can be limited using subsequent match parameters. They can be applied to any of the attributes associated by SWIG with the declarations appearing in its input. For example:
%rename("foo", match$name="bar") "";
can be used to achieve the same effect as the simpler
%rename("foo") bar;
and so is not very interesting on its own. However match can also be applied to the declaration type, for example match="class" restricts the match to class declarations only (in C++) and match="enumitem" restricts it to the enum elements. SWIG also provides convenience macros for such match expressions, for example
%rename("%(title)s", %$isenumitem) "";
will capitalize the names of all the enum elements but not change the case of the other declarations. Similarly, %$isclass, %$isfunction, %$isconstructor, %$isunion, %$istemplate, and %$isvariable can be used. Many other checks are possible and this documentation is not exhaustive, see the "%rename predicates" section in swig.swg for the full list of supported match expressions.
In addition to literally matching some string with match you can also use regexmatch or notregexmatch to match a string against a regular expression. For example, to ignore all functions having "Old" as a suffix you could use
%rename("$ignore", regexmatch$name="Old$") "";
For simple cases like this, specifying the regular expression for the declaration name directly can be preferable and can also be done using regextarget:
%rename("$ignore", regextarget=1) "Old$";
Notice that the check is done only against the name of the declaration itself, if you need to match the full name of a C++ declaration you must use fullname attribute:
%rename("$ignore", regextarget=1, fullname=1) "NameSpace::ClassName::.*Old$";
As for notregexmatch, it restricts the match only to the strings not matching the specified regular expression. So to rename all declarations to lower case except those consisting of capital letters only:
%rename("$(lower)s", notregexmatch$name="^[A-Z]+$") "";
Finally, variants of %rename and %ignore directives can be used to help wrap C++ overloaded functions and methods or C++ methods which use default arguments. This is described in the Ambiguity resolution and renaming section in the C++ chapter.
Using the techniques described above it is possible to ignore everything in a header and then selectively wrap a few chosen methods or classes. For example, consider a header, myheader.h which has many classes in it and just the one class called Star is wanted within this header, the following approach could be taken:
%ignore ""; // Ignore everything
// Unignore chosen class 'Star'
%rename("%s") Star;
// As the ignore everything will include the constructor, destructor, methods etc
// in the class, these have to be explicitly unignored too:
%rename("%s") Star::Star;
%rename("%s") Star::~Star;
%rename("%s") Star::shine; // named method
%include "myheader.h"
Another approach which might be more suitable as it does not require naming all the methods in the chosen class is to begin by ignoring just the classes. This does not add an explicit ignore to any members of the class, so when the chosen class is unignored, all of its methods will be wrapped.
%rename($ignore, %$isclass) ""; // Only ignore all classes
%rename("%s") Star; // Unignore 'Star'
%include "myheader.h"
SWIG supports default arguments in both C and C++ code. For example:
int plot(double x, double y, int color=WHITE);
In this case, SWIG generates wrapper code where the default arguments are optional in the target language. For example, this function could be used in Tcl as follows :
% plot -3.4 7.5 # Use default value % plot -3.4 7.5 10 # set color to 10 instead
Although the ANSI C standard does not allow default arguments, default arguments specified in a SWIG interface work with both C and C++.
Note: There is a subtle semantic issue concerning the use of default arguments and the SWIG generated wrapper code. When default arguments are used in C code, the default values are emitted into the wrappers and the function is invoked with a full set of arguments. This is different to when wrapping C++ where an overloaded wrapper method is generated for each defaulted argument. Please refer to the section on default arguments in the C++ chapter for further details.
Occasionally, a C library may include functions that expect to receive pointers to functions--possibly to serve as callbacks. SWIG provides full support for function pointers provided that the callback functions are defined in C and not in the target language. For example, consider a function like this:
int binary_op(int a, int b, int (*op)(int,int));
When you first wrap something like this into an extension module, you may find the function to be impossible to use. For instance, in Python:
>>> def add(x,y): ... return x+y ... >>> binary_op(3,4,add) Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: Type error. Expected _p_f_int_int__int >>>
The reason for this error is that SWIG doesn't know how to map a scripting language function into a C callback. However, existing C functions can be used as arguments provided you install them as constants. One way to do this is to use the %constant directive like this:
/* Function with a callback */ int binary_op(int a, int b, int (*op)(int,int)); /* Some callback functions */ %constant int add(int,int); %constant int sub(int,int); %constant int mul(int,int);
In this case, add, sub, and mul become function pointer constants in the target scripting language. This allows you to use them as follows:
>>> binary_op(3,4,add) 7 >>> binary_op(3,4,mul) 12 >>>
Unfortunately, by declaring the callback functions as constants, they are no longer accessible as functions. For example:
>>> add(3,4) Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: object is not callable: '_ff020efc_p_f_int_int__int' >>>
If you want to make a function available as both a callback function and a function, you can use the %callback and %nocallback directives like this:
/* Function with a callback */
int binary_op(int a, int b, int (*op)(int,int));
/* Some callback functions */
%callback("%s_cb");
int add(int,int);
int sub(int,int);
int mul(int,int);
%nocallback;
The argument to %callback is a printf-style format string that specifies the naming convention for the callback constants (%s gets replaced by the function name). The callback mode remains in effect until it is explicitly disabled using %nocallback. When you do this, the interface now works as follows:
>>> binary_op(3,4,add_cb) 7 >>> binary_op(3,4,mul_cb) 12 >>> add(3,4) 7 >>> mul(3,4) 12
Notice that when the function is used as a callback, special names such as add_cb are used instead. To call the function normally, just use the original function name such as add().
SWIG provides a number of extensions to standard C printf formatting that may be useful in this context. For instance, the following variation installs the callbacks as all upper case constants such as ADD, SUB, and MUL:
/* Some callback functions */
%callback("%(uppercase)s");
int add(int,int);
int sub(int,int);
int mul(int,int);
%nocallback;
A format string of "%(lowercase)s" converts all characters to lower case. A string of "%(title)s" capitalizes the first character and converts the rest to lower case.
And now, a final note about function pointer support. Although SWIG does not normally allow callback functions to be written in the target language, this can be accomplished with the use of typemaps and other advanced SWIG features. See the Typemaps chapter for more about typemaps and individual target language chapters for more on callbacks and the 'director' feature.
This section describes the behavior of SWIG when processing ANSI C structures and union declarations. Extensions to handle C++ are described in the next section.
If SWIG encounters the definition of a structure or union, it creates a set of accessor functions. Although SWIG does not need structure definitions to build an interface, providing definitions makes it possible to access structure members. The accessor functions generated by SWIG simply take a pointer to an object and allow access to an individual member. For example, the declaration :
struct Vector {
double x,y,z;
}
gets transformed into the following set of accessor functions :
double Vector_x_get(struct Vector *obj) {
return obj->x;
}
double Vector_y_get(struct Vector *obj) {
return obj->y;
}
double Vector_z_get(struct Vector *obj) {
return obj->z;
}
void Vector_x_set(struct Vector *obj, double value) {
obj->x = value;
}
void Vector_y_set(struct Vector *obj, double value) {
obj->y = value;
}
void Vector_z_set(struct Vector *obj, double value) {
obj->z = value;
}
In addition, SWIG creates default constructor and destructor functions if none are defined in the interface. For example:
struct Vector *new_Vector() {
return (Vector *) calloc(1,sizeof(struct Vector));
}
void delete_Vector(struct Vector *obj) {
free(obj);
}
Using these low-level accessor functions, an object can be minimally manipulated from the target language using code like this:
v = new_Vector() Vector_x_set(v,2) Vector_y_set(v,10) Vector_z_set(v,-5) ... delete_Vector(v)
However, most of SWIG's language modules also provide a high-level interface that is more convenient. Keep reading.
