reading vector

template <class Is>
std::vector<int> read(Is&& is) {
    int n = 0;
    is >> n;
    std::vector<int> v(n);
    for (auto& e : v) {
        is >> e;
    }
    return v;
}

void foo() {
    const auto v = read(std::cin);
//  const auto v = read(std::ifstream("input.txt"));
}