Need to call std::vector.clear() before assigning using operator=?

Question

I profiled my code recently with Visual Leak Detector for the first time, and it indicated a leak in a vector, which I wasn't expecting. The code is like so:

void func()
{
    std::vector<MsgUnit> msgVec;

    do
    {
        // msgVec.clear(); // do I need to do this to avoid a leak?
        msgVec = m_obj->returnMsgUnitVector();
    }
    while (someConditionNotMet);

    // process msgVec

    return;
}

MsgUnit has a copy constructor and destructor.

I haven't found the time to do in-depth testing, but a quick fix seems to indicate that uncommenting the clear() method removes the leak.

I'm wondering what the standard says about this behaviour. Do I need to clear the vector before assigning to it, to avoid a leak?

Answer 1

No, assignment will make the destination vector value-equivalent to the source vector. It will internally do what it needs to ensure this without leaking. [Assuming that the copy-constructor, assignment and destructors of your type don't leak]

Answer 2

MsgUnit has a copy constructor and destructor.

I guess it doesn't have an copy-assignment operator (per the Rule of Three), which is why you get leaks or worse when you reassign them. The implicitly-generated operator will simply assign each class member. If some of those members are dumb pointers to manually-managed resources, then you'll lose those pointers and leak the resources.

Either implement that or, better still, redesign MsgUnit to use a smart pointer (or similar) to manage its dynamic resources automatically with no need for you to mess around with destructors and the like.