Typ eines Ausdrucks erschließen:

decltype.cpp

#include <iostream>
 
template <typename T1, typename T2>
auto sum(T1 a, T2 b) -> decltype(a+b) { return a + b; }
 
int main()
{
  auto x = 3;
  decltype(x) y = x; 
 
  std::cout << x << ' ' << y << ' ' << sum(x,y) << '\n';
}