SWIG supports the following construct which is quite common in C programs :
typedef struct {
double x,y,z;
} Vector;
When encountered, SWIG assumes that the name of the object is `Vector' and creates accessor functions like before. The only difference is that the use of typedef allows SWIG to drop the struct keyword on its generated code. For example:
double Vector_x_get(Vector *obj) {
return obj->x;
}
If two different names are used like this :
typedef struct vector_struct {
double x,y,z;
} Vector;
the name Vector is used instead of vector_struct since this is more typical C programming style. If declarations defined later in the interface use the type struct vector_struct, SWIG knows that this is the same as Vector and it generates the appropriate type-checking code.
Structures involving character strings require some care. SWIG assumes that all members of type char * have been dynamically allocated using malloc() and that they are NULL-terminated ASCII strings. When such a member is modified, the previous contents will be released, and the new contents allocated. For example :
%module mymodule
...
struct Foo {
char *name;
...
}
This results in the following accessor functions :
char *Foo_name_get(Foo *obj) {
return Foo->name;
}
char *Foo_name_set(Foo *obj, char *c) {
if (obj->name)
free(obj->name);
obj->name = (char *) malloc(strlen(c)+1);
strcpy(obj->name,c);
return obj->name;
}
If this behavior differs from what you need in your applications, the SWIG "memberin" typemap can be used to change it. See the typemaps chapter for further details.
Note: If the -c++ option is used, new and delete are used to perform memory allocation.
Arrays may appear as the members of structures, but they will be read-only. SWIG will write an accessor function that returns the pointer to the first element of the array, but will not write a function to change the contents of the array itself. When this situation is detected, SWIG may generate a warning message such as the following :
interface.i:116. Warning. Array member will be read-only
To eliminate the warning message, typemaps can be used, but this is discussed in a later chapter. In many cases, the warning message is harmless.
Occasionally, a structure will contain data members that are themselves structures. For example:
typedef struct Foo {
int x;
} Foo;
typedef struct Bar {
int y;
Foo f; /* struct member */
} Bar;
When a structure member is wrapped, it is handled as a pointer, unless the %naturalvar directive is used where it is handled more like a C++ reference (see C++ Member data). The accessors to the member variable as a pointer are effectively wrapped as follows:
Foo *Bar_f_get(Bar *b) {
return &b->f;
}
void Bar_f_set(Bar *b, Foo *value) {
b->f = *value;
}
The reasons for this are somewhat subtle but have to do with the problem of modifying and accessing data inside the data member. For example, suppose you wanted to modify the value of f.x of a Bar object like this:
Bar *b; b->f.x = 37;
Translating this assignment to function calls (as would be used inside the scripting language interface) results in the following code:
Bar *b; Foo_x_set(Bar_f_get(b),37);
In this code, if the Bar_f_get() function were to return a Foo instead of a Foo *, then the resulting modification would be applied to a copy of f and not the data member f itself. Clearly that's not what you want!
It should be noted that this transformation to pointers only occurs if SWIG knows that a data member is a structure or class. For instance, if you had a structure like this,
struct Foo {
WORD w;
};
and nothing was known about WORD, then SWIG will generate more normal accessor functions like this:
WORD Foo_w_get(Foo *f) {
return f->w;
}
void Foo_w_set(FOO *f, WORD value) {
f->w = value;
}
Compatibility Note: SWIG-1.3.11 and earlier releases transformed all non-primitive member datatypes to pointers. Starting in SWIG-1.3.12, this transformation only occurs if a datatype is known to be a structure, class, or union. This is unlikely to break existing code. However, if you need to tell SWIG that an undeclared datatype is really a struct, simply use a forward struct declaration such as "struct Foo;".
When wrapping structures, it is generally useful to have a mechanism for creating and destroying objects. If you don't do anything, SWIG will automatically generate functions for creating and destroying objects using malloc() and free(). Note: the use of malloc() only applies when SWIG is used on C code (i.e., when the -c++ option is not supplied on the command line). C++ is handled differently.
If you don't want SWIG to generate default constructors for your interfaces, you can use the %nodefaultctor directive or the -nodefaultctor command line option. For example:
swig -nodefaultctor example.i
or
%module foo ... %nodefaultctor; // Don't create default constructors ... declarations ... %clearnodefaultctor; // Re-enable default constructors
If you need more precise control, %nodefaultctor can selectively target individual structure definitions. For example:
%nodefaultctor Foo; // No default constructor for Foo
...
struct Foo { // No default constructor generated.
};
struct Bar { // Default constructor generated.
};
Since ignoring the implicit or default destructors most of the time produces memory leaks, SWIG will always try to generate them. If needed, however, you can selectively disable the generation of the default/implicit destructor by using %nodefaultdtor
%nodefaultdtor Foo; // No default/implicit destructor for Foo
...
struct Foo { // No default destructor is generated.
};
struct Bar { // Default destructor generated.
};
Compatibility note: Prior to SWIG-1.3.7, SWIG did not generate default constructors or destructors unless you explicitly turned them on using -make_default. However, it appears that most users want to have constructor and destructor functions so it has now been enabled as the default behavior.
Note: There are also the -nodefault option and %nodefault directive, which disable both the default or implicit destructor generation. This could lead to memory leaks across the target languages, and it is highly recommended you don't use them.
Most languages provide a mechanism for creating classes and supporting object oriented programming. From a C standpoint, object oriented programming really just boils down to the process of attaching functions to structures. These functions normally operate on an instance of the structure (or object). Although there is a natural mapping of C++ to such a scheme, there is no direct mechanism for utilizing it with C code. However, SWIG provides a special %extend directive that makes it possible to attach methods to C structures for purposes of building an object oriented interface. Suppose you have a C header file with the following declaration :
/* file : vector.h */
...
typedef struct Vector {
double x,y,z;
} Vector;
You can make a Vector look a lot like a class by writing a SWIG interface like this:
// file : vector.i
%module mymodule
%{
#include "vector.h"
%}
%include "vector.h" // Just grab original C header file
%extend Vector { // Attach these functions to struct Vector
Vector(double x, double y, double z) {
Vector *v;
v = (Vector *) malloc(sizeof(Vector));
v->x = x;
v->y = y;
v->z = z;
return v;
}
~Vector() {
free($self);
}
double magnitude() {
return sqrt($self->x*$self->x+$self->y*$self->y+$self->z*$self->z);
}
void print() {
printf("Vector [%g, %g, %g]\n", $self->x,$self->y,$self->z);
}
};
Note the usage of the $self special variable. Its usage is identical to a C++ 'this' pointer and should be used whenever access to the struct instance is required. Also note that C++ constructor and destructor syntax has been used to simulate a constructor and destructor, even for C code. There is one subtle difference to a normal C++ constructor implementation though and that is although the constructor declaration is as per a normal C++ constructor, the newly constructed object must be returned as if the constructor declaration had a return value, a Vector * in this case.
Now, when used with proxy classes in Python, you can do things like this :
>>> v = Vector(3,4,0) # Create a new vector >>> print v.magnitude() # Print magnitude 5.0 >>> v.print() # Print it out [ 3, 4, 0 ] >>> del v # Destroy it
The %extend directive can also be used inside the definition of the Vector structure. For example:
// file : vector.i
%module mymodule
%{
#include "vector.h"
%}
typedef struct Vector {
double x,y,z;
%extend {
Vector(double x, double y, double z) { ... }
~Vector() { ... }
...
