Question

Given class:

class C
{
public:
    C()
    {
        cout << "Dflt ctor.";
    }
    C(C& obj)
    {
        cout << "Copy ctor.";
    }
    C(C&& obj)
    {
        cout << "Move ctor.";
    }
    C& operator=(C& obj)
    {
        cout << "operator=";
        return obj;
    }
    C& operator=(C&& obj)
    {
        cout << "Move operator=";
        return obj;
    }
};

and then in main:

int main(int argc, char* argv[])
{
    C c;
    C d = c;
    C e;
    e = c;
    return 0;
}

as you will see from the output the “regular” version of copy ctor and operator= are invoked but not those with rvalue args. So I would like to ask in what circumstances will the move ctor and operator=(C&&) be invoked?

Answer 1

The move constructor will be invoked when the right-hand side is a temporary, or something that has been explicitly cast to C&& either using static_cast<C&&> or std::move.

C c;
C d(std::move(c)); // move constructor
C e(static_cast<C&&>(c)); // move constructor
C f;
f=std::move(c); // move assignment
f=static_cast<C&&>(c); // move assignment
C g((C())); // move construct from temporary (extra parens needed for parsing)
f=C(); // move assign from temporary