More about Iterators

As we have already known, iterators are useful to traverse elements in a STL container. However, there are also other types of iterators that are commonly used.

iterator

iterator is the normal type of iterators we have learned before. It supports forward traversal of containers. We can use the deference to access the element it points to, and modify it as well.

int main() {
    vector<int> v;
    for (int i = 0; i < 10; i++) {
        v.push_back(i);
    }
    vector<int>::iterator it;
    for (it = v.begin(); it != v.end(); ++it) {
        *it += 1;
        cout << *it << " "; // 1 2 3 4 5 6 7 8 9 10
    }
    return 0;
}

const_iterator

const_iterator supports forward traversal of containers. We can use the deference to access the element it points to, but cannot modify it, since the deference returns a const reference to the element.

int main() {
    vector<int> v;
    for (int i = 0; i < 10; i++) {
        v.push_back(i);
    }
    vector<int>::const_iterator it;
    for (it = v.begin(); it != v.end(); ++it) {
        *it += 1;   // ERROR
        cout << *it << " ";
    }
    return 0;
}

reverse_iterator

reverse_iterator supports backward traversal of containers. We can use the deference to access the element it points to, and modify it as well.

Instead of begin() and end() in normal iterators, we use rbegin() and rend() in reverse iterators. rbegin() returns a reverse_iterator to the last element in the container, while rend() returns a reverse_iterator to the one before the first element in the container.

int main() {
    vector<int> v;
    for (int i = 0; i < 10; i++) {
        v.push_back(i);
    }
    vector<int>::reverse_iterator it;
    for (it = v.rbegin(); it != v.rend(); ++it) {
        *it += 1;
        cout << *it << " "; // 10 9 8 7 6 5 4 3 2 1
    }
    return 0;
}

const_reverse_iterator

reverse_iterator supports backward traversal of containers. We can use the deference to access the element it points to, but cannot modify it, since the deference returns a const reference to the element.

int main() {
    vector<int> v;
    for (int i = 0; i < 10; i++) {
        v.push_back(i);
    }
    vector<int>::const_reverse_iterator it;
    for (it = v.rbegin(); it != v.rend(); ++it) {
        *it += 1;   // ERROR
        cout << *it << " ";
    }
    return 0;
}