The vector Template Class

The vector template class is similar to the string class in that it is a dynamic array. You can set the size of a vector object during runtime, and you can append new data to the end or insert new data in the middle. Basically, it’s an alternative to using new to create a dynamic array. Actually, the vector class does use new and delete to manage memory, but it does so automatically.

At this time we won’t venture very deeply into what it means to be a template class. Instead, we’ll look at a few basic practical matters. First, to use a vector object, you need to include the vector header file. Second, the vector identifier is part of the std namespace, so you can use a using directive, a using declaration, or std::vector. Third, templates use a different syntax to indicate the type of data stored. Fourth, the vector class uses a different syntax to indicate the number of elements. Here are some examples:

#include <vector>
...
using namespace std;
vector<int> vi;        // create a zero-size array of int
int n;
cin >> n;
vector<double> vd(n);  // create an array of n doubles

We say that vi is an object of type vector<int> and that vd is an object of type vector<double>. Because vector objects resize automatically when you insert or add values to them, it’s okay for vi to start with 0 size. But for the resizing to work, you would use the various methods that are part of the vector package.

In general, the following declaration creates a vector object vt that can hold n_elem elements of type typeName:

vector<typeName> vt(n_elem);

The parameter n_elem can be an integer constant or an integer variable.