10 C++ Header Files

1. Purpose

The purpose of header files is not only for declarations, allowing you to use them across multiple .cpp files.

In the basics of C++, header files are traditionally used to declare certain function types so that they can be used throughout the program. For review: 06 How the C++ Compiler Works 07 How the C++ Linker Works.md^

2. pragma

#pragma is a preprocessor command (preprocessing)

#pragma once    // only include this file once

Therefore, it is essentially a header guard, preventing us from including the header file multiple times into a single translation unit (see AST, translation unit)

That is, you might have many header files, and without a header guard, it is very likely that including both header files in a .cpp file could lead to redefinition.

Example:

// #pragma once
struct Player{}    // struct type redefinition

3. Another Type of Guard

#ifndef _LOG_H   // if _LOG_H is not defined
#define _LOG_H  

void InitLog();
void Log(const char* messsge);

#endif

This was mainstream in the past, but now #pragma once is preferred.

4. Include Supplement

Including with " " is a relative path. If it is in the directory above the current file, you can use "../". Including with < > means the file must be in one of the include directories, but now quotes can also be used, meaning #include "iostream" works fine.

Why iostream has no extension

It was decided by the people who wrote the standard library, to distinguish between C Standard Library Header files and C++ Standard Library Header files.

C	C++
.h extension, e.g., stdlib.h	None, e.g., iostream