mixed-forwarding.cpp2

#include <iostream>
#include <utility>

struct X {
    int i;
    X(int i)         : i{     i} { std::cout << "+X "     << i << "\n"; }
    X(X const& that) : i{that.i} { std::cout << "copy X " << i << "\n"; }
    X(X &&     that) : i{that.i} { std::cout << "move X " << i << "\n"; }
};

copy_from: (copy _) = { }

use: (_) = {}

// invoking each of these with an rvalue std::pair argument ...
apply_implicit_forward: (forward t: std::pair<X, X>) = {
    copy_from(t.first);             // copies
    copy_from(t.second);            // moves
}
apply_explicit_forward: (forward t: std::pair<X, X>) = {
    copy_from(forward t.first);     // moves
    copy_from(forward t.second);    // moves
}

main: ()->int = {
    t1: std::pair<X,X> = (1,2);
    apply_implicit_forward(t1);
    use(t1);
    apply_implicit_forward(t1);

    t2: std::pair<X,X> = (3,4);
    apply_explicit_forward(t2);
    use(t2);
    apply_explicit_forward(t2);
}