#include <iostream>
#include <vector>

int main()
{
    std::vector<int> ages;
    ages.push_back(18);
    ages.push_back(19);
    ages.push_back(20);

    // type of iter would be std::vector<int>::iterator
    for (auto iter = ages.begin(); iter != ages.end(); ++iter) {
        (*iter)++; // OK
    }

    // type of iter would be std::vector<int>::const_iterator
    for (auto iter = ages.cbegin(); iter != ages.cend(); ++iter) {
        //(*iter)++; // NOT OK
    }

    // type of iter would be std::vector<int>::reverse_iterator
    for (auto iter = ages.rbegin(); iter != ages.rend(); ++iter) {
        std::cout << *iter << "\n"; // prints 20, 19, 18
    }

    // Can also use crbegin and crend
}