#include <iostream>
#include <math.h>

int main()
{
    double d = NAN;

    std::cout << isnan(d) << 'n';

    return 0;
}

$ g++ -std=c++98 main.cpp; ./a.out
1

$ g++ -std=c++11 main.cpp; ./a.out
1

$ g++ -std=c++14 main.cpp; ./a.out
1

#include <iostream>
#include <cmath>
#include <math.h>

int main()
{
    double d = NAN;

    std::cout << std::isnan(d) << 'n';
    std::cout << isnan(d) << 'n';

    return 0;
}

$ g++ -std=c++98 main.cpp; ./a.out
1
1

$ g++ -std=c++11 main.cpp
main.cpp: In function ‘int main()’:
main.cpp:10:25: error: ‘isnan’ was not declared in this scope
     std::cout << isnan(d) << 'n';
                         ^
main.cpp:10:25: note: suggested alternative:
In file included from main.cpp:3:0:
/usr/include/c++/5/cmath:641:5: note:   ‘std::isnan’
     isnan(_Tp __x)
     ^

$ g++ -std=c++14 main.cpp
main.cpp: In function ‘int main()’:
main.cpp:10:25: error: ‘isnan’ was not declared in this scope
     std::cout << isnan(d) << 'n';
                         ^
main.cpp:10:25: note: suggested alternative:
In file included from main.cpp:3:0:
/usr/include/c++/5/cmath:641:5: note:   ‘std::isnan’
     isnan(_Tp __x)
     ^

$ clang++-4.0 -std=c++98 main.cpp; ./a.out
1
1

$ clang++-4.0 -std=c++11 main.cpp
main.cpp:10:18: error: use of undeclared identifier 'isnan'
    std::cout << isnan(d) << 'n';
                 ^
1 error generated.