Portable alternative to #pragma once

Question

Can somebody tell me a workaround for #pragma once directive support for various compilers?

I want to use in my header something like:

#if _MSC_VER > ... || __GNUC__ > ... || ...

#pragma once

#endif

Maybe it already exists in boost sources or in your code?

Answer 1

Use include guards:

#ifndef MY_HEADER_H
#define MY_HEADER_H

// ...

#endif    // MY_HEADER_H

Sometimes you'll see these combined with the use of #pragma once:

#pragma once

#ifndef MY_HEADER_H
#define MY_HEADER_H

// ...

#endif    // MY_HEADER_H

#pragma once is pretty widely supported.

Answer 2

I like to use the traditional include guards myself (as recommended by the accepted answer). It's really not that much more work, and you get the benefit of 100% portability. If you are writing a library, posting code samples, etc, it's ideal to use that old school syntax to avoid anyone else running into trouble.

That said, it has been pointed out as well by others here that the vast majority of modern compilers respect the #pragma once directive, so it's relatively improbable you will encounter an issue using it in your own projects.

Wikipedia has a list of compilers supporting the directive:

https://en.wikipedia.org/wiki/Pragma_once#Portability