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Re: complex support when using -std=c++11


On Thu, 12 Nov 2015, D Haley wrote:

I am currently trying to understand an issue to do with complex number support in gcc.

Consider the following code:

#include <complex.h>
int main()
{
       float _Complex  a = _Complex_I;

}

Attempting to compile this with  these commands is fine:
$ g++ tmp.cpp -std=gnu++11
$ g++ tmp.cpp

Clang is also fine:
$ clang tmp.cpp -std=c++11

Not here, I am getting the same error with clang (or "use of undeclared identifier '_Complex_I'" with libc++). This probably depends more on your libc.

Attempting to compile with c++11 is not:
$ g++ tmp.cpp -std=c	++11
In file included from /usr/include/c++/5/complex.h:36:0,
                from tmp.cpp:2:
tmp.cpp: In function âint main()â:
tmp.cpp:5:29: error: unable to find numeric literal operator âoperator""iFâ
        float _Complex  a = _Complex_I;
                            ^
tmp.cpp:5:29: note: use -std=gnu++11 or -fext-numeric-literals to enable more built-in suffixes

I'm using debian testing's gcc:
$ gcc --version
gcc (Debian 5.2.1-17) 5.2.1 20150911
...


I discussed this on #gcc, and it was suggested (or I misunderstood) that this is intentional, and the library should not support c-type C++ primitives - however I can find no deprecation notice for this, nor does it appear that the c++11 standard (as far as I can see from a quick skim) has changed the behaviour in this regard.

Is this intended behaviour, or is this a bug? This behaviour was noticed when troubleshooting compilation behaviours in mathgl.

https://groups.google.com/forum/?_escaped_fragment_=topic/mathgl/cl4uYygPmOU#!topic/mathgl/cl4uYygPmOU

C++11, for some unknown reason, decided to hijack the C header complex.h and make it equivalent to the C++ header complex. The fact that you are still getting _Complex_I defined is already a gcc extension, as is providing _Complex in C++.

The C++ standard introduced User Defined Literals, which prevents the compiler from recognizing extra suffixes like iF in standard mode (why are so many people using c++11 and not gnu++11?).

Our support for complex.h in C++11 in gcc is kind of best-effort. In this case, I can think of a couple ways we could improve this

* _Complex_I is defined as (__extension__ 1.0iF). Maybe __extension__ could imply -fext-numeric-literals?

* glibc could define _Complex_I some other way, or libstdc++ could redefine it to some other safer form (for some reason __builtin_complex is currently C-only).

--
Marc Glisse


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