具有反向和pop / push_back成本的STL向量

Question

I am not exactly good to come up with algorithm costs, so here I am asking.

Here is a vector initially initialized with 1000 elements:

vector<unsigned int> mFreeIndexes(1000);

I will continuously pop_back/push_back elements to the vector, but never push_back over 1000 (so never force vector to reallocate).

In this case will the pop_back/push_back operations be O(1) or O(n)?

Answer 1

From the C++ standard 23.3.7.5:

void push_back(const T& x);

void push_back(T&& x);

Remarks: Causes reallocation if the new size is greater than the old capacity (...)

Note that it doesn't say that it can't reallocate in the other scenario but this would be a very unusual implementation of the standard. I think you can safely assume that push_back won't reallocate when there's still capacity.

The thing with pop_back is a bit more complicated. The standard does not say anything about reallocation in the pop_back context. But it seems to be a common implementation (with no know exception) that pop_back does not reallocate. There are some guarantees though, see this:

Can pop_back() ever reduce the capacity of a vector? (C++)

Anyway as long as you don't go over predefined size you are safe to assume that no reallocation happens and the complexity is indeed O(1).