Dynamic Arrays: std::vector and STL

In Plain English

Fixed arrays are restrictive because you must know their exact size in advance. The Standard Template Library (STL) provides std::vector—a dynamic array that automatically grows and shrinks as you add or remove elements.

The Mental Model

"If a fixed array is an unexpandable wooden egg carton with 6 slots, a std::vector is an accordion suitcase that automatically expands whenever you pack more items into it."

Deep Dive: How It Works

#include <vector>: Required header.

Declaration: std::vector<int> scores; (an empty vector of integers) or std::vector<std::string> names = {"Alex", "Ada"};.

.push_back(val): Appends an element to the end of the vector and expands memory automatically.

.size(): Returns the current number of elements.

.pop_back(): Removes the last element.

Range-based for loop: for (int x : vec) { std::cout << x; } gives clean, index-free iteration.

Syntax Blueprint

#include <vector>

std::vector<int> v;
v.push_back(10); // Adds 10
v.push_back(20); // Adds 20
for (int x : v) {
  std::cout << x << " ";
}

Dynamic resizable vector with push_back and range-based for loop.

Core Rules to Remember

Automatic Resizing: Grows dynamically as elements are added with push_back().

The Modern C++ Standard: Preferred over raw C-style arrays in 95% of modern C++ code.

Range-Based Loops: Use for (const auto &item : vec) for clean, high-performance iteration.

Common Beginner Traps & How to Fix Them

Mistake:Using [] on an empty vector: vec[0] = 5;

Why it happens: [] modifies an existing index, but cannot add a new slot to an empty vector.

How to fix: Use vec.push_back(5) to add new elements.

Pro Tip: If you know how many elements you'll insert, calling vec.reserve(1000) prevents intermediate reallocations!

Live Interactive Example

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Managing a Dynamic Inventory with std::vector

C++ 20

Output Console

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How it works: std::vector handles dynamic memory allocation and resizing behind the scenes.

Your Turn: Micro Challenge

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Micro Exercise

Build a numbers vector

Declare std::vector<int> nums;
Push 100 and 200 using push_back().
Print "Vector size: 2".

Sandbox Output

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