class Widget {
public:
  Widget();                                   // default ctor
  Widget(std::initializer_list<int> il);      // std::initializer_list ctor
  …                                           // no implicit conversion funcs
};

Widget w1;          // calls default ctor
Widget w2{};        // also calls default ctor
Widget w3();        // most vexing parse! declares a function!

Widget w4({});      // calls std::initializer_list ctor with empty list
Widget w5{{}};      // ditto

#include <iostream>
#include <initializer_list>

class Widget{
public:
    Widget(){

    }
    Widget(std::initializer_list<int> l){
        std::cout << "l.size = " << l.size() << std::endl;
    }
};

int main(){
    Widget w4({}); //l.size = 0
    Widget w5{{}}; //l.size = 1
}