std::strings's capacity(), reserve() & resize() functions

Question

I wan to use std::string simply to create a dynamic buffer and than iterate through it using an index. Is resize() the only function to actually allocate the buffer?

Answer 1

reserve(n) indeed allocates enough storage to hold at least n elements, but it doesn't actually fill the container with any elements. The string is still empty (has size 0), but you are guaranteed, that you can add (e.g. through push_back or insert) at least n elements before the string's internal buffer needs to be reallocated, whereas resize(n) really resizes the string to contain n elements (and deletes or adds new elements if neccessary).

So reserve is actually a mere optimization facility, when you know you are adding a bunch of elements to the container (e.g. in a push_back loop) and don't want it to reallocate the storage too often, which incurs memory allocation and copying costs. But it doesn't change the outside/client view of the string. It still stays empty (or keeps its current element count).

Likewise capacity returns the number of elements the string can hold until it needs to reallocate its internal storage, whereas size (and for string also length) returns the actual number of elements in the string.