When does vector::push_back increase capacity?

Question

I'm using a bunch of std::vectors by setting their capacity at the beginning and using push_back to slowly fill them up. Most of these vectors will have the same size (16 elements), although some might get larger. If I use push_back 16 times on a vector with size 0 and capacity 16 initially, can I be sure that capacity will be exactly 16 after the push_backs?

Answer 1

Yes -- once you reserve a specific capacity, the vector will not be reallocated until you exceed the capacity you've set¹. Exactly how many more items you may be able to push without reallocation isn't specified, but you are guaranteed at least that many.

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1. In particular, pointers and iterators into the vector are guaranteed to remain valid until you exceed the specified capacity.

Answer 2

23.3.6.5 [vector modifiers]

void push_back(const T& x);
void push_back(T&& x);

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

Pretty much self-explanatory.