11 replies to this topic

[Captacha]

Should I put the header guard statments before the other include statments in the file?

[Hodgman]

Yes.

[larspensjo]

So what type of guards are best?

I prefer "#pragma once", but I am uncertain of the support.

[Dragonsoulj]

As long as the compiler you are using supports it, you should be fine. The resulting executable doesn't worry about those.

[Tribad]

Even if #pragma once gets more and more supported and works well I am doing both types of guards.
First #pragma once and after that the pre-processor guards with #ifndef/#define/#endif. This way the probably faster solution with the pragma tries first and if it fails the old-style catches the double inclusion.

As I had to decide what way is the best it has been reported that #pragma once may fail if the same file has been opened via different pathes, with a crosslink somewhere. This should now be fixed for most of the compilers. But who knows?

[Hodgman]

pragma once is supported by all C++ compilers these days.

Modern compilers have built-in optimisations for both pragma once or the traditional ifndef/define/endif guards -- they'll both be just as fast as each other.

[Tribad]

You do not have modern compilers in all environments e.g. embedded systems.

[omercan]

I use "#pragma once" and I'm fine with it... but I think there are no really big differences.
But "'#pragma" is easier... Only one line without additional identifiers like "COLOR_H". It is more comfortable so I use it.
If you like them, use it...

pragma once is supported by all C++ compilers these days.

Yap, this is really true

You do not have modern compilers in all environments e.g. embedded systems.

True, too But when you develop an application or library for an embedded system you often only use one specific compiler for one architecture (if it is not a general library of course)... And often someone from outdoor don't recompile the code...

In my opinion: Use it when you like it. The compiler will tell his opinion.

[Álvaro]

You do not have modern compilers in all environments e.g. embedded systems.

Those compilers don't usually support the whole language either, and that doesn't mean that the features of the language they don't support should be avoided. I think using `#pragma once' should be fine these days. If you are programming for an embedded system, you need to be aware of several things you can't do, and this would be one of them.

Also, I got the impression that people tend to use gcc on pretty much every platform these days. But perhaps I am mistaken. Do you have an example of an embedded system for which people don't use gcc?

[larspensjo]

Also, I got the impression that people tend to use gcc on pretty much every platform these days. But perhaps I am mistaken. Do you have an example of an embedded system for which people don't use gcc?

I am working in the automotive business, and they have a lot of embedded systems these days. Not that I have seen more than a couple of them, but none had GCC.

[Álvaro]

I am working in the automotive business, and they have a lot of embedded systems these days. Not that I have seen more than a couple of them, but none had GCC.

Do the compilers for those platforms have decent support of all C++ features? Memory allocation? The functions in <cmath>? Templates? Exceptions? If the answer to all of those is "yes", then you have a point about warning about lack of support for `#pragma once'. If not, having to use header guards is one more oddity about this environment that you have to be aware of, and the OP should feel free to use `#pragma once' in other environments.

Edited by alvaro, 15 July 2012 - 01:35 PM.

[larspensjo]

I am working in the automotive business, and they have a lot of embedded systems these days. Not that I have seen more than a couple of them, but none had GCC.

Do the compilers for those platforms have decent support of all C++ features?

Not at all.

If not, having to use header guards is one more oddity about this environment that you have to be aware of, and the OP should feel free to use `#pragma once' in other environments.

I agree. Notice that OP didn't specify language, and I just wanted to point out that GCC probably isn't used ubiquitously in the embedded business (and neither is C++). But I suppose that, considering the sub set of C++, GCC will be more common.