#include <cassert>
#include <cstddef>
#include <vector>

int main() {
    std::vector<int> values{10, 20, 30};
    const std::size_t index = 1;
    int* selected = nullptr;
    assert(selected == nullptr);
    values.reserve(100);
    values.push_back(40);
    selected = &values.at(index);
    assert(*selected == 20);
    *selected = 25;
    assert(values.at(index) == 25);
}