Search code examples
c++c++17decltypeternary

Why do these two code snippets have the same effect?

template <typename T1, typename T2>
auto max (T1 a, T2 b) -> decltype(b<a?a:b);

template <typename T1, typename T2>
auto max (T1 a, T2 b) -> decltype(true?a:b);

I do not understand why these two code snippets can have the same effect. Plz give me some hint and a underlying explanation.

Cheers.

Solution

  • Because the type returned by a ternary operator is decided according the types of the second and third arguments, not according the value of the first.

    You can verify this with the following code

    #include <type_traits>

int main ()
 {
   auto x = true ? 1 : 2l;

   static_assert( std::is_same<decltype(x), long>::value, "!" );
 }

    Isn't important that true ? 1 : 2l return ever 1; the ternary operator return a common type between 1 (int) and 2l (long). That is long.

    In other words: there isn't (at the moment) a constexpr ternary operator.