Vector Basics: A Sequence That Can Change Size

A vector is an indexed sequence whose size can change

A vector stores elements in order and lets you access them by index, like an array. Unlike a fixed array length, a vector's current number of elements can change while your program runs. To use vector, include its standard library header, then write the element type inside angle brackets.

#include <vector>

std::vector<int> values = {4, 2, 7};

std::cout << values.size();  // 3

Here, values has the type vector<int>, so every element is an int. The braces initialize its three current elements in order: 4 at index 0, 2 at index 1, and 7 at index 2. size() returns the number of current elements, so values.size() is 3. It does not return the largest valid index.

Only current elements have valid indices

You can read or replace an element at an index that already exists. In the running vector, values[1] reads 2 because index 1 holds the second element. Assigning 6 to that same index replaces 2, giving [4, 6, 7]. The vector still has three elements, so its size has not changed.

std::vector<int> values = {4, 2, 7};

int middle = values[1];  // middle is 2
values[1] = 6;           // values is now [4, 6, 7]

std::cout << values.size();  // 3

When values.size() is 3, the valid indices are 0, 1, and 2. Index 3 is the next position outside the vector, not an empty fourth element waiting to be filled. Writing values[3] = 1 does not append anything. Through operator[], that out-of-range access causes undefined behavior, so the program cannot safely rely on what happens.

values[3] = 1;

push_back and pop_back change which indices exist

Use push_back when you want to create a new last element. Starting with [4, 6, 7], values.push_back(1) changes the vector to [4, 6, 7, 1]. Its size becomes 4, and index 3 now refers to the new 1. The operation changes both the contents and the set of valid indices.

std::vector<int> values = {4, 2, 7};
values[1] = 6;                 // [4, 6, 7], size 3

values.push_back(1);           // [4, 6, 7, 1], size 4
std::cout << values.size();

values.pop_back();              // [4, 6, 7], size 3
std::cout << values.size();

Use pop_back to remove the current last element. It removes 1 from [4, 6, 7, 1], returning the vector to [4, 6, 7] and reducing the size from 4 to 3. pop_back does not return the removed value. After removal, index 3 is outside the vector again. Calling pop_back on an empty vector causes undefined behavior.