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Table of Contents
Header guard
Header guards are a pattern used in C and C++ programming languages when writing header files. Header files (.h files) are files that are included in source files (.c and .cpp files) near the top (hence the name) using the #include <...> directive. They are used to allow multiple inclusions of the same file but prevent multiple definition errors.
Header guards follow the same general pattern:
#ifndef __HEADER_GUARD_H__ #define __HEADER_GUARD_H__ // ... #endif /* __HEADER_GUARD_H__ note: this is just a comment */
In C and C++ programming languages, #include is nothing but text substitution. If you include something multiple times
Example
A prototypical example of a header guard is the following.
// hello_world.h #ifndef __HELLO_WORLD_H__ #define __HELLO_WORLD_H__ struct Hello { const char *text = "world"; }; #endif /* __HELLO_WORLD_H__ */
This allows multiple inclusion of “helloworld.h”
c
// main.c
#include <hello_world.h>
#include <hello_world.h>
int main() {
Hello world;
return 0;
}
Without header guards,