Mir C++ Style Guide

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To guarantee uniqueness, they should be based on the full path in a project's source tree. For example, the file foo/src/bar/baz.h in project foo should have the following guard:

#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_

...

#endif  // FOO_BAR_BAZ_H_

When you include a header file you introduce a dependency that will cause your code to be recompiled whenever the header file changes. If your header file includes other header files, any change to those files will cause any code that includes your header to be recompiled. Therefore, we prefer to minimize includes, particularly includes of header files in other header files.

You can significantly reduce the number of header files you need to include in your own header files by using forward declarations. For example, if your header file uses the File class in ways that do not require access to the declaration of the File class, your header file can just forward declare class File; instead of having to #include "file/base/file.h".

How can we use a class Foo in a header file without access to its definition?

• We can declare data members of type Foo* or Foo&.
• We can declare (but not define) functions with arguments, and/or return values, of type Foo. (One exception is if an argument Foo or Foo const& has a non-explicit, one-argument constructor, in which case we need the full definition to support automatic type conversion.)
• We can declare static data members of type Foo. This is because static data members are defined outside the class definition.

On the other hand, you must include the header file for Foo if your class subclasses Foo or has a data member of type Foo.

Sometimes it makes sense to have pointer (or better, unique_ptr) members instead of object members. However, this complicates code readability and imposes a performance penalty, so avoid doing this transformation if the only purpose is to minimize includes in header files.

Of course, .cpp files typically do require the definitions of the classes they use, and usually have to include several header files.

Note: If you use a symbol Foo in your source file, you should bring in a definition for Foo yourself, either via an #include or via a forward declaration. Do not depend on the symbol being brought in transitively via headers not directly included. One exception is if Foo is used in myfile.cpp, it's ok to #include (or forward-declare) Foo in myfile.h, instead of myfile.cpp.

Definition: You can declare functions in a way that allows the compiler to expand them inline rather than calling them through the usual function call mechanism.

Pros: Inlining a function can generate more efficient object code, as long as the inlined function is small. Feel free to inline accessors and mutators, and other short, performance-critical functions.

Cons: Overuse of inlining can actually make programs slower. Depending on a function's size, inlining it can cause the code size to increase or decrease. Inlining a very small accessor function will usually decrease code size while inlining a very large function can dramatically increase code size. On modern processors smaller code usually runs faster due to better use of the instruction cache.

Decision:

A decent rule of thumb is to not inline a function if it is more than 10 lines long. Beware of destructors, which are often longer than they appear because of implicit member- and base-destructor calls!

Another useful rule of thumb: it's typically not cost effective to inline functions with loops or switch statements (unless, in the common case, the loop or switch statement is never executed).

It is important to know that functions are not always inlined even if they are declared as such; for example, virtual and recursive functions are not normally inlined. Usually recursive functions should not be inline. The main reason for making a virtual function inline is to place its definition in the class, either for convenience or to document its behavior, e.g., for accessors and mutators.

The definition of an inline function needs to be in a header file, so that the compiler has the definition available for inlining at the call sites. However, implementation code properly belongs in .cpp files, and we do not like to have much actual code in .h files unless there is a readability or performance advantage.

If an inline function definition is short, with very little, if any, logic in it, you should put the code in your .h file. For example, accessors and mutators should certainly be inside a class definition. More complex inline functions may also be put in a .h file for the convenience of the implementer and callers, though if this makes the .h file too unwieldy you can instead put that code in a separate -inl.h file. This separates the implementation from the class definition, while still allowing the implementation to be included where necessary.

Another use of -inl.h files is for definitions of function templates. This can be used to keep your template definitions easy to read.

Do not forget that a -inl.h file requires a #define guard just like any other header file.

Parameters to C/C++ functions are either input to the function, output from the function, or both. Input parameters are usually values or const references, while output and input/output parameters will be non-const references or pointers to non-const. When ordering function parameters, put all output parameters before any input-only parameters. In particular, do not add new parameters to the end of the function just because they are new; place new output parameters before the input-only parameters.

This is not a hard-and-fast rule. Parameters that are both input and output (often classes/structs) muddy the waters, and, as always, consistency with related functions may require you to bend the rule.

All of a project's header files should be listed as descendants of the project's source directory without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory). For example, my-awesome-project/src/base/logging.h should be included as

#include "base/logging.h"

In dir/foo.cpp or dir/foo_test.cpp, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows:

1. dir2/foo2.h (preferred location — see details below).
2. Your project's public .h files.
3. Your project's private .h files.
4. Other libraries' .h files.
5. C system files.
6. C++ system files.

