STL Vector Class

Dr. Mark J. Sebern

Version 1.5 (12/14/1999)

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The vector container resembles a C++ array in that it holds zero or more objects of the same type, and that each of these objects can be accessed individually. (Be sure to study the requirements for objects that are stored in STL containers.) The vector container is defined as a template class, meaning that it can be customized to hold objects of any type. Here is a simple example:

#include <vector>
using namespace std;
// Define a vector of integers.
// The template name is "vector" and the type of object
// it contains is "int"; the fully specified container
// data type is "vector<int>".
vector<int> vec_one;

int a = 2;     // Some integer data objects
int b = -5;

vec_one.push_back(a); // Add item at end of vector

// vec_one now contains three int values: 2, 9, -5

unsigned int indx;
for (indx = 0; indx < vec_one.size(); indx++)
  cout << vec_one[indx] << endl; Write out vector item

Member functions

Some commonly used member functions of the vector class are:

size size_type size() const;
Returns the number of items (elements) currently stored in the vector. The size_type type is an unsigned integral value.
empty bool empty() const;
Returns a true value if the number of elements is zero, false otherwise.
push_back void push_back(const T& x);
Adds the element x at the end of the vector. (T is the data type of the vector's elements.)
begin iterator begin();
const_iterator begin() const;

Returns an iterator (a special kind of object) that references the beginning of the vector. Although the iterator can be used for many things, for now we will just consider its use with erase and sort.
end iterator end();
const_iterator end() const;
Returns an iterator (a special kind of object) that references a position past the end of the vector. Like begin(), we will just consider its use with erase and sort.
erase void erase(iterator first, iterator last);
Erase (remove) elements from a vector. For now, we will consider only the case of removing all elements from a vector (see clear() for an alternate way to do the same thing):
vector<int> a;
a.erase(a.begin(),a.end()); // Remove all elements. 
clear void clear ();
Erase all elements from a vector.

vector<int> a;
a.clear(); // Remove all elements.


Some of the operators defined for the vector container are:

= The assignment operator replaces the target vector's contents with that of the source vector:
vector<int> a;
vector<int> b;



b = a;
// The vector b now contains two elements: 5, 10 
== Tests whether two vectors have the same content (element-by-element comparison for all elements).
[] The subscript operator returns a reference to an element of the vector. A subscript value of zero returns a reference to the first element, and so on. The subscript must be between zero and size()-1. See the example above to see how the subscript operator is used in a loop to access elements of a vector. The subscripted vector may appear on the left or right sides of an assignment (the returned reference is an lvalue):
vector<double> vec;

vec[1] = vec[0] + 5.0;
vec[0] = 2.7; // Vector now has two elements: 2.7, 6.2 

To sort a vector, see the STL sort algorithm.

This page was last updated on December 14, 1999; send comments to Mark Sebern.