#include <iostream>

using namespace std;

class A
{
public:
    A() : a(0) {}
    A(int a) : a(a) {}
    bool operator == (const A &rhs) const { cout << "A==" << endl; return this->a == rhs.a; }
private:
    int a;
};

class B
{
    friend bool operator == (const B &lhs, const B &rhs);
public:
    B() : a(0) {}
    B(int a) : a(a) {}
private:
    int a;
};
bool operator == (const B &lhs, const B &rhs) { cout << "B==" << endl; return lhs.a == rhs.a; }

int main()
{
    A a1; A a2(1);
    B b1; B b2(0);
    cout << (a1 == a2) << endl; // OK
    cout << (b1 == b2) << endl; // OK также
    cout << (b1 == 1) << endl; // OK, 1 --> B(1)
    cout << (1 == b1) << endl; // OK, 1 --> B(1) тоже
    cout << (a1 == 0) << endl; // OK, 0 --> A(0)
    // cout << (1 == a2) << endl; --> error: no match for operator==(int, A)
    return 0;
}