Why is forwarding reference constructor called instead of copy constructor?

Question

Given the following code

#include <iostream>

using namespace std;

template <typename Type>
struct Something {
    Something() {
        cout << "Something()" << endl;
    }

    template <typename SomethingType>
    Something(SomethingType&&) {
        cout << "Something(SomethingType&&)" << endl;
    }
};

int main() {
    Something<int> something_else{Something<int>{}};
    auto something = Something<int>{};
    Something<int>{something};
    return 0;
}

I get the following output

Something()
Something()
Something(SomethingType&&)

Why is the copy constructor being resolved to the templated forwarding reference constructor but not the move constructor? I am guessing that it's because the move constructor was implicitly defined but not the copy constructor. But I am still confused after reading the cases for where the copy constructor is not implicitly defined in stack overflow.

Brittney

This is a decently common blog topic: akrzemi1.wordpress.com/2013/10/10/too-perfect-forwarding – Kilar

Answer 1

I am guessing that it's because the move constructor was implicitly defined but not the copy constructor.

No, both are implicitly defined for class Something.

Why is the copy constructor being resolved to the templated forwarding reference constructor

Because the copy constructor takes const Something& as its parameter. That means for the copy constructor to be called, implicit conversion is needed for adding const qualifier. But the forwarding reference constructor could be instantiated to take Something& as its parameter, then it's an exact match and wins in the overload resolution.

So if you make something const, the implicitly defined copy constructor will be invoked for the 3rd case instead of the forwarding reference constructor.

LIVE

but not the move constructor?

Because for the move constructor the above issue doesn't matter. For the invocation of the 1st and 2nd case, both implicitly defined move constructor and forwarding reference constructor are exact match, then non-template move constructor wins.

Aerodynamics

A book I've found very useful in understanding these kinds of issues is Effective Modern C++ by Scott Meyers. – Potbelly