}
} Vector;
Note that %extend can be used to access externally written functions provided they follow the naming convention used in this example :
/* File : vector.c */
/* Vector methods */
#include "vector.h"
Vector *new_Vector(double x, double y, double z) {
Vector *v;
v = (Vector *) malloc(sizeof(Vector));
v->x = x;
v->y = y;
v->z = z;
return v;
}
void delete_Vector(Vector *v) {
free(v);
}
double Vector_magnitude(Vector *v) {
return sqrt(v->x*v->x+v->y*v->y+v->z*v->z);
}
// File : vector.i
// Interface file
%module mymodule
%{
#include "vector.h"
%}
typedef struct Vector {
double x,y,z;
%extend {
Vector(int,int,int); // This calls new_Vector()
~Vector(); // This calls delete_Vector()
double magnitude(); // This will call Vector_magnitude()
...
}
} Vector;
The name used for %extend should be the name of the struct and not the name of any typedef to the struct. For example:
typedef struct Integer {
int value;
} Int;
%extend Integer { ... } /* Correct name */
%extend Int { ... } /* Incorrect name */
struct Float {
float value;
};
typedef struct Float FloatValue;
%extend Float { ... } /* Correct name */
%extend FloatValue { ... } /* Incorrect name */
There is one exception to this rule and that is when the struct is anonymously named such as:
typedef struct {
double value;
} Double;
%extend Double { ... } /* Okay */
A little known feature of the %extend directive is that it can also be used to add synthesized attributes or to modify the behavior of existing data attributes. For example, suppose you wanted to make magnitude a read-only attribute of Vector instead of a method. To do this, you might write some code like this:
// Add a new attribute to Vector
%extend Vector {
const double magnitude;
}
// Now supply the implementation of the Vector_magnitude_get function
%{
const double Vector_magnitude_get(Vector *v) {
return (const double) sqrt(v->x*v->x+v->y*v->y+v->z*v->z);
}
%}
Now, for all practical purposes, magnitude will appear like an attribute of the object.
A similar technique can also be used to work with data members that you want to process. For example, consider this interface:
typedef struct Person {
char name[50];
...
} Person;
Say you wanted to ensure name was always upper case, you can rewrite the interface as follows to ensure this occurs whenever a name is read or written to:
typedef struct Person {
%extend {
char name[50];
}
...
} Person;
%{
#include <string.h>
#include <ctype.h>
void make_upper(char *name) {
char *c;
for (c = name; *c; ++c)
*c = (char)toupper((int)*c);
}
/* Specific implementation of set/get functions forcing capitalization */
char *Person_name_get(Person *p) {
make_upper(p->name);
return p->name;
}
void Person_name_set(Person *p, char *val) {
strncpy(p->name,val,50);
make_upper(p->name);
}
%}
Finally, it should be stressed that even though %extend can be used to add new data members, these new members can not require the allocation of additional storage in the object (e.g., their values must be entirely synthesized from existing attributes of the structure or obtained elsewhere).
Compatibility note: The %extend directive is a new name for the %addmethods directive. Since %addmethods could be used to extend a structure with more than just methods, a more suitable directive name has been chosen.
Occasionally, a C program will involve structures like this :
typedef struct Object {
int objtype;
union {
int ivalue;
double dvalue;
char *strvalue;
void *ptrvalue;
} intRep;
} Object;
When SWIG encounters this, it performs a structure splitting operation that transforms the declaration into the equivalent of the following:
typedef union {
int ivalue;
double dvalue;
char *strvalue;
void *ptrvalue;
} Object_intRep;
typedef struct Object {
int objType;
Object_intRep intRep;
} Object;
SWIG will then create an Object_intRep structure for use inside the interface file. Accessor functions will be created for both structures. In this case, functions like this would be created :
Object_intRep *Object_intRep_get(Object *o) {
return (Object_intRep *) &o->intRep;
}
int Object_intRep_ivalue_get(Object_intRep *o) {
return o->ivalue;
}
int Object_intRep_ivalue_set(Object_intRep *o, int value) {
return (o->ivalue = value);
}
double Object_intRep_dvalue_get(Object_intRep *o) {
return o->dvalue;
}
... etc ...
Although this process is a little hairy, it works like you would expect in the target scripting language--especially when proxy classes are used. For instance, in Perl:
# Perl5 script for accessing nested member
$o = CreateObject(); # Create an object somehow
$o->{intRep}->{ivalue} = 7 # Change value of o.intRep.ivalue
If you have a lot of nested structure declarations, it is advisable to double-check them after running SWIG. Although, there is a good chance that they will work, you may have to modify the interface file in certain cases.
Finally, note that nesting is handled differently in C++ mode, see Nested classes.
SWIG doesn't care if the declaration of a structure in a .i file exactly matches that used in the underlying C code (except in the case of nested structures). For this reason, there are no problems omitting problematic members or simply omitting the structure definition altogether. If you are happy passing pointers around, this can be done without ever giving SWIG a structure definition.
Starting with SWIG1.3, a number of improvements have been made to SWIG's code generator. Specifically, even though structure access has been described in terms of high-level accessor functions such as this,
double Vector_x_get(Vector *v) {
return v->x;
}
most of the generated code is actually inlined directly into wrapper functions. Therefore, no function Vector_x_get() actually exists in the generated wrapper file. For example, when creating a Tcl module, the following function is generated instead:
static int
_wrap_Vector_x_get(ClientData clientData, Tcl_Interp *interp,
int objc, Tcl_Obj *CONST objv[]) {
struct Vector *arg1 ;
double result ;
if (SWIG_GetArgs(interp, objc, objv,"p:Vector_x_get self ",&arg0,
SWIGTYPE_p_Vector) == TCL_ERROR)
return TCL_ERROR;
result = (double ) (arg1->x);
Tcl_SetObjResult(interp,Tcl_NewDoubleObj((double) result));
return TCL_OK;
}
The only exception to this rule are methods defined with %extend . In this case, the added code is contained in a separate function.
Finally, it is important to note that most language modules may choose to build a more advanced interface. Although you may never use the low-level interface described here, most of SWIG's language modules use it in some way or another.
Sometimes it is necessary to insert special code into the resulting wrapper file generated by SWIG. For example, you may want to include additional C code to perform initialization or other operations. There are four common ways to insert code, but it's useful to know how the output of SWIG is structured first.
When SWIG creates its output file, it is broken up into five sections corresponding to runtime code, headers, wrapper functions, and module initialization code (in that order).
Code is inserted into the appropriate code section by using one of the code insertion directives listed below. The order of the sections in the wrapper file is as shown:
%begin %{
... code in begin section ...
%}
%runtime %{
... code in runtime section ...
%}
%header %{
... code in header section ...
%}
%wrapper %{
... code in wrapper section ...
%}
%init %{
... code in init section ...
%}
The bare %{ ... %} directive is a shortcut that is the same as %header %{ ... %}.
The %begin section is effectively empty as it just contains the SWIG banner by default. This section is provided as a way for users to insert code at the top of the wrapper file before any other code is generated. Everything in a code insertion block is copied verbatim into the output file and is not parsed by SWIG. Most SWIG input files have at least one such block to include header files and support C code. Additional code blocks may be placed anywhere in a SWIG file as needed.
%module mymodule
%{
#include "my_header.h"
%}
... Declare functions here
%{
void some_extra_function() {
...
