friend_name_injection.cpp

// Friend name injection, a.k.a. Barton–Nackman trick

#include <iostream>

struct X {
    // both of these functions are injected into the global namespace
    // however only f(const X&) is found via ADL
    friend void f(const X&) { std::cout << __PRETTY_FUNCTION__ << std::endl; }
    friend void f() { std::cout << __PRETTY_FUNCTION__ << std::endl; }
};

int main() {
    f(X{}); // OK, in global namespace via ADL

    // f(); // doesn't compile, function is not found via regular name look-up
    // X::f(); // same
    // ::f(); // same
}