std::abs for "unsigned long long int" data type

Question

Why should I get this error

 C2668: 'abs' : ambiguous call to overloaded function

For a simple code like this

#include <iostream>
#include <cmath>
int main()
{
  unsigned long long int a = 10000000000000;
  unsigned long long int b = 20000000000000;
  std::cout << std::abs(a-b) << "\n";   // ERROR
  return 0;
}

The error still presents after removing std::. However if I use int data type (with smaller values) there is no problem.

The traditional solution is to check that manually

std::cout << (a<b) ? (b-a) : (a-b) << "\n";

Is that the only solution?

Answer 1

The check seem the only really good solution. Alternatives require type bigger than yours and nonstandard extension to use it.

You can go with solutions casting to signed long long if your range fits. I would hardly suggest that way, especially if the implementation is placed in a function that does only that.

Answer 2

You are including <cmath> and thus using the "floating-point abs".

The "integer abs" is declared in <cstdlib>.

However, there is no overload for unsigned long long int (both a and b are, thus a-b is, too), and the overload for long long int only exists since C++11.