Header file in C/C++

Purpose?

To hold declarations for other files to use.
Typically only contain declarations, do not define how something is implemented.

Standard library header files

Standard libary function is implemented in the C++ runtime support library, which is automatically linked into the program during the link phase.

iostream-library-linking

Use header file instead of forward declaration

We need a forward declaration so that the compiler would know about the function (even it is implemented in the another file) when compiling.
However, writing forward declarations for every function that lives in another file can is tedious.
A header file only has to be written once, and it can be included in as many files as needed.

Header guard

Header guards prevent a given header file from being included more than once in the same file.

#ifndef ADD_H   // can be any unique name
#define ADD_H

int add(int x, int y);

#endif

How Compiler work with header files?

When the compiler compiles the #include "some_header.h", It simply copies the contents of some_header.h into the current file at that point. Consequently, program will compile and link correctly.

compile-and-linking-with-header

`<header.h>` vs `"header.h"`

Angled brackets(<>): compiler looks for the header in the system directories.
Double-quotes(""): compiler looks for the header file in the current directory containing the source code. If not found, it will check any other include paths specified in compiler/IDE settings. That failing, it will fall back to checking the system directories.

Best practices

Always include header guards.
Do not define variables in header files unless they are constants. Header files should generally only be used for declarations.
Do not define functions in header files.
Each header file should have a specific job, and be as independent as possible. For example, you might put all your declarations related to functionality A in A.h and all your declarations related to functionality B in B.h. That way if you only care about A later, you can just include A.h and not get any of the stuff related to B.
Give your header files the same name as the source files they’re associated with (e.g. grades.h goes with grades.cpp).
Try to minimize the number of other header files you include in your header files. Only include what is necessary. Do not include .cpp files.