Declaring vectors

Learn to create dynamic arrays with std::vector.

What is std::vector?

std::vector is the dynamic-size array container of the C++ standard library. Unlike C-style arrays, its size can grow and shrink at runtime; it expands automatically as you add elements with push_back.

To use it you must add the #include <vector> header. The declaration looks like std::vector<int> nums;: the type goes inside angle brackets, and you do not have to give a size. Creating an empty vector and filling it later is a common pattern.

Empty vector and initial values

You can declare a vector in two common ways. The first is to start empty: std::vector<int> nums; — here nums is empty, with size zero. The second is with initial values: std::vector<int> nums = {10, 20, 30}; — the braces produce a three-element vector.

Whether you start empty or filled, the size can change later. Starting empty and filling with push_back is a common technique when working with user data.

Exercise Declare an empty vector and print its size

Declare an empty std::vector<int> nums; and print its size with size() using std::cout.

Show example solution

#include <iostream>
#include <vector>

int main() {
    std::vector<int> nums;
    std::cout << nums.size() << std::endl;
    return 0;
}

Test yourself

Which header must be added to use std::vector?
```
#include <???>
```
- <iostream>
- <string>
- <array>
- <vector>
Show answer

<vector> — the <vector> header brings in the std::vector template.

Initialize the vector

std::vector<int> nums ____ {10, 20, 30};

Show answer

std::vector<int> nums = {10, 20, 30};

Match the vector parts

std::vector

Container name
<int>

Element type
nums

Vector name
{10, 20, 30}

Initial values

Next lesson: push_back

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