#include <iostream>     // std::cout
#include <algorithm>    // std::remove_if
#include <set>

using namespace std;

int main () {

    set<int> numbers = {1,2,3,4,5,6};

    for(auto it = numbers.begin(); it != numbers.end();) {
        if ((*it) > 2) {
            numbers.erase(it);
        } else {
            ++it;
        }
    }

    return 0;
}