How to check if a float is a true number in c++?

Question

How to check if a float number is a true number? That is: it is not infinity, negative infinity, NaN ...

float f;
???

Answer 1

Simpler than std::fpclassify() is to use std::isfinite()

Determines if the given floating point number arg has finite value i.e. it is normal, subnormal or zero, but not infinite or NaN.

Answer 2

std::isnormal() does what you want but it also checks for 0.0. So you might check that case extra:

float f;
bool isNumber = (std::isnormal(f) || f == 0.0);

Edit: as pointed out by user2079303 isnormal also returns false for a subnormal number which the OP probably does not want.

However, maybe std::isfinite does the right thing.

float f;
bool isNumber = std::isfinite(f) ;

it returns false for NaN and Inf.