Confusion around undefined behaviour when compiling for multi-platform

Question

I've recently discovered that it's undefined behaviour to start anything with two underscores in C++.

So I've been fixing all my include guards.

But I've encountered code like

#ifdef __WINDOWS___
#endif
#ifdef __GNUC__
#endif

But isn't using __GNUC__ on a windows compiler UB and vice-versa? What should I do here?

Double underscores are reserved for the implementation to define. These macros are defined (or not defined) by the implementation. — Barmar, Jun 10 '16 at 07:11

score 5 · Accepted Answer · answered Jun 10 '16 at 07:10

You misunderstand the rules slightly.

It's fine for you to use something that has (or hasn't) been #DEFINEd by your toolchain, even if it starts with two underscores.

It's certainly not fine for your to #DEFINE something starting with two underscores yourself.

This is a useful convention; it means that your source code cannot clash with the way your compiler implements the C++ standard.

And I'm glad you're fixing your include guards!

score 1 · Answer 2 · edited Jun 10 '16 at 08:33

1

First of all it would look better if you write:

#ifdef _WIN32
...
#endif

because _WIN32 is defined for all Windows C++/C compilers. Next Windows doesn't support GCC per se. It does have a minimalistic port called MinGW. And because of that you should write:

#ifdef __MINGW32__
...
#endif

edited Jun 10 '16 at 08:33

rbaleksandar

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answered Jun 10 '16 at 07:16

Andrew Bezzubtsev

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Thank you but I don't think I should mess about with the header too much! – Fitzwilliam Bennet-Darcy Jun 10 '16 at 07:16
I don't think it's very lot... And, there is some platform-specific macros data: http://stackoverflow.com/questions/5919996/how-to-detect-reliably-mac-os-x-ios-linux-windows-in-c-preprocessor – Andrew Bezzubtsev Jun 10 '16 at 07:21
Thanks for this, I've voted up but had to accept only one answer. – Fitzwilliam Bennet-Darcy Jun 10 '16 at 07:37

Confusion around undefined behaviour when compiling for multi-platform

2 Answers2