In the C++ standard vector.capacity section, it defines two overloads for resize(). (see also https://en.cppreference.com/w/cpp/container/vector/resize)
This overload requires that the type T is MoveInsertable and DefaultInsertable:
constexpr void resize(size_type sz);
The other overload requires that the type T is CopyInsertable:
constexpr void resize(size_type sz, const T& c);
My question is that why doesn't the second overload try to move from the existing values from the old vector, and further copy-insert new values from the supplied c argument. Wouldn't that be more efficient?
I agree that the second parameter could be an rvalue reference - this saves one copy, when it is subsequently copied from the first newly appended item. I assume what you have in mind is something like this:
std::vector<LargeObject> v;
// ...
LargeObject obj = setupLotsOfResources();
// Now do 1 move and 9 copies instead of 10 copies
v.resize(10, std::move(obj));
However, I would consider this an edge case, and working with rvalue references that are used for 1 move construction and N-1 copies is quite a confusing API. As you are free to use the std::vector API with what is already there such that for the above example, you will have 1 move and N-1 copies, I believe the rationale behind the existing function signature is ease of use and a straightforward signature that doesn't require much studying of the specs to understand what it does.