}
%}
A common use for code blocks is to write "helper" functions. These are functions that are used specifically for the purpose of building an interface, but which are generally not visible to the normal C program. For example :
%{
/* Create a new vector */
static Vector *new_Vector() {
return (Vector *) malloc(sizeof(Vector));
}
%}
// Now wrap it
Vector *new_Vector();
Since the process of writing helper functions is fairly common, there is a special inlined form of code block that is used as follows :
%inline %{
/* Create a new vector */
Vector *new_Vector() {
return (Vector *) malloc(sizeof(Vector));
}
%}
The %inline directive inserts all of the code that follows verbatim into the header portion of an interface file. The code is then parsed by both the SWIG preprocessor and parser. Thus, the above example creates a new command new_Vector using only one declaration. Since the code inside an %inline %{ ... %} block is given to both the C compiler and SWIG, it is illegal to include any SWIG directives inside a %{ ... %} block.
When code is included in the %init section, it is copied directly into the module initialization function. For example, if you needed to perform some extra initialization on module loading, you could write this:
%init %{
init_variables();
%}
This section describes the general approach for building interfaces with SWIG. The specifics related to a particular scripting language are found in later chapters.
SWIG doesn't require modifications to your C code, but if you feed it a collection of raw C header files or source code, the results might not be what you expect---in fact, they might be awful. Here's a series of steps you can follow to make an interface for a C program :
Although this may sound complicated, the process turns out to be fairly easy once you get the hang of it.
In the process of building an interface, SWIG may encounter syntax errors or other problems. The best way to deal with this is to simply copy the offending code into a separate interface file and edit it. However, the SWIG developers have worked very hard to improve the SWIG parser--you should report parsing errors to the swig-devel mailing list or to the SWIG bug tracker.
The preferred method of using SWIG is to generate a separate interface file. Suppose you have the following C header file :
/* File : header.h */ #include <stdio.h> #include <math.h> extern int foo(double); extern double bar(int, int); extern void dump(FILE *f);
A typical SWIG interface file for this header file would look like the following :
/* File : interface.i */
%module mymodule
%{
#include "header.h"
%}
extern int foo(double);
extern double bar(int, int);
extern void dump(FILE *f);
Of course, in this case, our header file is pretty simple so we could use a simpler approach and use an interface file like this:
/* File : interface.i */
%module mymodule
%{
#include "header.h"
%}
%include "header.h"
The main advantage of this approach is minimal maintenance of an interface file for when the header file changes in the future. In more complex projects, an interface file containing numerous %include and #include statements like this is one of the most common approaches to interface file design due to lower maintenance overhead.
Although SWIG can parse many header files, it is more common to write a special .i file defining the interface to a package. There are several reasons why you might want to do this:
Sometimes, it is necessary to use certain header files in order for the code generated by SWIG to compile properly. Make sure you include certain header files by using a %{,%} block like this:
%module graphics
%{
#include <GL/gl.h>
#include <GL/glu.h>
%}
// Put the rest of the declarations here
...
If your program defines a main() function, you may need to get rid of it or rename it in order to use a scripting language. Most scripting languages define their own main() procedure that is called instead. main() also makes no sense when working with dynamic loading. There are a few approaches to solving the main() conflict :
Getting rid of main() may cause potential initialization problems of a program. To handle this problem, you may consider writing a special function called program_init() that initializes your program upon startup. This function could then be called either from the scripting language as the first operation, or when the SWIG generated module is loaded.
As a general note, many C programs only use the main() function to parse command line options and to set parameters. However, by using a scripting language, you are probably trying to create a program that is more interactive. In many cases, the old main() program can be completely replaced by a Perl, Python, or Tcl script.
Note: In some cases, you might be inclined to create a scripting language wrapper for main(). If you do this, the compilation will probably work and your module might even load correctly. The only trouble is that when you call your main() wrapper, you will find that it actually invokes the main() of the scripting language interpreter itself! This behavior is a side effect of the symbol binding mechanism used in the dynamic linker. The bottom line: don't do this.
This chapter describes SWIG's support for wrapping C++. As a prerequisite, you should first read the chapter SWIG Basics to see how SWIG wraps ANSI C. Support for C++ builds upon ANSI C wrapping and that material will be useful in understanding this chapter.
Because of its complexity and the fact that C++ can be difficult to integrate with itself let alone other languages, SWIG only provides support for a subset of C++ features. Fortunately, this is now a rather large subset.
In part, the problem with C++ wrapping is that there is no semantically obvious (or automatic ) way to map many of its advanced features into other languages. As a simple example, consider the problem of wrapping C++ multiple inheritance to a target language with no such support. Similarly, the use of overloaded operators and overloaded functions can be problematic when no such capability exists in a target language.
A more subtle issue with C++ has to do with the way that some C++ programmers think about programming libraries. In the world of SWIG, you are really trying to create binary-level software components for use in other languages. In order for this to work, a "component" has to contain real executable instructions and there has to be some kind of binary linking mechanism for accessing its functionality. In contrast, C++ has increasingly relied upon generic programming and templates for much of its functionality. Although templates are a powerful feature, they are largely orthogonal to the whole notion of binary components and libraries. For example, an STL vector does not define any kind of binary object for which SWIG can just create a wrapper. To further complicate matters, these libraries often utilize a lot of behind the scenes magic in which the semantics of seemingly basic operations (e.g., pointer dereferencing, procedure call, etc.) can be changed in dramatic and sometimes non-obvious ways. Although this "magic" may present few problems in a C++-only universe, it greatly complicates the problem of crossing language boundaries and provides many opportunities to shoot yourself in the foot. You will just have to be careful.
To wrap C++, SWIG uses a layered approach to code generation. At the lowest level, SWIG generates a collection of procedural ANSI-C style wrappers. These wrappers take care of basic type conversion, type checking, error handling, and other low-level details of the C++ binding. These wrappers are also sufficient to bind C++ into any target language that supports built-in procedures. In some sense, you might view this layer of wrapping as providing a C library interface to C++. On top of the low-level procedural (flattened) interface, SWIG generates proxy classes that provide a natural object-oriented (OO) interface to the underlying code. The proxy classes are typically written in the target language itself. For instance, in Python, a real Python class is used to provide a wrapper around the underlying C++ object.
It is important to emphasize that SWIG takes a deliberately conservative and non-intrusive approach to C++ wrapping. SWIG does not encapsulate C++ classes inside a special C++ adaptor, it does not rely upon templates, nor does it add in additional C++ inheritance when generating wrappers. The last thing that most C++ programs need is even more compiler magic. Therefore, SWIG tries to maintain a very strict and clean separation between the implementation of your C++ application and the resulting wrapper code. You might say that SWIG has been written to follow the principle of least surprise--it does not play sneaky tricks with the C++ type system, it doesn't mess with your class hierarchies, and it doesn't introduce new semantics. Although this approach might not provide the most seamless integration with C++, it is safe, simple, portable, and debuggable.
Some of this chapter focuses on the low-level procedural interface to C++ that is used as the foundation for all language modules. Keep in mind that the target languages also provide the high-level OO interface via proxy classes. More detailed coverage can be found in the documentation for each target language.
SWIG currently supports most C++ features including the following:
The following C++ features are not currently supported:
As a rule of thumb, SWIG should not be used on raw C++ source files, use header files only.
SWIG's C++ support is an ongoing project so some of these limitations may be lifted in future releases. However, we make no promises. Also, submitting a bug report is a very good way to get problems fixed (wink).
When wrapping C++ code, it is critical that SWIG be called with the `-c++' option. This changes the way a number of critical features such as memory management are handled. It also enables the recognition of C++ keywords. Without the -c++ flag, SWIG will either issue a warning or a large number of syntax errors if it encounters C++ code in an interface file.
