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Adding an include guard breaks the build

Answer 1

+2 A:

If you add an #ifndef for a constant that's already defined, it will always validate to true. You say "the file is declared", but files do not get declared. It is really the constant that you place after #ifndef that you should be checking. Do a simple search through the whole source tree and double-check in what order your current #define appears.

And of course: is your code correct? Try with a rubbish name as constant, and place #endif right after it: if it still errors, you have typos (paste your code, if so). See also this post.

PS: I see that David Thornley was typing similar advice in a comment... sorry if this duplicates info in this thread.

Abel 2009-11-16 19:10:02

as I said, everything compiles fine when I put rubbish as the constant.

yan bellavance 2009-11-16 19:34:45

then, sounds to me you should use rubbish indeed (the name really doesn't matter) as the name causing trouble is apparently declared elsewhere already. As mentioned in the other answer, the constant can be any name. If the scope is only local, then don't bother and keep your working rubbish-like name.

Abel 2009-11-16 20:07:59

I was hoping to understand why this is so.

yan bellavance 2009-11-16 21:16:32

Maybe I wasn't clear. Make sure to take a good look at Pate's answer, it's rather extensive. If `XYZ` is defined, and you have `#ifndef XYZ`, it will always fail. If you want the reverse, use a name that's not defined. That's all the *"why"* there is to it, I'm afraid.

Abel 2009-11-16 23:55:58

ok I will look into it..sorry i am at work and didnt have much time today to look at it. Thx for your answers though

yan bellavance 2009-11-17 01:54:15

Answer 2

+13 A:

Roger Pate 2009-11-16 19:26:04

#pragma once - the best include guard (if supported by your target compilers)

sbk 2009-11-16 20:34:20

Except the #pragma introduces the debate about support and being non-standard, with no additional benefit. It's easy to use include guards.

Roger Pate 2009-11-25 20:53:46

A potential additional benefit is that the compiler could build a collection of header file names that have had #pragma once used in them and never even open the file again rather than having to open the file, read it all, parse it and ignore it. That said I use both and have the #pragma once protected by a compiler version check ...

Len Holgate 2009-11-25 20:57:48

The compiler can do that with include guards. GCC has done this for years, and I heard MSVC does now too. A significant problem is how to handle things like links (both symbolic and hard)--the #pragma fails, but include guards work.

Roger Pate 2009-11-25 21:06:11

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Adding an include guard breaks the build

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