The preferred ordering reduces hidden dependencies. We want every header file to be compilable on its own. The easiest way to achieve this is to make sure that every one of them is the first .h file #included in some .cpp.

dir/foo.cpp and dir2/foo2.h are often in the same directory (e.g. base/test_basictypes.cpp and base/basictypes.h), but can be in different directories too.

Within each section it is nice to order the includes alphabetically.

For example, the includes in my-awesome-project/src/foo/internal/fooserver.cpp might look like this:

        #include "foo/public/fooserver.h"  // Preferred location.

	#include "base/basictypes.h"
        #include "base/commandlineflags.h"
        #include "foo/public/bar.h"

        #include <sys/types.h>
        #include <unistd.h>

        #include <hash_map>
        #include <vector>

Definition: Namespaces subdivide the global scope into distinct, named scopes, and so are useful for preventing name collisions in the global scope.

Pros:

Namespaces provide a (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provided by classes.

For example, if two different projects have a class Foo in the global scope, these symbols may collide at compile time or at runtime. If each project places their code in a namespace, project1::Foo and project2::Foo are now distinct symbols that do not collide.

Cons:

Namespaces can be confusing, because they provide an additional (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provided by classes.

Use of unnamed spaces in header files can easily cause violations of the C++ One Definition Rule (ODR).

Decision:

Use namespaces according to the policy described below.

Unnamed Namespaces

• Unnamed namespaces are allowed and even encouraged in .cpp files, to avoid runtime naming conflicts:

  namespace                           // This is in a .cpp file.
  {
  // The content of a namespace is not indented
  enum { UNUSED, EOF, ERROR };       // Commonly used tokens.
  bool AtEof() { return pos_ == EOF; }  // Uses our namespace's EOF.
  
  }  // namespace

  However, file-scope declarations that are associated with a particular class may be declared in that class as types, static data members or static member functions rather than as members of an unnamed namespace. Terminate the unnamed namespace as shown, with a comment // namespace.

• Do not use unnamed namespaces in .h files.

Named Namespaces

Named namespaces should be used as follows:

• Namespaces wrap the entire source file after includes, gflags definitions/declarations, and forward declarations of classes from other namespaces:

  // In the .h file
  namespace mynamespace
  {
  
  // All declarations are within the namespace scope.
  // Notice the lack of indentation.
  class MyClass
  {
  public:
      ...
      void foo();
  };
  
  }  // namespace mynamespace

  // In the .cpp file
  namespace mynamespace
  {
  
  // Definition of functions is within scope of the namespace.
  void MyClass::foo()
  {
      ...
  }
  
  }  // namespace mynamespace

  The typical .cpp file might have more complex detail, including the need to reference classes in other namespaces.

  #include "a.h"
  
  DEFINE_BOOL(someflag, false, "dummy flag");
  
  class C;  // Forward declaration of class C in the global namespace.
  namespace a { class A; }  // Forward declaration of a::A.
  
  namespace b
  {
  
  ...code for b...         // Code goes against the left margin.
  
  }  // namespace b

• Do not declare anything in namespace std, not even forward declarations of standard library classes. Declaring entities in namespace std is undefined behavior, i.e., not portable. To declare entities from the standard library, include the appropriate header file.
• You may use a using-directive to make all names from a namespace available, but only in a source file.
• You may use a using-declaration anywhere in a .cpp file, and in functions, methods or classes in .h files.

  // OK in .cpp files.
  // Must be in a function, method or class in .h files.
  using ::foo::bar;

• Namespace aliases are allowed anywhere in a .cpp file, anywhere inside the named namespace that wraps an entire .h file, and in functions and methods.

  // Shorten access to some commonly used names in .cpp files.
  namespace fbz = ::foo::bar::baz;
  
  // Shorten access to some commonly used names (in a .h file).
  namespace librarian
  {
  // The following alias is available to all files including
  // this header (in namespace librarian):
  // alias names should therefore be chosen consistently
  // within a project.
  namespace pd_s = ::pipeline_diagnostics::sidetable;
  
  inline void my_inline_function()
  {
      // namespace alias local to a function (or method).
      namespace fbz = ::foo::bar::baz;
      ...
  }
  }  // namespace librarian

  Note that an alias in a .h file is visible to everyone #including that file, so public headers (those available outside a project) and headers transitively #included by them, should avoid defining aliases, as part of the general goal of keeping public APIs as small as possible.