When compiling and linking the resulting wrapper file, it is normal to use the C++ compiler. For example:
$ swig -c++ -tcl example.i $ c++ -fPIC -c example_wrap.cxx $ c++ example_wrap.o $(OBJS) -o example.so
Unfortunately, the process varies slightly on each platform. Make sure you refer to the documentation on each target language for further details. The SWIG Wiki also has further details.
Compatibility Note: Early versions of SWIG generated just a flattened low-level C style API to C++ classes by default. The -noproxy commandline option is recognised by many target languages and will generate just this interface as in earlier versions.In order to provide a natural mapping from C++ classes to the target language classes, SWIG's target languages mostly wrap C++ classes with special proxy classes. These proxy classes are typically implemented in the target language itself. For example, if you're building a Python module, each C++ class is wrapped by a Python proxy class. Or if you're building a Java module, each C++ class is wrapped by a Java proxy class.
Proxy classes are always constructed as an extra layer of wrapping that uses low-level accessor functions. To illustrate, suppose you had a C++ class like this:
class Foo {
public:
Foo();
~Foo();
int bar(int x);
int x;
};
Using C++ as pseudocode, a proxy class looks something like this:
class FooProxy {
private:
Foo *self;
public:
FooProxy() {
self = new_Foo();
}
~FooProxy() {
delete_Foo(self);
}
int bar(int x) {
return Foo_bar(self,x);
}
int x_get() {
return Foo_x_get(self);
}
void x_set(int x) {
Foo_x_set(self,x);
}
};
Of course, always keep in mind that the real proxy class is written in the target language. For example, in Python, the proxy might look roughly like this:
class Foo:
def __init__(self):
self.this = new_Foo()
def __del__(self):
delete_Foo(self.this)
def bar(self,x):
return Foo_bar(self.this,x)
def __getattr__(self,name):
if name == 'x':
return Foo_x_get(self.this)
...
def __setattr__(self,name,value):
if name == 'x':
Foo_x_set(self.this,value)
...
Again, it's important to emphasize that the low-level accessor functions are always used by the proxy classes. Whenever possible, proxies try to take advantage of language features that are similar to C++. This might include operator overloading, exception handling, and other features.
A major issue with proxies concerns the memory management of wrapped objects. Consider the following C++ code:
class Foo {
public:
Foo();
~Foo();
int bar(int x);
int x;
};
class Spam {
public:
Foo *value;
...
};
Consider some script code that uses these classes:
f = Foo() # Creates a new Foo s = Spam() # Creates a new Spam s.value = f # Stores a reference to f inside s g = s.value # Returns stored reference g = 4 # Reassign g to some other value del f # Destroy f
Now, ponder the resulting memory management issues. When objects are created in the script, the objects are wrapped by newly created proxy classes. That is, there is both a new proxy class instance and a new instance of the underlying C++ class. In this example, both f and s are created in this way. However, the statement s.value is rather curious---when executed, a pointer to f is stored inside another object. This means that the scripting proxy class AND another C++ class share a reference to the same object. To make matters even more interesting, consider the statement g = s.value. When executed, this creates a new proxy class g that provides a wrapper around the C++ object stored in s.value . In general, there is no way to know where this object came from---it could have been created by the script, but it could also have been generated internally. In this particular example, the assignment of g results in a second proxy class for f. In other words, a reference to f is now shared by two proxy classes and a C++ class.
Finally, consider what happens when objects are destroyed. In the statement, g=4, the variable g is reassigned. In many languages, this makes the old value of g available for garbage collection. Therefore, this causes one of the proxy classes to be destroyed. Later on, the statement del f destroys the other proxy class. Of course, there is still a reference to the original object stored inside another C++ object. What happens to it? Is the object still valid?
To deal with memory management problems, proxy classes provide an API for controlling ownership. In C++ pseudocode, ownership control might look roughly like this:
class FooProxy {
public:
Foo *self;
int thisown;
FooProxy() {
self = new_Foo();
thisown = 1; // Newly created object
}
~FooProxy() {
if (thisown) delete_Foo(self);
}
...
// Ownership control API
void disown() {
thisown = 0;
}
void acquire() {
thisown = 1;
}
};
class FooPtrProxy: public FooProxy {
public:
FooPtrProxy(Foo *s) {
self = s;
thisown = 0;
}
};
class SpamProxy {
...
FooProxy *value_get() {
return FooPtrProxy(Spam_value_get(self));
}
void value_set(FooProxy *v) {
Spam_value_set(self,v->self);
v->disown();
}
...
};
Looking at this code, there are a few central features:
Given the tricky nature of C++ memory management, it is impossible for proxy classes to automatically handle every possible memory management problem. However, proxies do provide a mechanism for manual control that can be used (if necessary) to address some of the more tricky memory management problems.
Language specific details on proxy classes are contained in the chapters describing each target language. This chapter has merely introduced the topic in a very general way.
The following code shows a SWIG interface file for a simple C++ class.
%module list
%{
#include "list.h"
%}
// Very simple C++ example for linked list
class List {
public:
List();
~List();
int search(char *value);
void insert(char *);
void remove(char *);
char *get(int n);
int length;
static void print(List *l);
};
To generate wrappers for this class, SWIG first reduces the class to a collection of low-level C-style accessor functions which are then used by the proxy classes.
C++ constructors and destructors are translated into accessor functions such as the following :
List * new_List(void) {
return new List;
}
void delete_List(List *l) {
delete l;
}
Following the C++ rules for implicit constructor and destructors, SWIG will automatically assume there is one even when they are not explicitly declared in the class interface.
In general then:
And as in C++, a few rules that alters the previous behavior:
SWIG should never generate a default constructor, copy constructor or default destructor wrapper for a class in which it is illegal to do so. In some cases, however, it could be necessary (if the complete class declaration is not visible from SWIG, and one of the above rules is violated) or desired (to reduce the size of the final interface) by manually disabling the implicit constructor/destructor generation.
To manually disable these, the %nodefaultctor and %nodefaultdtor feature flag directives can be used. Note that these directives only affects the implicit generation, and they have no effect if the default/copy constructors or destructor are explicitly declared in the class interface.
For example:
%nodefaultctor Foo; // Disable the default constructor for class Foo.
class Foo { // No default constructor is generated, unless one is declared
...
};
class Bar { // A default constructor is generated, if possible
...
};
The directive %nodefaultctor can also be applied "globally", as in:
%nodefaultctor; // Disable creation of default constructors
class Foo { // No default constructor is generated, unless one is declared
...
};
class Bar {
public:
Bar(); // The default constructor is generated, since one is declared
};
%clearnodefaultctor; // Enable the creation of default constructors again
The corresponding %nodefaultdtor directive can be used to disable the generation of the default or implicit destructor, if needed. Be aware, however, that this could lead to memory leaks in the target language. Hence, it is recommended to use this directive only in well known cases. For example:
%nodefaultdtor Foo; // Disable the implicit/default destructor for class Foo.
class Foo { // No destructor is generated, unless one is declared
...
};
Compatibility Note: The generation of default constructors/implicit destructors was made the default behavior in SWIG 1.3.7. This may break certain older modules, but the old behavior can be easily restored using %nodefault or the -nodefault command line option. Furthermore, in order for SWIG to properly generate (or not generate) default constructors, it must be able to gather information from both the private and protected sections (specifically, it needs to know if a private or protected constructor/destructor is defined). In older versions of SWIG, it was fairly common to simply remove or comment out the private and protected sections of a class due to parser limitations. However, this removal may now cause SWIG to erroneously generate constructors for classes that define a constructor in those sections. Consider restoring those sections in the interface or using %nodefault to fix the problem.
