Unexpected Output Type From decltype

Question

I've been experimenting with decltype, trying to figure out the potential benefits of it. I ran this very simple function:

template<class T, class U>
auto MyFunc(T t, U u) -> decltype(t < u ? t : u)
{
  return t < u ? t : u;
}

when calling the function:

auto d = MyFunc(3,3.5);

std::cout << d<<'\n';
std::cout<<"type: "<<typeid(d).name();

I get the following result:

3
type: d

My question is why the returned type is d (decimal) not i (integer)? Thanks!

Surprisingly an almost exact duplicate question was asked a few days ago — Borgleader, Dec 25 '19 at 17:08

score 0 · Answer 1 · answered Dec 25 '19 at 16:55

0

You can't return two different types depending on the values of the input. Therefore it choses the widest compatible type, here double

answered Dec 25 '19 at 16:55

Vivick

2,051
2
6
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score 0 · Answer 2 · answered Dec 25 '19 at 17:11

The compiler does not dynamically look up the types in this context. It needs to create the template instance so it chooses the most suitable type, which is double because if it would choose int, there might be a data loss conversion from double to int if it would be the case.

Unexpected Output Type From decltype

2 Answers2