std::fdim(C++11)

namespace std {
  float fdim(float x, float y);
  double fdim(double x, double y);
  long double fdim(long double x, long double y);

  Promoted fdim(Arithmetic1 x, Arithmetic2 y);

  float fdimf(float x, float y);                   // C++17 から
  long double fdiml(long double x, long double y); // C++17 から
}

#include <cmath>
#include <iostream>

int main() {
  std::cout << std::showpos;
  std::cout << "fdim(-1.0, 0.0) = " << std::fdim(-1.0, 0.0) << std::endl;
  std::cout << "fdim( 0.0, 0.0) = " << std::fdim( 0.0, 0.0) << std::endl;
  std::cout << "fdim(+1.0, 0.0) = " << std::fdim(+1.0, 0.0) << std::endl;
}