Note: The %nodefault directive/-nodefault options described above, which disable both the default constructor and the implicit destructors, could lead to memory leaks, and so it is strongly recommended to not use them.
If a class defines a constructor, SWIG normally tries to generate a wrapper for it. However, SWIG will not generate a constructor wrapper if it thinks that it will result in illegal wrapper code. There are really two cases where this might show up.
First, SWIG won't generate wrappers for protected or private constructors. For example:
class Foo {
protected:
Foo(); // Not wrapped.
public:
...
};
Next, SWIG won't generate wrappers for a class if it appears to be abstract--that is, it has undefined pure virtual methods. Here are some examples:
class Bar {
public:
Bar(); // Not wrapped. Bar is abstract.
virtual void spam(void) = 0;
};
class Grok : public Bar {
public:
Grok(); // Not wrapped. No implementation of abstract spam().
};
Some users are surprised (or confused) to find missing constructor wrappers in their interfaces. In almost all cases, this is caused when classes are determined to be abstract. To see if this is the case, run SWIG with all of its warnings turned on:
% swig -Wall -python module.i
In this mode, SWIG will issue a warning for all abstract classes. It is possible to force a class to be non-abstract using this:
%feature("notabstract") Foo;
class Foo : public Bar {
public:
Foo(); // Generated no matter what---not abstract.
...
};
More information about %feature can be found in the Customization features chapter.
If a class defines more than one constructor, its behavior depends on the capabilities of the target language. If overloading is supported, the copy constructor is accessible using the normal constructor function. For example, if you have this:
class List {
public:
List();
List(const List &); // Copy constructor
...
};
then the copy constructor can be used as follows:
x = List() # Create a list y = List(x) # Copy list x
If the target language does not support overloading, then the copy constructor is available through a special function like this:
List *copy_List(List *f) {
return new List(*f);
}
Note: For a class X, SWIG only treats a constructor as a copy constructor if it can be applied to an object of type X or X *. If more than one copy constructor is defined, only the first definition that appears is used as the copy constructor--other definitions will result in a name-clash. Constructors such as X(const X &), X(X &), and X(X *) are handled as copy constructors in SWIG.
Note: SWIG does not generate a copy constructor wrapper unless one is explicitly declared in the class. This differs from the treatment of default constructors and destructors. However, copy constructor wrappers can be generated if using the copyctor feature flag. For example:
%copyctor List;
class List {
public:
List();
};
Will generate a copy constructor wrapper for List.
Compatibility note: Special support for copy constructors was not added until SWIG-1.3.12. In previous versions, copy constructors could be wrapped, but they had to be renamed. For example:
class Foo {
public:
Foo();
%name(CopyFoo) Foo(const Foo &);
...
};
For backwards compatibility, SWIG does not perform any special copy-constructor handling if the constructor has been manually renamed. For instance, in the above example, the name of the constructor is set to new_CopyFoo(). This is the same as in older versions.
All member functions are roughly translated into accessor functions like this :
int List_search(List *obj, char *value) {
return obj->search(value);
}
This translation is the same even if the member function has been declared as virtual.
It should be noted that SWIG does not actually create a C accessor function in the code it generates. Instead, member access such as obj->search(value) is directly inlined into the generated wrapper functions. However, the name and calling convention of the low-level procedural wrappers match the accessor function prototype described above.
Static member functions are called directly without making any special transformations. For example, the static member function print(List *l) directly invokes List::print(List *l) in the generated wrapper code.
Member data is handled in exactly the same manner as for C structures. A pair of accessor functions are effectively created. For example :
int List_length_get(List *obj) {
return obj->length;
}
int List_length_set(List *obj, int value) {
obj->length = value;
return value;
}
A read-only member can be created using the %immutable and %mutable feature flag directive. For example, we probably wouldn't want the user to change the length of a list so we could do the following to make the value available, but read-only.
class List {
public:
...
%immutable;
int length;
%mutable;
...
};
Alternatively, you can specify an immutable member in advance like this:
%immutable List::length;
...
class List {
...
int length; // Immutable by above directive
...
};
Similarly, all data attributes declared as const are wrapped as read-only members.
By default, SWIG uses the const reference typemaps for members that are primitive types. There are some subtle issues when wrapping data members that are not primitive types, such as classes. For instance, if you had another class like this,
class Foo {
public:
List items;
...
then the low-level accessor to the items member actually uses pointers. For example:
List *Foo_items_get(Foo *self) {
return &self->items;
}
void Foo_items_set(Foo *self, List *value) {
self->items = *value;
}
More information about this can be found in the SWIG Basics chapter, Structure data members section.
The wrapper code to generate the accessors for classes comes from the pointer typemaps. This can be somewhat unnatural for some types. For example, a user would expect the STL std::string class member variables to be wrapped as a string in the target language, rather than a pointer to this class. The const reference typemaps offer this type of marshalling, so there is a feature to tell SWIG to use the const reference typemaps rather than the pointer typemaps. It is the naturalvar feature and can be used to effectively change the way accessors are generated to the following:
const List &Foo_items_get(Foo *self) {
return self->items;
}
void Foo_items_set(Foo *self, const List &value) {
self->items = value;
}
The %naturalvar directive is a macro for, and hence equivalent to, %feature("naturalvar"). It can be used as follows:
// All List variables will use const List& typemaps
%naturalvar List;
// Only Foo::myList will use const List& typemaps
%naturalvar Foo::myList;
struct Foo {
List myList;
};
// All non-primitive types will use const reference typemaps
%naturalvar;
The observant reader will notice that %naturalvar works like any other feature flag directive but with some extra flexibility. The first of the example usages above shows %naturalvar attaching to the myList 's variable type, that is the List class. The second usage shows %naturalvar attaching to the variable name. Hence the naturalvar feature can be used on either the variable's name or type. Note that using the naturalvar feature on a variable's name overrides any naturalvar feature attached to the variable's type.
It is generally a good idea to use this feature globally as the reference typemaps have extra NULL checking compared to the pointer typemaps. A pointer can be NULL, whereas a reference cannot, so the extra checking ensures that the target language user does not pass in a value that translates to a NULL pointer and thereby preventing any potential NULL pointer dereferences. The %naturalvar feature will apply to global variables in addition to member variables in some language modules, eg C# and Java.
The naturalvar behavior can also be turned on as a global setting via the -naturalvar commandline option or the module mode option, %module(naturalvar=1). However, any use of %feature("naturalvar") will override the global setting.
Compatibility note: The %naturalvar feature was introduced in SWIG-1.3.28, prior to which it was necessary to manually apply the const reference typemaps, eg %apply const std::string & { std::string * }, but this example would also apply the typemaps to methods taking a std::string pointer.
Compatibility note: Read-only access used to be controlled by a pair of directives %readonly and %readwrite. Although these directives still work, they generate a warning message. Simply change the directives to %immutable; and %mutable; to silence the warning. Don't forget the extra semicolon!
Compatibility note: Prior to SWIG-1.3.12, all members of unknown type were wrapped into accessor functions using pointers. For example, if you had a structure like this
struct Foo {
size_t len;
};
and nothing was known about size_t, then accessors would be written to work with size_t *. Starting in SWIG-1.3.12, this behavior has been modified. Specifically, pointers will only be used if SWIG knows that a datatype corresponds to a structure or class. Therefore, the above code would be wrapped into accessors involving size_t. This change is subtle, but it smooths over a few problems related to structure wrapping and some of SWIG's customization features.
