Header should be included once

In header files I've seen two main ways for defining macro to avoid including the file mo开发者_如何学编程re than once.

1.

#ifndef SOME_CLASS
#define SOME_CLASS
//code ...
#endif

2.

#ifndef SOME_CLASS
//code...
#define SOME_CLASS
#endif

Which is more preferable and why?

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I prefer the first method, because it doesn't matter what happens after the ifndef because it will be defined straight after.

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The first option is commonly optimized by compilers to behave like the non-standard #pragma once.

It is also safer in case of recursive includes. If, in the //code... part, you include another header which includes .... which includes the header you're currently editing, then the second version won't work.

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I'd go for the first one.

Reason: If you ever want to change the guard name (say, SOME_CLASS to SOMECLASS), you don't have to scroll all the way down to the end of file to change it too.

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The best option is to use #pragma once. With #define you must be very careful when using multiple libraries as the guard name may not be unique.

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I prefer the first option. Suppose you include more files, and these files in turn include the file containing #ifndef SOME_CLASS.

I think it's fairly easy to spot include errors, if the #define SOME_CLASS isn't adjacent to #ifndef SOME_CLASS.

// SomeClass.h
#ifndef SOME_CLASS
#include "OtherFile.h" // will eventually lead to #include "SomeClass.h"
#define SOME_CLASS

... boat load of code here...

#endif // SOME_CLASS