Question

Possible Duplicate:
Why is this “min” template of cpp-next at fault?

From another question I got this function template:

template <class T, class U>
auto min(T x, U y) -> decltype(x < y ? x : y) {
    return x < y ? x : y;
}

It compiles and seems to work fine, but I’m unsure why it works. How can the return type be deduced at compile-time? — I would think it can be either T or U depending on which argument is smaller, and that can only be determined at run-time.

Answer 1

An expression of the form a ? b : c always returns the same type whether a is true-valued or not. If b and c are of different types, then type promotion occurs, just like when 3 + 4.2 evaluates to 7.2 (via double(3) + 4.2).