Iterators in C++

An iterator is an object that points to an element of a container and can move to the next one. Every STL container provides iterators, and every STL algorithm works with them – which is what lets one std::sort or std::find work with any container. Think of them as generalised pointers.

begin, end and half-open ranges

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v{10, 20, 30, 40};
    for (auto it = v.begin(); it != v.end(); ++it) {    // what range-for does for you
        *it += 1;                                        // * gives the element
        std::cout << *it << ' ';
    }
    std::cout << '\n';

    auto it = v.begin() + 2;                             // jump (vector iterators are random access)
    std::cout << "third: " << *it << ", index " << it - v.begin() << '\n';
    std::cout << "elements: " << v.end() - v.begin() << '\n';
    return 0;
}

11 21 31 41 
third: 31, index 2
elements: 4

Half-open ranges make an empty range simply begin == end, and the size end − begin. Every algorithm takes ranges this way: std::sort(v.begin(), v.end()), or just part of a container: std::sort(v.begin(), v.begin() + 3).

Iterators with algorithms

#include <algorithm>
#include <iostream>
#include <list>
#include <string>
#include <vector>

int main() {
    std::vector<std::string> cities{"Pune", "Mumbai", "Delhi", "Kochi"};
    auto it = std::find(cities.begin(), cities.end(), "Kochi");
    if (it != cities.end()) std::cout << "found at index " << it - cities.begin() << '\n';

    std::list<int> scores{72, 95, 41, 88};                 // a different container, same algorithms
    auto best = std::max_element(scores.begin(), scores.end());
    std::cout << "best " << *best << ", count above 70: "
              << std::count_if(scores.begin(), scores.end(), [](int s) { return s > 70; }) << '\n';

    std::sort(cities.begin() + 1, cities.end());           // sort only part of the range
    for (const auto& c : cities) std::cout << c << ' ';
    std::cout << '\n';
    return 0;
}

found at index 3
best 95, count above 70: 3
Pune Delhi Kochi Mumbai 

Reverse and const iterators

#include <iostream>
#include <string>
#include <vector>

int main() {
    std::vector<int> v{1, 2, 3, 4, 5};
    for (auto it = v.rbegin(); it != v.rend(); ++it) std::cout << *it << ' ';   // backwards
    std::cout << '\n';

    for (auto it = v.cbegin(); it != v.cend(); ++it) {
        // *it = 0;                // error: cbegin gives a const_iterator (read-only)
        std::cout << *it * 10 << ' ';
    }
    std::cout << '\n';

    std::string s = "stressed";
    std::cout << std::string(s.rbegin(), s.rend()) << '\n';    // build a string from a reversed range
    return 0;
}

5 4 3 2 1 
10 20 30 40 50 
desserts

Iterator categories

Algorithms state what they need. std::sort needs random-access iterators, so it doesn't work on a std::list – which has its own list.sort() member instead. std::next(it, n) and std::distance(a, b) work with any category.

Practice

Summary