Adding smallest possible float to a float

Question

I want to add the smallest possible value of a float to a float. So, for example, I tried doing this to get 1.0 + the smallest possible float:

float result =         


        

Answer 1

To increase/decrement a floating point value by the smallest possible amount, use nextafter towards +/- infinity().

If you just use next_after(x,std::numeric_limits::max()), the result with be wrong in case x is infinity.

#include 
#include 
#include 

template
T next_above(const T& v){
    return std::nextafter(v,std::numeric_limits::infinity()) ;
}
template
T next_below(const T& v){
    return std::nextafter(v,-std::numeric_limits::infinity()) ;
}

int main(){
  std::cout << "eps   : "<::epsilon()<< std::endl; // gives eps

  std::cout << "after : "<::infinity(),
     std::numeric_limits::infinity()) << std::endl; // gives inf

  // while this is probably not what you need:
  std::cout << std::nextafter(std::numeric_limits::infinity(),
     std::numeric_limits::max()) << std::endl; // gives 1.79769e+308

}