Most vexing parse C++11

Question

I'm confused about Y y {X{}}; what exactly this line does and what is its connection to the most vexing parse. A brief explanation is appreciated:

#include <iostream>

struct X {
    X() { std::cout << "X"; }
};
struct Y {
    Y(const X &x) { std::cout << "Y"; }
    void f() { std::cout << "f"; }
};
int main() {
    Y y { X{} };
    y.f();
}

Answer 1

what exactly this line does

It creates a temporary X, value-initialising it by calling the default constructor, and then uses that to initialise a Y variable, calling the const X& conversion constructor.

where is connection to Most vexing parse

If you were to try to write this using old-school initialisation syntax

Y y (X());

then the so-called "most vexing parse" would interpret this as a function, rather than a variable, declaration: a function called y, with return type Y and a single parameter, whose type is a (pointer to a) function returning X.

You could add extra parentheses, so that it can't be interpreted as a function declaration:

Y y ((X()));

or, since C++11, you can use brace-initialisation as your example does.