Arrays & Vectors

C++ inherits C's fixed-size array, and it comes with the same limitation: once you declare its size, it can never grow. The standard library's std::vector fixes that — it's an array that resizes itself as you add and remove elements — and in real C++ code, it's what you reach for almost every time.

A C-style array

The syntax is identical to C: a type, a name, and a size in square brackets:

#include <iostream>

int main() {
    int scores[4] = {88, 92, 79, 95};

    for (int i = 0; i < 4; i++) {
        std::cout << "scores[" << i << "] = " << scores[i] << std::endl;
    }
    return 0;
}

scores[0] = 88
scores[1] = 92
scores[2] = 79
scores[3] = 95

That works, but scores can never hold more than 4 elements, and like in C, C++ won't stop you from reading or writing past the end of it — scores[10] compiles and silently touches memory it has no business touching.

std::vector: an array that grows

std::vector lives in the <vector> header. You create one, and .push_back() appends a new element, resizing automatically as needed:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> scores = {88, 92, 79};
    scores.push_back(95);
    scores.push_back(100);

    std::cout << "Count: " << scores.size() << std::endl;
    std::cout << "First: " << scores[0] << std::endl;
    std::cout << "Last: " << scores[scores.size() - 1] << std::endl;
    return 0;
}

Count: 5
First: 88
Last: 100

scores started with 3 elements, and each push_back() call grew it — no size to declare up front, no risk of running out of room. .size() always reports the current element count, so scores[scores.size() - 1] is a reliable way to reach the last element regardless of how many push_back() calls happened before it.

Iterating with range-based for

Vectors work naturally with the range-based for loop from the previous lesson:

#include <iostream>
#include <vector>

int main() {
    std::vector<std::string> fruits = {"apple", "banana", "cherry"};

    for (const std::string& fruit : fruits) {
        std::cout << "- " << fruit << std::endl;
    }
    return 0;
}

- apple
- banana
- cherry

const std::string& fruit gives each loop iteration a reference to the element instead of a fresh copy of it — for a vector of strings or other larger objects, copying every element on every iteration would be wasteful, and const makes clear the loop won't modify the original vector.