include_guards

Concept: Include Guards

Include guards are a preprocessor technique used to prevent multiple inclusions of the same header file in a single compilation unit. They are essential for avoiding duplicate definitions and potential compilation errors.

Example 1: Basic Usage

Explanation

Here's a basic example of how to use include guards:

// myheader.h
#ifndef MYHEADER_H
#define MYHEADER_H

// Header content goes here
struct MyStruct {
    int x;
    int y;
};

#endif // MYHEADER_H

• The #ifndef MYHEADER_H checks if MYHEADER_H is not defined.
• If it's not defined, we define it with #define MYHEADER_H.
• The header content is placed between these guards.
• The #endif closes the conditional block.

Example 2: Multiple Inclusions

Explanation

Let's see how include guards prevent multiple inclusions:

// main.cpp
#include "myheader.h"
#include "myheader.h" // This inclusion will be ignored

int main() {
    MyStruct s;
    s.x = 10;
    s.y = 20;
    return 0;
}

• The first #include "myheader.h" processes the header normally.
• The second inclusion is effectively ignored because MYHEADER_H is already defined.
• This prevents duplicate definitions of MyStruct.

Alternative: #pragma once

An alternative to traditional include guards is the #pragma once directive:

// myheader.h
#pragma once

struct MyStruct {
    int x;
    int y;
};

• #pragma once is widely supported by modern compilers.
• It's shorter and less error-prone than traditional include guards.
• However, it's not part of the C++ standard, so it may not work on all compilers.

Summary

Include guards are crucial for preventing multiple inclusions of header files, which can lead to compilation errors due to duplicate definitions. They work by defining a unique macro for each header file and checking if that macro is already defined before processing the file contents. While traditional include guards using #ifndef, #define, and #endif are standard and widely supported, the #pragma once directive offers a simpler alternative in many modern compilers. Proper use of include guards is essential for writing robust and maintainable C++ code.

Citations:

[1] https://acodersjourney.com/top-10-c-header-file-mistakes-and-how-to-fix-them/ [2] https://64.github.io/cpp-faq/include-guards-pragma-once/ [3] https://upcoder.com/20/cc-include-guidelines/ [4] https://www.cs.kent.edu/~nmadi/CS2/Notes/include_guard.html [5] https://en.wikipedia.org/wiki/Include_guard [6] https://www.reddit.com/r/C_Programming/comments/wq9u5h/include_guards/ [7] https://computingonplains.wordpress.com/2023/12/05/include-guard-vs-pragma-once-in-c-and-c/ [8] https://stackoverflow.com/questions/27810115/what-exactly-do-c-include-guards-do

Related

• How do include guards prevent circular references
• What are the differences between include guards and #pragma once
• Can include guards affect build times in large projects
• Are there any performance benefits to using #pragma once over include guards
• How do include guards work in multi-file projects