STL Iterators

An iterator identifies a position in a range. A half-open range [first, last) includes first and excludes last, so an empty range has first == last. Dereferencing the end iterator is invalid.

#include <iostream>
#include <vector>

std::vector<int> values{2, 4, 6};
for (auto it = values.cbegin(); it != values.cend(); ++it) {
    std::cout << *it << '\n';
}

Use begin/end for mutable traversal and cbegin/cend for read-only traversal. Range-for expands to iterator operations and is clearer when positions are unneeded.

Capabilities and adapters

Input iterators support single-pass reading, output iterators writing, forward iterators multipass traversal, bidirectional iterators decrement, and random-access iterators support arithmetic. Contiguous iterators additionally guarantee adjacent storage. Algorithms require only the weakest category they need.

std::back_inserter turns assignments into push_back; reverse iterators traverse backward; std::next, std::advance, and std::distance work across categories, though cost varies. C++20 ranges may use different iterator and sentinel types for an endpoint.

Iterators are non-owning and inherit container lifetime rules. Mutation can invalidate them: vector growth moves storage, erase invalidates defined regions, and unordered rehash changes bucket structure. Capture the iterator returned by erase when deleting during traversal. Never compare iterators from unrelated containers.