Thread: Question about #pragma once

Normally I make a header file like this:

Code:

#ifndef HEADERFILE_H_
#define HEADERFILE_H_

...

#endif HEADERFILE_H_

However using the VS2008 Express IDE's Project->Add class wizard it does this:

Code:

#pragma once
...

So my question is, does this do the same thing as the above code?

Yes.
It has the added "bonus" feature of locking you into your compiler vendor.

It doesn't work reliably anyway. Use normal ifdef guards.

Ok thanks, is there any way to make the VS compiler use the normal guards or do I just have to manually type it (not like I mind, but doing a project which is going to use a lot of classes so anything to make it faster is nice)

There's no built-in feature, though some 3rd party plugins may provide the means to do so.
Or you could create a VS macro.

ah ok thanks Elysia

The #pragma once directive on MS compilers is said to speed up compilation in some situation. There's a difference between it and standard include guards. Being a pragma, there's not much to worry about portability since pragmas are ignored if the compiler don't understand them (well, you might have a warning message in those case).

So it's not that much of a portability issue in my opinion. Nevertheless, I wouldn't consider using #pragma once in my code except in really special case.

Being a pragma, there's not much to worry about portability

The worry is that if you use it to the exclusion of guards, the code will fail to compile on compilers not supporting #pragma once.
Also, the code becomes invalid under the rules of the C++ standard.

As for speeding up compilation, it does, but only because the MS compiler still does not support the simple include guard recognition trick that pretty much every other compiler has supported for years and years.

Originally Posted by CornedBee

Also, the code becomes invalid under the rules of the C++ standard.

I believe that the code merely behaves in an implementation defined manner, which is not the same as saying that it is invalid as per the Standard.

Hmm.

Possibly. Yeah, basically, if the code contains any pragma, anywhere, its entire semantics are completely implementation-defined.

Not a very comforting thought.