Question

I’m using std::vector to store an image in my Image class. I’m having a bit trouble understanding how they work. A function which rotates the image:

void Image :: resize (int width, int height)
{
    //the image in the object is "image"

    std::vector<uint8_t> vec;  //new vector to store rotated image

    // rotate "image" and store in "vec"

    image = vec; // copy "vec" to "image" (right?)

    //vec destructs itself on going out of scope
}

Is there any way to prevent the last copy? Like in Java, just by switching references? It would be nice if any copying is prevented.

Answer 1

You can use std::vector::swap:

image.swap(vec);

This is a essentially a pointer swap, the contents are transferred rather than copied. It is perfectly valid since you don’t care about the contents of vec after the swap.

In C++11 you can “move” the contents of vec into image:

image = std::move(vec);

This operation has essentially the same effect, except that the state of vec is less well defined (it in a self consistent state but you cannot make any assumptions about its contents… but you don’t care anyway because you know you are discarding it immediately).