C++ fastest way to clear or erase a vector

Question

I have a code where I routinely fill a vector with between 0 and 5000 elements. I know the maximum never exceeds 5000. Instead of initializing vector multiple times, I would

Answer 1

The cost of clear() depends greately on what the stored objects are, and in particular whether they have a trivial destructor. If the type does not have a trivial destructor, then the call must destroy all stored objects and it is in fact an O(n) operation, but you cannot really do anything better.

Now, if the stored elements have trivial destructors, then the implementation can optimize the cost away and clear() becomes a cheap O(1) operation (just resetting the size --end pointer).

Remember that to understand asymptotic complexity you need to know what it talks about. In the case of clear() it represents the number of destructors called, but if the cost (hidden) is 0, then the operation is a no-op.