Memory de-allocation with std::vector re-assignment

Question

Suppose I have VectorA and VectorB are two std::vector<SameType>, both initilized (i mean VectorA.size() > 0 and VectorB.size() > 0)

If I do:

VectorA = VectorB;

the memory previosly allocated for VectorA is automatically freed?

Answer 1

It is freed in the sense that the destructors of all contained objects are called, and the vector no longer owns the memory.¹

But really, it's just returned to the allocator, which may or may not actually return it to the OS.

So long as there isn't a bug in the allocator being used, you haven't created a memory leak, if that's what your concern is.

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_{1. As @David points out in the comment below, the memory isn't necessarily deallocated, depending on whether the size needs to change or not.}

Answer 2

In general, not necessarily. When you assign one vector to the other, the post condition is that both arrays will contain equivalent objects at the end of the operation.

If the capacity of the destination vector is enough, the operation can be achieved by calling the assignment operator on the set of min( v1.size(), v2.size() ) elements, and then either destructing the rest of the elements if the destination vector held more elements, or else copy-constructing the extra elements at the end. In this case no memory release or allocation will be done.

If the destination vector does not have enough capacity, then it will create a new buffer with enough capacity and copy-construct the elements in the new buffer from the source vector. It will then swap the old and new buffers, destroy all old objects and release the old buffer. In this case, the old objects are destroyed and the old memory released, but this is just one case.