Iterators

Overview

Iterators are objects that behave like pointers. They traverse STL containers securely and connect containers to algorithms.

Syntax

cpp

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

// Standard iterator
std::vector<int>::iterator it;
for (it = v.begin(); it != v.end(); ++it) {
    cout << *it; // Dereference to get value
}

// C++11 auto (Much cleaner)
for (auto it = v.begin(); it != v.end(); ++it) {
    cout << *it;
}

Common Pitfalls

Erasing an element from a container while iterating through it. This invalidates the iterator and causes crashes. (Correct way: `it = vec.erase(it);`).

Interview Tips

`v.end()` does NOT point to the last element. It points to one past the last element (out of bounds). This is why loops run while `it != v.end()`.
Understand Iterator Invalidation: adding elements to a vector might cause it to reallocate memory to a new location, making all your old iterators point to garbage.

Real-World Example

Using iterators with maps.

example

cpp

#include <iostream>
#include <map>
using namespace std;

int main() {
    map<int, string> m = {{1, "A"}, {2, "B"}};
    
    // Iterating a map returns std::pair
    for (auto it = m.begin(); it != m.end(); ++it) {
        cout << "Key: " << it->first << " Val: " << it->second << endl;
    }
    return 0;
}

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