// Function fun has a default value 1.0 for the second parameter b
#include <iostream>

double fun(double a, double b= 1.0); //IMPORTANT LINE
int main(){
  double a=5,b=10;
  fun(a);
  fun(a,b);
}
double fun(double a,double b) {
  if(b==1) {
    std::cout <<"a="<< a<<" default case b=1"<<"\n";
  }
  else {
    std::cout <<"a="<< a<<" not default case, b="<<b<<"\n";
  }
  return (1.0);// just test
}