How to determine if double/float is NaN?

<cmath> in C++11 supports std::isan(). Returns true if the factor is NaN, or false if not.

bool isnan( float arg ); (since C++11)
bool isnan( double arg ); (since C++11)
bool isnan( long double arg ); (since C++11)

According to IEEE standard, NaN value is always false when comparing NaN value.

This means that f!=f is always true for float/doublef.

In principle, this method should work for a compiler that follows IEEE floating point I'm not sure that optimization is followed. It is recommended that you check and write in advance with the compiler.