cin to function arguments without variable

You can still create wrapper function:

template <typename T>
T get_input(std::istream& cin)
{
    T res;
    std::cin >> res;
    return res;
}

And then:

const int len = get_input<int>(std::cin);
for (int i = 0; i < len; i++) {
    foo(get_input<int>(std::cin));
}