Vector C++ Memory Allocation

Question

I want to create a vector of elements representing a certain structure.

The thing is that I don't know how many elements the structure will have, since the number will change very often, and I don't really know how to create a vector.

How to make that?

In order to make it more clear:

I saw that when creating a vector, you do something like this:

std::vector<structureType> vectorName(nrOfElements);

I don't know the number of elements and what to write there, between brackets.

Answer 1

If you default construct the vector, you get an empty one:

std::vector<structureType> vectorName; // holds 0 elements

then you can push elements into the vector, increasing its size (see also other vector modifiers):

vectorName.push_back(someStructureTypeInstance);

This might suit your needs. If you are worried about future memory re-allocations, you can use std::vector::reserve after constructing the vector.

std::vector<structureType> vectorName; // holds 0 elements
vectorName.reserve(100); // still 0 elements, but capacity for 100