#include <vector>
#include <string>
#include <iostream>
#include <algorithm>
 
int main() {
  	const std::string str { "123456789" };
    std::vector<char> sub;
    auto iter = std::begin(str);
    // use advance to make this clear when you read.
    std::advance(iter, 3);
    // from where the advance left the iterator, and go fw 4 positions.
    std::copy_n(iter, 4, std::back_inserter(sub));
    for (auto c: sub) {
        std::cout << c << "\n";
    }
}

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