if (sz > size())
    insert(end(), sz-size(), c);
else if (sz < size())
    erase(begin()+sz, end());
else
    ; //do nothing

void clear()
{
    erase(begin(), end());
}

iterator erase(iterator position);
iterator erase(iterator first, iterator last);

template<typename C> void shrinkContainer(C &container) {
    if (container.size() != container.capacity()) {
        C tmp = container;
        swap(container, tmp);
    }
    //container.size() == container.capacity()
}