SWIG will wrap all types of functions that have default arguments. For example member functions:
class Foo {
public:
void bar(int x, int y = 3, int z = 4);
};
SWIG handles default arguments by generating an extra overloaded method for each defaulted argument. SWIG is effectively handling methods with default arguments as if it was wrapping the equivalent overloaded methods. Thus for the example above, it is as if we had instead given the following to SWIG:
class Foo {
public:
void bar(int x, int y, int z);
void bar(int x, int y);
void bar(int x);
};
The wrappers produced are exactly the same as if the above code was instead fed into SWIG. Details of this are covered later in the Wrapping Overloaded Functions and Methods section. This approach allows SWIG to wrap all possible default arguments, but can be verbose. For example if a method has ten default arguments, then eleven wrapper methods are generated.
Please see the Features and default arguments section for more information on using %feature with functions with default arguments. The Ambiguity resolution and renaming section also deals with using %rename and %ignore on methods with default arguments. If you are writing your own typemaps for types used in methods with default arguments, you may also need to write a typecheck typemap. See the Typemaps and overloading section for details or otherwise use the compactdefaultargs feature flag as mentioned below.
Compatibility note: Versions of SWIG prior to SWIG-1.3.23 wrapped default arguments slightly differently. Instead a single wrapper method was generated and the default values were copied into the C++ wrappers so that the method being wrapped was then called with all the arguments specified. If the size of the wrappers are a concern then this approach to wrapping methods with default arguments can be re-activated by using the compactdefaultargs feature flag.
%feature("compactdefaultargs") Foo::bar;
class Foo {
public:
void bar(int x, int y = 3, int z = 4);
};
This is great for reducing the size of the wrappers, but the caveat is it does not work for the statically typed languages, such as C# and Java, which don't have optional arguments in the language, Another restriction of this feature is that it cannot handle default arguments that are not public. The following example illustrates this:
class Foo {
private:
static const int spam;
public:
void bar(int x, int y = spam); // Won't work with %feature("compactdefaultargs") -
// private default value
};
This produces uncompilable wrapper code because default values in C++ are evaluated in the same scope as the member function whereas SWIG evaluates them in the scope of a wrapper function (meaning that the values have to be public).
The compactdefaultargs feature is automatically turned on when wrapping C code with default arguments. Some target languages will also automatically turn on this feature if the keyword arguments feature (kwargs) is specified for either C or C++ functions, and the target language supports kwargs, the compactdefaultargs feature is also automatically turned on. Keyword arguments are a language feature of some scripting languages, for example Ruby and Python. SWIG is unable to support kwargs when wrapping overloaded methods, so the default approach cannot be used.
SWIG wraps class members that are public following the C++ conventions, i.e., by explicit public declaration or by the use of the using directive. In general, anything specified in a private or protected section will be ignored, although the internal code generator sometimes looks at the contents of the private and protected sections so that it can properly generate code for default constructors and destructors. Directors could also modify the way non-public virtual protected members are treated.
By default, members of a class definition are assumed to be private until you explicitly give a `public:' declaration (This is the same convention used by C++).
Enumerations and constants are handled differently by the different language modules and are described in detail in the appropriate language chapter. However, many languages map enums and constants in a class definition into constants with the classname as a prefix. For example :
class Swig {
public:
enum {ALE, LAGER, PORTER, STOUT};
};
Generates the following set of constants in the target scripting language :
Swig_ALE = Swig::ALE Swig_LAGER = Swig::LAGER Swig_PORTER = Swig::PORTER Swig_STOUT = Swig::STOUT
Members declared as const are wrapped as read-only members and do not create constants.
Friend declarations are recognised by SWIG. For example, if you have this code:
class Foo {
public:
...
friend void blah(Foo *f);
...
};
then the friend declaration does result in a wrapper code equivalent to one generated for the following declaration
class Foo {
public:
...
};
void blah(Foo *f);
A friend declaration, as in C++, is understood to be in the same scope where the class is declared, hence, you can have
%ignore bar::blah(Foo *f);
namespace bar {
class Foo {
public:
...
friend void blah(Foo *f);
...
};
}
and a wrapper for the method 'blah' will not be generated.
C++ references are supported, but SWIG transforms them back into pointers. For example, a declaration like this :
class Foo {
public:
double bar(double &a);
}
has a low-level accessor
double Foo_bar(Foo *obj, double *a) {
obj->bar(*a);
}
As a special case, most language modules pass const references to primitive datatypes (int, short, float, etc.) by value instead of pointers. For example, if you have a function like this,
void foo(const int &x);
it is called from a script as follows:
foo(3) # Notice pass by value
Functions that return a reference are remapped to return a pointer instead. For example:
class Bar {
public:
Foo &spam();
};
Generates an accessor like this:
Foo *Bar_spam(Bar *obj) {
Foo &result = obj->spam();
return &result;
}
However, functions that return const references to primitive datatypes (int, short, etc.) normally return the result as a value rather than a pointer. For example, a function like this,
const int &bar();
will return integers such as 37 or 42 in the target scripting language rather than a pointer to an integer.
Don't return references to objects allocated as local variables on the stack. SWIG doesn't make a copy of the objects so this will probably cause your program to crash.
Note: The special treatment for references to primitive datatypes is necessary to provide more seamless integration with more advanced C++ wrapping applications---especially related to templates and the STL. This was first added in SWIG-1.3.12.
Occasionally, a C++ program will pass and return class objects by value. For example, a function like this might appear:
Vector cross_product(Vector a, Vector b);
If no information is supplied about Vector, SWIG creates a wrapper function similar to the following:
Vector *wrap_cross_product(Vector *a, Vector *b) {
Vector x = *a;
Vector y = *b;
Vector r = cross_product(x,y);
return new Vector(r);
}
In order for the wrapper code to compile, Vector must define a copy constructor and a default constructor.
If Vector is defined as a class in the interface, but it does not support a default constructor, SWIG changes the wrapper code by encapsulating the arguments inside a special C++ template wrapper class, through a process called the "Fulton Transform". This produces a wrapper that looks like this:
Vector cross_product(Vector *a, Vector *b) {
SwigValueWrapper<Vector> x = *a;
SwigValueWrapper<Vector> y = *b;
SwigValueWrapper<Vector> r = cross_product(x,y);
return new Vector(r);
}
This transformation is a little sneaky, but it provides support for pass-by-value even when a class does not provide a default constructor and it makes it possible to properly support a number of SWIG's customization options. The definition of SwigValueWrapper can be found by reading the SWIG wrapper code. This class is really nothing more than a thin wrapper around a pointer.
Although SWIG usually detects the classes to which the Fulton Transform should be applied, in some situations it's necessary to override it. That's done with %feature("valuewrapper") to ensure it is used and %feature("novaluewrapper") to ensure it is not used:
%feature("novaluewrapper") A;
class A;
%feature("valuewrapper") B;
struct B {
B();
// ....
};
It is well worth considering turning this feature on for classes that do have a default constructor. It will remove a redundant constructor call at the point of the variable declaration in the wrapper, so will generate notably better performance for large objects or for classes with expensive construction. Alternatively consider returning a reference or a pointer.
Note: this transformation has no effect on typemaps or any other part of SWIG---it should be transparent except that you may see this code when reading the SWIG output file.
Note: This template transformation is new in SWIG-1.3.11 and may be refined in future SWIG releases. In practice, it is only absolutely necessary to do this for classes that don't define a default constructor.
Note: The use of this template only occurs when objects are passed or returned by value. It is not used for C++ pointers or references.
SWIG supports C++ inheritance of classes and allows both single and multiple inheritance, as limited or allowed by the target language. The SWIG type-checker knows about the relationship between base and derived classes and allows pointers to any object of a derived class to be used in functions of a base class. The type-checker properly casts pointer values and is safe to use with multiple inheritance.
