initializer_list and default constructor overload resolution

Question

#include <initializer_list>
#include <iostream>
using namespace std;

struct Y {};

struct X
{
    X(initializer_list<Y>) { cout << "yay" << endl; }
    explicit X() { cout << "boo" << endl; }
};

X f()
{
    return {};
}

int main()
{
    f();

    return 0;
}

This prints out "boo". Why doesn't it print out "yay" ?

Is there anyway to differentiate the following two constructions:

1. X()
2. X{}

or

1. return X();
2. return {};

or

void g(const X&)

1. g(X())
2. g({})

Thanks.

Answer 1

Is there anyway to differentiate the following two constructions:

No. They are not different constructs.

The primary purpose of the {} constructor syntax was to introduce uniform initialization, to have initialization work the same everywhere. If there was a difference between them, it wouldn't be uniform.

If you want to use the empty initializer list constructor, you have to state that you're passing an initializer list explicitly. Like this: return initializer_list<Y>{}; Of course, one of the other purposes of uniform initialization is to not have to type out typenames so much, so you can achieve the same effect with return {{}};

Answer 2

return {}; will always use a default constructor if there is one.

return X({}); or return {{}}; will construct from an empty initialiser list.