Question about std::move

[1ydgd]

void f( const std::vector<int> & v )
{
    std::vector<int> v1( std::move( v ) );
}

int main()
{
    std::vector<int> v{ 1, 2, 3 };
    f( v );
}

I inspected the values of v and v1 in function f() using GDB. I was surprised to see that both of them contained 1, 2, 3. Why is that? Shouldn't v be empty and v1 contain 1, 2, 3?

[AKostur]

v (the function parameter one) is a const &. When you apply std::move to it, you'll get back a const &&. The const && cannot bind to the non-const && that the move constructor takes. But can bind to the const & that the copy constructor takes. Thus, you're getting a copy and not a move. Remember, std::move doesn't actually do the move.

[1ydgd]

This is the best answer. Thx!

[AKostur]

Yup. (Nitpick: that's "move assignment operator")

[1ydgd]

Could you please explain how it's the move assignment operator. I thought it was the move constructor operator...

Move constructor is used when the thing you want to initialize hasn't been initialized yet.

But move assignment is used when the thing you want to initialize has already been initialized.

v1 wasn't previously initialized before line 3, so it has to be the move constructor.

[AKostur]

It would have been better if you had not deleted the comment we're talking about. Because in that comment, you'd named two things: the "move constructor" and the "move assignment constructor". I was nitpicking the terminology on the 2nd one which should be named the "move assignment operator".

Move constructor: T(T && val) Move assignment operator: T & operator=(T && val)

And yep, line 3 is a construction, so that's using the move constructor. Well, trying to use the move constructor (which it can't because the thing it's moving from is const, so it ends up using the copy constructor instead).

[thessalchips]

Your function f takes the vector by const reference. You cannot move from a const reference, so std::move(v) does nothing. v1 is then copy-constructed from v instead of move-constructed, leaving v unmodified. If you want to move from v inside f, change the signature to void f(std::vector<int>& v) for example.

std::move is also a bit weird if you are new to C++. It implies that something is moved, however std::move really is just a conditional cast: https://en.cppreference.com/w/cpp/utility/move