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Pragma Once and Header Guards

We saw how .h header files are used in a previous lesson, but before we go further let's talk about a potential issue you might run into: multiple inclusions. If the same header file gets included more than once, you can end up with some nasty errors caused by redefining things like functions or structs.

Pragma Once

One simple solution (and the one we'll use for the rest of this course) is #pragma once. Adding this line to the top of a header file tells the compiler to include the file only once, even if it's referenced multiple times across your program.

// my_header.h

#pragma once

struct Point {
    int x;
    int y;
};

Header Guards

Another common way to avoid multiple inclusions is with include guards, which use preprocessor directives like this:

#ifndef MY_HEADER_H
#define MY_HEADER_H

// some cool code

#endif

This method works by defining a unique macro for the header file. If it's already been included, the guard prevents it from being processed again.

Throughout this course, you'll see #pragma once in our header files. It's quicker and less error-prone than traditional include guards, and it works well with most modern compilers.