Iterators & Iterator Categories

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Read: ~30 min 14 practice problems

1. What Is an Iterator?

An iterator is a generalized pointer. It supports *, ++, and (depending on category) more.

iter.cpp

#include <iostream>
#include <vector>
#include <list>

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

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

    // Same pattern works for std::list
    std::list<int> L = {1, 2, 3};
    for (auto it = L.begin(); it != L.end(); ++it)
        std::cout << *it << " ";
}

2. The Six Categories

Category	Operations	Example
Input	`++ * ==` (read-once)	`std::istream_iterator`
Output	`++ *` (write-once)	`std::ostream_iterator`
Forward	Input + multi-pass	`std::forward_list`
Bidirectional	Forward + `--`	`std::list`, `std::map`
Random Access	Bidirectional + `+ - [i]`	`std::vector`, `std::deque`
Contiguous (C++20)	Random + memory contiguous	`std::vector`, raw arrays

3. `begin()`, `end()` & Reverse Iterators

reverse.cpp

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {1, 2, 3, 4, 5};

    // reverse iteration
    for (auto it = v.rbegin(); it != v.rend(); ++it)
        std::cout << *it << " ";
    std::cout << "\n";

    // const iteration
    for (auto it = v.cbegin(); it != v.cend(); ++it)
        std::cout << *it << " ";
}

4. Iterator Invalidation

Container	What invalidates
vector	Reallocation invalidates all; insert/erase invalidates from that point on
deque	Insert at middle invalidates everything; at ends, only iterators
list / forward_list	Only invalidates the erased iterator
map / set	Only the erased iterator
unordered_map / set	Rehashing invalidates iterators (not references)

Common bug Iterating with one iterator while modifying the container with another. Always check invalidation rules.

5. Common Iterator Operations

std::next(it, n) / std::prev(it, n)
std::distance(a, b) — number of steps from a to b
std::advance(it, n) — move iterator

6. Practice Problems

Iterate a vector with iterators (not range-for).

✨ Show Answer
See section 1.
Why does v.end() point past the last element?

✨ Show Answer
Half-open range convention [begin, end). Lets end - begin equal size, and lets empty ranges be begin == end.
Print elements in reverse using rbegin/rend.

✨ Show Answer
See section 3.
Why use cbegin/cend?

✨ Show Answer
Returns a const_iterator regardless of the variable's constness. Useful for safety in templates.
What category is std::list iterator?

✨ Show Answer
Bidirectional. ++ and -- work, but no random it + 5.
Why does std::sort require random-access iterators?

✨ Show Answer
Quicksort partitions need to swap elements at arbitrary positions in O(1). Lists can't do this efficiently — they have list::sort instead.
Compute distance between two iterators.
✨ Show Answer
```
auto n = std::distance(it1, it2);
```

Advance an iterator by 3.

✨ Show Answer

std::advance(it, 3); // or auto j = std::next(it, 3);

Erase elements while iterating safely.

✨ Show Answer

for (auto it = v.begin(); it != v.end(); ) {
    if (*it < 0) it = v.erase(it);   // erase returns next valid
    else            ++it;
}

Use ostream_iterator to print.

✨ Show Answer

std::copy(v.begin(), v.end(),
    std::ostream_iterator<int>(std::cout, " "));

When does push_back invalidate vector iterators?

✨ Show Answer
When it triggers a reallocation (size exceeds capacity). All previous iterators are invalidated.
Build a list iterator iterating from end.
✨ Show Answer
```
for (auto it = L.rbegin(); it != L.rend(); ++it) ...
```
What's a const_iterator vs iterator?

✨ Show Answer
const_iterator gives read-only access (*it is const). Use it when you don't need to modify.
Why does v.begin() + 1 work but not on list?

✨ Show Answer
Vector iterators are random-access; arithmetic is O(1). List iterators are bidirectional; you can only ++/-- in O(1).

Summary

Iterators are the bridge between containers and algorithms. Six categories define what operations they support. Use range-for for simple iteration; use raw iterators when you need to erase, insert, or pass to STL algorithms. Always know your iterator invalidation rules — they cause subtle bugs.

Next Module → Lambda Expressions & std::function.

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1. What Is an Iterator?

2. The Six Categories

3. begin(), end() & Reverse Iterators

4. Iterator Invalidation

5. Common Iterator Operations

6. Practice Problems

Summary

3. `begin()`, `end()` & Reverse Iterators