An iterator is a generalised pointer: it points to an element, *it reads it, ++it moves on. Iterators are the glue that lets any algorithm run on any container.

Basics

C++

#include <iostream>
#include <iterator>
#include <vector>

int main() {
    std::vector<int> v{10, 20, 30, 40};
    for (auto it = v.begin(); it != v.end(); ++it) std::cout << *it << ' ';   // [begin, end)
    std::cout << '\n';
    for (auto it = v.rbegin(); it != v.rend(); ++it) std::cout << *it << ' '; // reverse
    std::cout << '\n';

    auto it = v.begin() + 2;                    // random access
    std::cout << *it << ' ' << it[1] << ' ' << (it - v.begin()) << '\n';
    std::cout << *std::next(v.begin()) << ' ' << *std::prev(v.end()) << ' ' << std::distance(v.begin(), v.end()) << '\n';

    for (auto cit = v.cbegin(); cit != v.cend(); ++cit) { /* *cit = 0; would not compile */ }
}

Output

10 20 30 40
40 30 20 10
30 40 2
20 40 4

Iterator categories

Category	Supports	Example containers
Input / Output	Single pass: `++`, `*` (read or write)	`istream_iterator` / `ostream_iterator`, `back_inserter`
Forward	Multi-pass `++`	`forward_list`, `unordered_*`
Bidirectional	+ `--`	`list`, `set`, `map`
Random access	+ `it + n`, `it[n]`, `<`, `it2 - it1`	`deque`
Contiguous (C++20)	+ elements adjacent in memory	`vector`, `array`, `string`, raw arrays

Algorithms state what they need: std::sort requires random access (so std::list has its own sort()), std::reverse needs bidirectional, std::find just input.

Iterator adaptors

C++

#include <algorithm>
#include <iostream>
#include <iterator>
#include <sstream>
#include <vector>

int main() {
    std::istringstream in("5 3 8 1");
    std::vector<int> v(std::istream_iterator<int>{in}, std::istream_iterator<int>{});   // read all ints

    std::vector<int> evens;
    std::copy_if(v.begin(), v.end(), std::back_inserter(evens), [](int x) { return x % 2 == 0; });

    std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, ","));   // print with separator
    std::cout << " evens=" << evens.size() << '\n';
}

Output

5,3,8,1, evens=1

Making your own class iterable

C++

#include <iostream>

class Range {                                   // like Python's range(lo, hi)
    int lo_, hi_;
public:
    Range(int lo, int hi) : lo_(lo), hi_(hi) {}
    struct iterator {
        int cur;
        int operator*() const { return cur; }
        iterator& operator++() { ++cur; return *this; }
        bool operator==(const iterator&) const = default;
    };
    iterator begin() const { return {lo_}; }
    iterator end() const { return {hi_}; }
};

int main() {
    for (int i : Range(3, 7)) std::cout << i << ' ';
    std::cout << '\n';
}

Output

3 4 5 6

Why does the STL use half-open ranges [begin, end)?