c++ - STL vector with reverse and pop/push_back cost -

i not come algorithm costs, here asking.

here vector initialized 1000 elements:

vector<unsigned int> mfreeindexes(1000); 

i continuously pop_back/push_back elements vector, never push_back on 1000 (so never force vector reallocate).

in case pop_back/push_back operations o(1) or o(n)?

from c++ standard 23.3.7.5:

void push_back(const t& x);

void push_back(t&& x);

remarks: causes reallocation if new size greater old capacity (...)

note doesn't can't reallocate in other scenario unusual implementation of standard. think can safely assume push_back won't reallocate when there's still capacity.

the thing pop_back bit more complicated. standard not reallocation in pop_back context. seems common implementation (with no know exception) pop_back not reallocate. there guarantees though, see this:

can pop_back() ever reduce capacity of vector? (c++)

anyway long don't go on predefined size safe assume no reallocation happens , complexity indeed o(1).