#include <iostream>

struct A { };

struct B {
    B(){}
    explicit B(const A& a) {
        std::cout << "constructor" << std::endl;
    }

    //B& operator=(const A& a) {
    //  cout << "assignment operator" << endl;
    //  return *this;
    //}
};

int main() {
    A a;
    B b;
    b = a;

    return 0;
}