C++ Iterators

🔁 C++ Iterators

Iterators in C++ act like pointers that allow you to traverse and access elements of STL containers (vector, list, map, set, etc.).
They form the bridge between containers and algorithms.

🔹 1. Why Iterators?

  • Traverse containers in a uniform way

  • Work with STL algorithms (sort, find, count, etc.)

  • Abstract container internals

  • Type-safe and flexible

🔹 2. Basic Iterator Syntax

container_type::iterator it;

Example:

vector<int>::iterator it;

Modern way:

auto it = v.begin();

🔹 3. Common Iterator Functions

FunctionDescription
begin()Points to first element
end()Points after last element
rbegin()Reverse begin
rend()Reverse end

🔹 4. Iterating a Container

Vector Example

vector<int> v = {10, 20, 30};

for (auto it = v.begin(); it != v.end(); ++it) {
cout << *it << " ";
}

Reverse Iteration

for (auto it = v.rbegin(); it != v.rend(); ++it) {
cout << *it << " ";
}

🔹 5. Iterators with map

map<int, string> m = {{1, "A"}, {2, "B"}};

for (auto it = m.begin(); it != m.end(); ++it) {
cout << it->first << " " << it->second << endl;
}

🔹 6. Types of Iterators

1️⃣ Input Iterator

  • Read only

  • One-direction

2️⃣ Output Iterator

  • Write only

3️⃣ Forward Iterator

  • Read & write

  • One-direction

4️⃣ Bidirectional Iterator

  • Forward + backward

  • (list, set, map)

5️⃣ Random Access Iterator

  • Jump to any position

  • (vector, deque, array)

🔹 7. const_iterator

Prevents modification of elements.

vector<int>::const_iterator it;

for (it = v.cbegin(); it != v.cend(); ++it) {
cout << *it;
}

🔹 8. Iterator Arithmetic

✔ Only for random-access iterators:

auto it = v.begin();
it += 2; // valid for vector

❌ Invalid for list or set

🔹 9. Iterator with Algorithms

#include <algorithm>

auto it = find(v.begin(), v.end(), 20);
if (it != v.end()) {
cout << "Found";
}

🔹 10. Invalidated Iterators (Important!)

Some operations invalidate iterators.

vector<int> v = {1, 2, 3};
auto it = v.begin();
v.push_back(4); // it may become invalid

✔ Safer with list

🔹 11. Range-based Loop vs Iterators

for (int x : v) {
cout << x;
}

✔ Simpler
❌ Less control than iterators

❌ Common Mistakes

auto it = v.end();
cout << *it; // ❌ invalid (end() points past last)

📌 Summary

  • Iterators act like pointers

  • Used to traverse STL containers

  • Required for STL algorithms

  • Different containers support different iterator types

  • Be careful with iterator invalidation

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