(C++) Header file

A file called in your program, using the #include statement.

Header files often consist of functions or class ?declarations and or definitons. Often they have the .h or .hpp extension. Header files from the STL must be called without this .h extension.

Sometimes a header (.h) file also has an implementation (.cpp) file with the same name. When this is the case, the header file contains declarations and the implementation file the definitions. A header file and its implementation file are sometimes called a Unit.

include <iostream>              //STL header 

include <myOwnCoolHeaderFile.h> //Header in standard folder

include "myOwnCoolHeaderFile.h" //Local header

Note that the third #include first checks if the header file is in the same folder as the program. If it is not present, it will check the standard header folder for it.

Always use #include guards [1].

Use of header and implementation files

There are three ways to put your code in header (.h) and implementation (.cpp) files:

1) Use only .h files
2) Add .cpp files to project
3) #include all .cpp files from the file that contains main()

1) Use only .h files

This option is used by many libraries (most notably the STL and most parts of Boost) put all definitions in the .h files. This has the advantage that they are easy to call: you only need to #include the necessary .h files.

//Example.h
ifndef __EXAMPLE_H

define __EXAMPLE_H


include <iostream>


struct Example
{
  void sayHello() const 
  {
    std::cout << "Hello" << std::endl; //Definition in .h file
  }
};
endif

//Main.cpp
include "Example.h"

int main()
{
  Example myExample;
  myExample.sayHello();
}

2) Add .cpp files to project

This option is used in larger non-library code. The header files contain the declarations, the implementation files the definitions. The .cpp files must be 'Added to Project'. How this is done is dependent on the programming environment. The advantage is that compiling is quickest.

//Example.h
ifndef __EXAMPLE_H

define __EXAMPLE_H


struct Example
{
  void sayHello() const;
};
endif

//Example.cpp
include "Example.h"

include <iostream>


void Example::sayHello() const 
{
  std::cout << "Hello" << std::endl;
}

//Main.cpp
include "Example.h"

int main()
{
  Example myExample;
  myExample.sayHello();
}

3) #include all .cpp files from the file that contains main()

This option can be used in example source code. It enables newbies to compile-and-run.

//Example.h
ifndef __EXAMPLE_H

define __EXAMPLE_H


struct Example
{
  void sayHello() const;
};
endif

//Example.cpp
include "Example.h"

include <iostream>


void Example::sayHello() const 
{
  std::cout << "Hello" << std::endl;
}

//Main.cpp
#include "Example.cpp" //Calling .cpp file
int main()
{
  Example myExample;
  myExample.sayHello();
}

C++ Library headers

International C++ Standard, table 11

C++ Headers for C library facilities

International C++ Standard, table 12

Incomplete list of non-standard header files

'Header' links

Reference

1) Herb Sutter and Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Chapter 24: 'Always write internal #include guards. Never write external #include guards'.

last edited (November 27, 2006) by bilderbikkel, Number of views: 12310, Current Rev: 30 (Diff)

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