Moving: what does it take?

Question

What does it take to use the move assignment operator of std::string (in VC11)?

I hoped it'd be used automatically as v isn't needed after the assignment anymore. Is std::move required in this case? If so, I might as well use the non-C++11 swap.

#include <string>

struct user_t
{
    void set_name(std::string v)
    {
        name_ = v;
        // swap(name_, v);
        // name_ = std::move(v);
    }

    std::string name_;
};

int main()
{
    user_t u;
    u.set_name("Olaf");
    return 0;
}

Answer 1

I hoped it'd be used automatically as v isn't needed after the assignment anymore. Is std::move required in this case?

Movement always must be explicitly stated for lvalues, unless they are being returned (by value) from a function.

This prevents accidentally moving something. Remember: movement is a destructive act; you don't want it to just happen.

Also, it would be strange if the semantics of name_ = v; changed based on whether this was the last line in a function. After all, this is perfectly legal code:

name_ = v;
v[0] = 5; //Assuming v has at least one character.

Why should the first line execute a copy sometimes and a move other times?

If so, I might as well use the non-C++11 swap.

You can do as you like, but std::move is more obvious as to the intent. We know what it means and what you're doing with it.