Adding an explicit constructor makes a construction fail

Question

I would like to understand how the compiler is selecting constructor in the following situation

class Y
{
public:
    Y(int) {}
};

class X
{
public:
    X(int, int, Y) { cout << "Y\n"; }
    //explicit X(int, int, int) { cout << "3 ints\n"; }
};


int main()
{
    X x({ 1, 2, 3 });  // Y
}

so that if we uncomment the explicit constructor, there will be no suitable constructor to call.

Answer 1

X x({ 1, 2, 3 }); is direct initialization.

The behavior of your program can be explained using copy initialization's documentation notes which states:

In addition, the implicit conversion in copy-initialization must produce T directly from the initializer, while, e.g. direct-initialization expects an implicit conversion from the initializer to an argument of T's constructor.

(emphasis mine)

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And since in your given example there is no implicit conversion(as you've made that constructor explcit) from the initializer list, you get the mentioned error saying:

error: converting to ‘X’ from initializer list would use explicit constructor ‘X::X(int, int, int)’

You can resolve this error by removing the explicit used for the concerned constructor X::X(int, int, int) as then there will be an implicit conversion available from the initializer list.

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but why can't X x({ 1, 2, 3 }); keep using X::X(int, int, Y) when X::X(int, int, int) is explicit?

Because the explicit constructor X::X(int, int, int) that we provide also take part in overload resolution and it is a better match than the other constructor X::X(int, int, Y). And since it is a better match it is preferred but since it is explicit, we get the mentioned error.