#ifndef or #pragma once? 1

[McBugzz]

For more than 2 years I have used the folowing construction to prevent files from multiple including:

#ifndef BLA_BLA_BLA
#define BLA_BLA_BLA

// ...

#endif

Recently I descovered the

#pragma once

and even though this seems to be an easier statement, I don't recall EVER seeing it in ANY project, personal or company, big or small - never.

Why is #pragma once not being used all around as it is shorter?

 

[xwb]

#pragma once is a note to the compiler to only open the file once. This is basically to speed up compilation. It is not a standard C++ feature: it is just MSVC specific.

Most people try to keep to standard C++, even when the only products they produce are for the MS platform.

 

[EarlAllen]

I also recently came across the #pragma once command when I had a multiple function definition error. When I eventually replaced the pragma with the #ifndef syntax, it solved the compiler problem. There is a functional difference between the two and it looks as though the #ifndef approach is safer.

 

[qednick]

I actually believe #pragma once was Metrowerks specific in the CodeWarrior compilers but was later adopted by Microsoft in VC++.
Also, when you add a new class to a VC++ project, the statement appears towards the head of the header file as such:
[tt]
#if _MSC_VER > 1000
#pragma once
#endif // _MSC_VER > 1000
[/tt]

[sup]programmer (prog'ram'er), n A hot-headed, anorak wearing, pimple-faced computer geek.[/sup]​