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Re: Strange C++ function pointer test


On 31 December 2015 at 11:34, Dominik Vogt wrote:
> On Thu, Dec 31, 2015 at 10:11:55AM +0000, Jonathan Wakely wrote:
>> <cmath> is required to declare std::abs and it's unspecified whether
>> it also declares it as ::abs.
>>
>> <math.h> is required to declare ::abs and it's unspecified whether it
>> also declares it as std::abs.
>
> (*abs should be in <cstdlib> or <stdlib.h>).

For the integer overloads, yes, but in C++ the floating point
overloads are in <cmath> and <math.h>. I did double-check the standard
before posting, even though I know what I'm talking about quite well
:-)

Again, please see [c.math].

> I see.  abs and ::abs are the same in this case, and std::abs
> refers to the "double" variant, so that does not compile either.

No, it refers to an overload set, which includes the float overload.

> The minimal failing program is
>
> -- abs.C --
> #include <stdlib.h>
> static float (*p1_)(float) = abs;
> -- abs.C --
>
> G++-6.0.0, Glibc-2.20 on S390x, compiled with
>
>   $ g++ -std=c++11 abs.C
>
> The test program from the xvs suite includes stdlib.h and math.h,
> but not cstdlib, cmath (or ctgmath).  It's not clear to me which
> headers g++ includes automatically, but this seems to be correct
> behaviour, or is it not?  Does it depend on the Glibc version?

G++ doesn't include any headers automatically, but again, you need
<cmath> or <math.h> for the floating point overloads.


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