[Bug libstdc++/48891] std functions conflicts with C functions when building with c++0x support (and using namespace std)

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48891

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-05-05 22:03:55 UTC ---
(In reply to comment #0)
> 
> and build it with : g++ main.cpp -std=c++0x -std=gnu++0x -o test

There's no point specifying two -std options, only the last one takes effect.

> If I deactivate the c++0x support it works.

Because in C++98 there is no std::isnan, so you only get the version in the
global namespace from <math.h>

> The real issue is that the c++0x standard removes the prohibition on C++
> headers declaring C names in the global namespace. The problem here is that
> math.h is included therefore the declarations are in the global namespace.

I don't think that's the problem, because libstdc++ has always declared the
names in the global namespace even though it wasn't valid in C++03 - we haven't
changed that for C++0x (all that happened is the standard was relaxed to
reflect the reality of actual implementations)


> I'm not really sure how the compiler can solve that but this new "feature" of
> c++0x seems to be very annoying. I could solve it by not using namespace std
> but let say the project is huge, it will requires lot of modifications.
> 
> Basically any time you use using namespace std, you may have conflicts with the
> underlaying C libraries, it's even more annoying with your own namespace
> because your functions can conflict with all the stuff in the global namespace
> put by C libraries and it's very common in a cpp file to use "using namespace
> foo;"
> 
> Any suggestions on how I could "workaround" that?

Qualify isnan explicitly, by calling either ::isnan or std::isnan