SWIG treats private or protected inheritance as close to the C++ spirit, and target language capabilities, as possible. In most cases, this means that SWIG will parse the non-public inheritance declarations, but that will have no effect in the generated code, besides the implicit policies derived for constructors and destructors.
The following example shows how SWIG handles inheritance. For clarity, the full C++ code has been omitted.
// shapes.i
%module shapes
%{
#include "shapes.h"
%}
class Shape {
public:
double x,y;
virtual double area() = 0;
virtual double perimeter() = 0;
void set_location(double x, double y);
};
class Circle : public Shape {
public:
Circle(double radius);
~Circle();
double area();
double perimeter();
};
class Square : public Shape {
public:
Square(double size);
~Square();
double area();
double perimeter();
}
When wrapped into Python, we can perform the following operations (shown using the low level Python accessors):
$ python >>> import shapes >>> circle = shapes.new_Circle(7) >>> square = shapes.new_Square(10) >>> print shapes.Circle_area(circle) 153.93804004599999757 >>> print shapes.Shape_area(circle) 153.93804004599999757 >>> print shapes.Shape_area(square) 100.00000000000000000 >>> shapes.Shape_set_location(square,2,-3) >>> print shapes.Shape_perimeter(square) 40.00000000000000000 >>>
In this example, Circle and Square objects have been created. Member functions can be invoked on each object by making calls to Circle_area, Square_area, and so on. However, the same results can be accomplished by simply using the Shape_area function on either object.
One important point concerning inheritance is that the low-level accessor functions are only generated for classes in which they are actually declared. For instance, in the above example, the method set_location() is only accessible as Shape_set_location() and not as Circle_set_location() or Square_set_location() . Of course, the Shape_set_location() function will accept any kind of object derived from Shape. Similarly, accessor functions for the attributes x and y are generated as Shape_x_get(), Shape_x_set(), Shape_y_get(), and Shape_y_set(). Functions such as Circle_x_get() are not available--instead you should use Shape_x_get().
Note that there is a one to one correlation between the low-level accessor functions and the proxy methods and therefore there is also a one to one correlation between the C++ class methods and the generated proxy class methods.
Note: For the best results, SWIG requires all base classes to be defined in an interface. Otherwise, you may get a warning message like this:
example.i:18: Warning 401: Nothing known about base class 'Foo'. Ignored.
If any base class is undefined, SWIG still generates correct type relationships. For instance, a function accepting a Foo * will accept any object derived from Foo regardless of whether or not SWIG actually wrapped the Foo class. If you really don't want to generate wrappers for the base class, but you want to silence the warning, you might consider using the %import directive to include the file that defines Foo. %import simply gathers type information, but doesn't generate wrappers. Alternatively, you could just define Foo as an empty class in the SWIG interface or use warning suppression .
Note: typedef-names can be used as base classes. For example:
class Foo {
...
};
typedef Foo FooObj;
class Bar : public FooObj { // Ok. Base class is Foo
...
};
Similarly, typedef allows unnamed structures to be used as base classes. For example:
typedef struct {
...
} Foo;
class Bar : public Foo { // Ok.
...
};
Compatibility Note: Starting in version 1.3.7, SWIG only generates low-level accessor wrappers for the declarations that are actually defined in each class. This differs from SWIG1.1 which used to inherit all of the declarations defined in base classes and regenerate specialized accessor functions such as Circle_x_get(), Square_x_get(), Circle_set_location(), and Square_set_location(). This behavior resulted in huge amounts of replicated code for large class hierarchies and made it awkward to build applications spread across multiple modules (since accessor functions are duplicated in every single module). It is also unnecessary to have such wrappers when advanced features like proxy classes are used. Note: Further optimizations are enabled when using the -fvirtual option, which avoids the regenerating of wrapper functions for virtual members that are already defined in a base class.
When a target scripting language refers to a C++ object, it normally uses a tagged pointer object that contains both the value of the pointer and a type string. For example, in Tcl, a C++ pointer might be encoded as a string like this:
_808fea88_p_Circle
A somewhat common question is whether or not the type-tag could be safely removed from the pointer. For instance, to get better performance, could you strip all type tags and just use simple integers instead?
In general, the answer to this question is no. In the wrappers, all pointers are converted into a common data representation in the target language. Typically this is the equivalent of casting a pointer to void *. This means that any C++ type information associated with the pointer is lost in the conversion.
The problem with losing type information is that it is needed to properly support many advanced C++ features--especially multiple inheritance. For example, suppose you had code like this:
class A {
public:
int x;
};
class B {
public:
int y;
};
class C : public A, public B {
};
int A_function(A *a) {
return a->x;
}
int B_function(B *b) {
return b->y;
}
Now, consider the following code that uses void *.
C *c = new C(); void *p = (void *) c; ... int x = A_function((A *) p); int y = B_function((B *) p);
In this code, both A_function() and B_function() may legally accept an object of type C * (via inheritance). However, one of the functions will always return the wrong result when used as shown. The reason for this is that even though p points to an object of type C, the casting operation doesn't work like you would expect. Internally, this has to do with the data representation of C. With multiple inheritance, the data from each base class is stacked together. For example:
------------ <--- (C *), (A *)
| A |
|------------| <--- (B *)
| B |
------------
Because of this stacking, a pointer of type C * may change value when it is converted to a A * or B *. However, this adjustment does not occur if you are converting from a void *.
The use of type tags marks all pointers with the real type of the underlying object. This extra information is then used by SWIG generated wrappers to correctly cast pointer values under inheritance (avoiding the above problem).
Some of the language modules are able to solve the problem by storing multiple instances of the pointer, for example, A *, in the A proxy class as well as C * in the C proxy class. The correct cast can then be made by choosing the correct void * pointer to use and is guaranteed to work as the cast to a void pointer and back to the same type does not lose any type information:
C *c = new C(); void *p = (void *) c; void *pA = (void *) c; void *pB = (void *) c; ... int x = A_function((A *) pA); int y = B_function((B *) pB);
In practice, the pointer is held as an integral number in the target language proxy class.
In many language modules, SWIG provides partial support for overloaded functions, methods, and constructors. For example, if you supply SWIG with overloaded functions like this:
void foo(int x) {
printf("x is %d\n", x);
}
void foo(char *x) {
printf("x is '%s'\n", x);
}
The function is used in a completely natural way. For example:
>>> foo(3)
x is 3
>>> foo("hello")
x is 'hello'
>>>
Overloading works in a similar manner for methods and constructors. For example if you have this code,
class Foo {
public:
Foo();
Foo(const Foo &); // Copy constructor
void bar(int x);
void bar(char *s, int y);
};
it might be used like this
>>> f = Foo() # Create a Foo
>>> f.bar(3)
>>> g = Foo(f) # Copy Foo
>>> f.bar("hello",2)
The implementation of overloaded functions and methods is somewhat complicated due to the dynamic nature of scripting languages. Unlike C++, which binds overloaded methods at compile time, SWIG must determine the proper function as a runtime check for scripting language targets. This check is further complicated by the typeless nature of certain scripting languages. For instance, in Tcl, all types are simply strings. Therefore, if you have two overloaded functions like this,
void foo(char *x); void foo(int x);
the order in which the arguments are checked plays a rather critical role.
For statically typed languages, SWIG uses the language's method overloading mechanism. To implement overloading for the scripting languages, SWIG generates a dispatch function that checks the number of passed arguments and their types. To create this function, SWIG first examines all of the overloaded methods and ranks them according to the following rules:
Argument type precedence. All C++ datatypes are assigned a numeric type precedence value (which is determined by t