Writing Our Own Header Files

To allow programs to be broken up into logical parts, C++ supports what is commonly known as separate compilation. Separate compilation lets us compose a program from several files. To support separate compilation.

Designing Our Own Headers

A header provides a centralized location for related declarations. Headers normally contain class definitions, extern variable declarations, and function declarations.
 
Proper use of header files can provide two benefits: All files are guaranteed to use the same declaration for a given entity; and should a declaration require change, only the header needs to be updated.
 
Some care should be taken in designing headers. The declarations in a header should logically belong together. A header takes time to compile. If it is too large programmers may be reluctant to incur the compile-time cost of including it.
Let’s See how to do it :
We have used Dev C++ to create this projects

Step 1

Create a new Dev C++ console application

Step 2

Create Three Files as shown in the following image
1) main.cpp (to compile and run main file)
2) myHeader.h (to declare function)
3) myHeaderFile.cpp ( to define function body)

Step 3

Open myHeader.h to write function declaration as :

int multipliedBy3(int);

Step 4

Open myHeaderFile.cpp to write function definition or body of the funciton as :

#include "myHeader.h"
int multipliedBy3(int a)
{
	return a * 3;
}

Step 5

Open main.cpp and copy/type code to call the function defined and declared in other file as :

#include <iostream>
#include "myHeader.h"

int main()
{
	std::cout << multipliedBy3(2);
	getchar();
	return 0;
}

Compile and run the main.cpp file and you are done
Remember : Headers Are for Declarations, Not Definitions