矢量C ++内存分配

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

Answer 2

I don''t know what to write there between brackets

Write nothing )) In this case you'll create an empty vector, wich could be grown with std::vector::push_back()

Update: Do not forget to remove empty () to avoid vexing parse

Answer 3

You can change the number of elements the vector contains, by inserting and/or removing elements. You are specifically looking for vector's methods insert , push_back / emplace_back , resize , pop_back , erase .

You'll find descriptions of the methods in any C++ reference (eg have a look here in the "Modifiers" section) and in the C++ beginner's book of your choice.