Definition: A class can define another class within it; this is also called a member class.

class Foo
{
private:
    // Bar is a member class, nested within Foo.
    class Bar
    {
      ...
    };

};

Pros: This is useful when the nested (or member) class is only used by the enclosing class; making it a member puts it in the enclosing class scope rather than polluting the outer scope with the class name. Nested classes can be forward declared within the enclosing class and then defined in the .cpp file to avoid including the nested class definition in the enclosing class declaration, since the nested class definition is usually only relevant to the implementation.

Cons: Nested classes can be forward-declared only within the definition of the enclosing class. Thus, any header file manipulating a Foo::Bar* pointer will have to include the full class declaration for Foo.

Decision: Do not make nested classes public unless they are actually part of the interface, e.g., a class that holds a set of options for some method.

Pros: Nonmember and static member functions can be useful in some situations. Putting nonmember functions in a namespace avoids polluting the global namespace.

Cons: Nonmember and static member functions may make more sense as members of a new class, especially if they access external resources or have significant dependencies.

Decision:

Sometimes it is useful, or even necessary, to define a function not bound to a class instance. Such a function can be either a static member or a nonmember function. Nonmember functions should not depend on external variables, and should nearly always exist in a namespace. Rather than creating classes only to group static member functions which do not share static data, use namespaces instead.

Functions defined in the same compilation unit as production classes may introduce unnecessary coupling and link-time dependencies when directly called from other compilation units; static member functions are particularly susceptible to this. Consider extracting a new class, or placing the functions in a namespace possibly in a separate library.

If you must define a nonmember function and it is only needed in its .cpp file, use an unnamed namespace or static linkage (eg static int foo() {...}) to limit its scope.

C++ allows you to declare variables anywhere in a function. We encourage you to declare them in as local a scope as possible, and as close to the first use as possible. This makes it easier for the reader to find the declaration and see what type the variable is and what it was initialized to. In particular, initialization should be used instead of declaration and assignment, e.g.

int i;
i = f();      // Bad -- initialization separate from declaration.

int j = g();  // Good -- declaration has initialization.

Note that gcc implements for (int i = 0; i < 10; ++i) correctly (the scope of i is only the scope of the for loop), so you can then reuse i in another for loop in the same scope. It also correctly scopes declarations in if and while statements, e.g.

while (char const* p = strchr(str, '/')) str = p + 1;

There is one caveat: if the variable is an object, its constructor is invoked every time it enters scope and is created, and its destructor is invoked every time it goes out of scope.

// Inefficient implementation:
for (int i = 0; i < 1000000; ++i)
{
    Foo f;  // My ctor and dtor get called 1000000 times each.
    f.do_something(i);
}

It may be more efficient to declare such a variable used in a loop outside that loop:

Foo f;  // My ctor and dtor get called once each.
for (int i = 0; i < 1000000; ++i)
{
    f.do_something(i);
}

Definition: It is possible to perform initialization in the body of the constructor.

Pros: Convenience in typing. No need to worry about whether the class has been initialized or not.

Cons: The problems with doing work in constructors are:

• If the work calls virtual functions, these calls will not get dispatched to the subclass implementations. Future modification to your class can quietly introduce this problem even if your class is not currently subclassed, causing much confusion.
• If someone creates a global variable of this type (which is against the rules, but still), the constructor code will be called before main(), possibly breaking some implicit assumptions in the constructor code.

Decision: Constructors should not make virtual calls to functions, access potentially uninitialized global variables, etc.

Definition: The default constructor is called when we create a class object with no arguments. It is always called when calling new[] (for arrays).

Pros: Initializing structures by default makes debugging much easier.

Cons: Extra work for you, the code writer.

Decision:

If your class defines POD member variables and has no other constructors you must define a default constructor (one that takes no arguments). It should initialize the object in such a way that its internal state is consistent and valid.

The reason for this is that if you have no other constructors and do not define a default constructor, the compiler will generate one for you. This compiler generated constructor may not initialize your object sensibly.

If your class is composed from and/or inherits from an existing class or classes but you add no new member variables, you are not required to have a default constructor.

If your class has value semantics then consider making the class invariants such that the default constructor is cheap. For example, initialising member pointers to nullptr and allocating on first use.

Definition: Normally, if a constructor takes one argument, it can be used as a conversion. For instance, if you define Foo::Foo(string name) and then pass a string to a function that expects a Foo, the constructor will be called to convert the string into a Foo and will pass the Foo to your function for you. This can be convenient but is also a source of trouble when things get converted and new objects created without you meaning them to. Declaring a constructor explicit prevents it from being invoked implicitly as a conversion.

Pros: Avoids undesirable conversions.

Cons: Avoids desirable conversions.

