A forward declaration is the declaration of a data type the compiler will encounter further on. Because this lets the compiler check less code, forward declarations speed up compilation.

Never #include a header file when a forward declaration will suffice [1].

A forward declaration of a class can be used when nothing needs to be known about that class:

The member functions of the class are not called
Only a reference to the class is used (all references occupy the same space in memory)
Only a pointer to the class is used (all pointers occupy the same space in memory)

For a list of VCL forward declarations, go to the VCL forward declaration page.

Example

This example shows the header file of a class before and after using as much forward declarations as possible.

Before


#ifndef Unit1H

#define Unit1H



#include <iostream>

#include "UnitX.h"

#include "UnitY.h"

#include "UnitZ.h"



struct MyClass

{

  MyClass(X &x) : m_x(x) {}

  X& m_x;

  Y* m_y;

  Z m_z;

};



std::ostream& operator<<(std::ostream& os, const Unit1& y);



#endif

After


#ifndef Unit1H

#define Unit1H



#include <iosfwd>

struct X;

struct Y;

#include "UnitZ.h"



struct MyClass

{

  MyClass(X &x) : m_x(x) {}

  X& m_x;

  Y* m_y;

  Z m_z;

};



std::ostream& operator<<(std::ostream& os, const Unit1& y);

#endif

References

Herb Sutter. Exceptional C++. ISBN: 0-201-61562-2. Item 26: 'Never #include a header when a forward declaration will suffice'

Go back to Richel Bilderbeek's C++ page.

Go back to Richel Bilderbeek's homepage.

(C++) forward declaration

Example

Before

After

References