漠丶涯

The Header File of C++

1.Definition of a class

- in c++ , separated .h and .cpp files are used to define one class.

- Class declaration and prototypes in that class are in the header file(.h).

- All the bodies of these functions are in the source file(.cpp)

2.The header files 

- if a function is declared in a header file , you must include the header file everywhere the function is used and where the function is defined.

- if a class is declared in a header file , you must include the header file everywhere the class is used and where class member functions are defined.

3.Header = interface 

- The header is a contract between you and the user of your code.

- The compile enforces the contract by requiring you to declare all structures and functions before they are used.

4.Declarations vs definitions

- A .cpp file is a compile unit 

- only declarations are allowed to be in .h

- extern variables 

- function prototypes

- class/struct declaration

5.#include 

- #include is to insert the included file into the .cpp file at where the #include statement is .

- #include "xx.h" : first search in the current directory , then directories declared somewhere

- #include <xx.h> :search in the specified directories 

- #include <xx>:same as #include <xx.h>

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