Decision:

We require all single argument constructors to be explicit. Always put explicit in front of one-argument constructors in the class definition: explicit Foo(string name);

The exception is copy constructors, which, in the rare cases when we allow them, should probably not be explicit. Classes that are intended to be transparent wrappers around other classes are also exceptions. Such exceptions should be clearly marked with comments.

Definition: The copy constructor and assignment operator are used to create copies of objects. The copy constructor is implicitly invoked by the compiler in some situations, e.g. passing objects by value.

Pros: Copy constructors make it easy to copy objects. STL containers require that all contents be copyable and assignable. Copy constructors can be more efficient than CopyFrom()-style workarounds because they combine construction with copying, the compiler can elide them in some contexts, and they make it easier to avoid heap allocation.

Cons: Implicit copying of objects in C++ is a rich source of bugs and of performance problems. It also reduces readability, as it becomes hard to track which objects are being passed around by value as opposed to by reference, and therefore where changes to an object are reflected.

Decision:

Few classes need to be copyable. Most should have neither a copy constructor nor an assignment operator. In many situations, a pointer or reference will work just as well as a copied value, with better performance. For example, you can pass function parameters by reference or pointer instead of by value, and you can store pointers rather than objects in an STL container.

If your class needs to be copyable, prefer providing a copy method, such as CopyFrom() or Clone(), rather than a copy constructor, because such methods cannot be invoked implicitly. If a copy method is insufficient in your situation (e.g. for performance reasons, or because your class needs to be stored by value in an STL container), provide both a copy constructor and assignment operator.

If your class does not need a copy constructor or assignment operator, you must explicitly disable them.

The struct and class keywords behave almost identically in C++. We add our own semantic meanings to each keyword, so you should use the appropriate keyword for the data-type you're defining.

structs should be used for passive objects that carry data, and may have associated constants, but lack any functionality other than access/setting the data members. The accessing/setting of fields is done by directly accessing the fields rather than through method invocations. Methods should not provide behavior but should only be used to set up the data members, e.g., constructor, destructor, initialize(), reset(), validate().

If more functionality is required, a class is more appropriate. If in doubt, make it a class.

For consistency with STL, you can use struct instead of class for functors and traits.

Definition: When a sub-class inherits from a base class, it includes the definitions of all the data and operations that the parent base class defines. In practice, inheritance is used in two major ways in C++: implementation inheritance, in which actual code is inherited by the child, and interface inheritance, in which only method names are inherited.

Pros: Implementation inheritance reduces code size by re-using the base class code as it specializes an existing type. Because inheritance is a compile-time declaration, you and the compiler can understand the operation and detect errors. Interface inheritance can be used to programmatically enforce that a class expose a particular API. Again, the compiler can detect errors, in this case, when a class does not define a necessary method of the API.

Cons: For implementation inheritance, because the code implementing a sub-class is spread between the base and the sub-class, it can be more difficult to understand an implementation. The sub-class cannot override functions that are not virtual, so the sub-class cannot change implementation. The base class may also define some data members, so that specifies physical layout of the base class.

Decision:

All inheritance should be public. If you want to do private inheritance, you should be including an instance of the base class as a member instead.

Do not overuse implementation inheritance. Composition is often more appropriate. Try to restrict use of inheritance to the "is-a" case: Bar subclasses Foo if it can reasonably be said that Bar "is a kind of" Foo.

Make your destructor virtual if necessary. If your class has virtual methods, its destructor should be virtual.

Limit the use of protected to those member functions that might need to be accessed from subclasses. Note that data members should be private.

When redefining an inherited virtual method (both pure and non-pure), explicitly declare it override in the declaration of the derived class. Rationale: using override allows the compiler to consistently detect attempts to override methods that have been changed or completely removed. It also makes it straightforward for a reader to determine if a method is virtual or not.

Definition: Multiple inheritance allows a sub-class to have more than one base class. We distinguish between base classes that are interfaces and those that have an implementation.

Pros: Multiple implementation inheritance may let you re-use even more code than single inheritance (see Inheritance).

Cons: Only very rarely is multiple implementation inheritance actually useful. When multiple implementation inheritance seems like the solution, you can usually find a different, more explicit, and cleaner solution.

Decision: Multiple inheritance is allowed only when all superclasses, with the possible exception of the first one, are interfaces.

Definition:

A class is an interface if it meets the following requirements:

• It has only public pure virtual ("= 0") methods and static methods (but see below for destructor).
• It may not have non-static data members.
• It need not have any constructors defined. If a constructor is provided, it must take no arguments and it must be protected.
• If it is a subclass, it may only be derived from classes that satisfy these conditions.

An interface class can never be directly instantiated because of the pure virtual method(s) it declares. To make sure all imp