Defined in header <cmath>        float       fdim( float x, float y );   (1)   (since C++11)   double      fdim( double x, double y );   (2)   (since C++11)   long double fdim( long double x, long double y );   (3)   (since C++11)   Promoted    fdim( Arithmetic1 x, Arithmetic2 y );   (4)   (since C++11)  
 1-3) Returns the positive difference between x and y, that is, if x>y, returns x-y, otherwise (if x≤y), returns +0.
 4) A set of overloads or a function template for all combinatio