performance - 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 like to do just once

vector<struct> myvector;
myvector.reserve(5000);

However, to fill the vector again, I have to clear the vector first without altering its capacity. So usually I call myvector.clear();

This is a O(n) operation. Is there something simple I can do to increase the performance of this or is this about the best that it will get?

question from:https://stackoverflow.com/questions/16420357/c-fastest-way-to-clear-or-erase-a-vector

Answer 1

If your struct has a non-trivial destructor, then that needs to be called for all the elements of the vector regardless of how it is emptied. If your struct only has a trivial destructor, the compiler or the standard library implementation is allowed to optimize away the destruction process and give you a O(1) operation.