[Bug libstdc++/48891] New: std functions conflicts with C functions when building with c++0x support.

["alexis.menard at openbossa dot org" <gcc-bugzilla@gcc.gnu.org>]

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

           Summary: std functions conflicts with C functions when building
                    with c++0x support.
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: alexis.menard@openbossa.org


Created attachment 24193
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24193
Small test case.

Hello,

I'm not sure it's a real bug (though that example builds fine with gcc 4.5.0)
but at least perhaps I'll get help.

Consider :

#include <stdlib.h>
#include <cmath>
#include <stdio.h>

using namespace std;

int main(int argc, char** argv)
{
    double number = 0;
    if (isnan(number))
    {
        printf("Nan\n");
    }
    return 0;
}

and build it with : g++ main.cpp -std=c++0x -std=gnu++0x -o test

It fails to compile : 

main.cpp: In function ‘int main(int, char**)’:
main.cpp:10:21: error: call of overloaded ‘isnan(double&)’ is ambiguous
main.cpp:10:21: note: candidates are:
/usr/include/bits/mathcalls.h:235:1: note: int isnan(double)
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0/cmath:558:3:
note: bool std::isnan(long double)
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0/cmath:554:3:
note: bool std::isnan(double)
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0/cmath:550:3:
note: bool std::isnan(float)

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

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'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?