resize(size() + N), resize(size() - N) should do the trick too, unless default constructing your elements is costly or not defined.

But I wonder what use case there is where you can't just push_back() one by one? The only potential issue here is that there might be a reallocation happening while append this range of N elements, but can't imagine what might break.

It's a perf leak, not a correctness issue, as with std::unordered_map. So yes, you can just push_back() each item, and as with std::unordered_map the worst case is linearly worse than the Right Thing™.

This is a Quality of Implementation issue in the library standard itself.

Ok - if the N elements you want to add would cause more than 1 reallocation by constant size factor, yes I can see that. (also I just saw that resize() has the same allocation behaviour as reserve() has, so scratch my suggestion above).

Never had a problem myself with that though, my main use case for std::vector is with reserve() right at the beginning and never provoking a reallocation (so I often end up coding up my own container to assert reallocation can't happen accidentally).

Sometimes (infrequently) I use the script-style push_back()-only pattern. But I feel very uneasy about uncontrolled reallocations in larger systems, and anyway they constrain the implementation space because of pointer and iterator invaliation.