CODEWITHSUNDEEP
c++stdmovemove-semantics
jorgez
jorgez

Reputation: 41

failed to move contents with std::move

I'm trying to understand how std::move with a simple example (below). Basically I'm trying to move to contents of p1 to p2, so p1 is empty after that, which doesn't happen though.

I guess I'm not using std::move properly. I would appreciate a lot if anybody could explain that to me.

#include <iostream>
#include <utility>

int main()
{
    int * p1 = new int[10];
    for(int i = 0; i < 10; ++i)
        p1[i]=i;
    // moving contents of p1 to p2  
    int * p2 = std::move(p1);   
    // I was expeting p1 now to be empty but it's not...
    if(p1 != NULL)
        std::cout << "I'M NOT EMPTY\n";
    // prints I'M NOT EMPTY     
}

Upvotes: 2

Views: 670

Answers (3)

Craig Scott
Craig Scott

Reputation: 10137

Unfortunately, std::move doesn't do what people new to C++11 often expect. It is defined by the C++ standard simply as a static_cast, so on its own it doesn't actually modify the object it is given. There is often an expectation that std::move will somehow clear or zero out the object being moved from, but as explained here, that's not at all what the C++ standard requires.

Also, riv's answer is only partially correct. The guarantee he talks about is not really to do with std::move, it is more related to rvalue references and the guarantee is typically discussed in conjunction with move constructors and move assignment. If you want more background on std::move, rvalues, etc., I highly recommend Thomas Becker's discussion which is one of the internet's most referenced sites on the subject.

Upvotes: 1

wesley.mesquita
wesley.mesquita

Reputation: 785

Maybe this
Why does moving a pointer variable not set it to null? can clarify what´s going on in your code. As riv pointed out, there is no reason to std::move assign NULL to your pointer.

This reference http://en.cppreference.com/w/cpp/utility/move has good examples of undefined behaviors when using std::move.

Upvotes: 1

riv
riv

Reputation: 7324

The move function is not guaranteed to clear the contents of the source object. It is, however, guaranteed to be left in such a state that you can safely destroy it or assign a new value to it.

Upvotes: 2

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