C++17: Keep only some members when tuple unpacking

Question

Let\'s imagine you need to call the following method:

std::tuple foo();

In C++17, you can call the function and unpack

Answer 1

You could write a helper function that only gives you back certain indices of a std::tuple:

template 
auto take_only(Tuple&& tuple) {
    using T = std::remove_reference_t;

    return std::tuple...>(
        std::get(std::forward(tuple))...);
}

auto [b, c] = take_only<1, 2>(foo());

Or drops the head or something:

template 
auto drop_head_impl(Tuple&& tuple, std::index_sequence<0, Is...> ) {
    return take_only(std::forward(tuple));
}

template 
auto drop_head(Tuple&& tuple) {
    return drop_head_impl(std::forward(tuple),
        std::make_index_sequence>>());
}

auto [b, c] = drop_head(foo());

But the above implementations almost certainly have some lifetime complexity issues that directly using structured bindings won't - since there isn't any lifetime extension here.

So just, do what Vittorio says:

auto [a, b, c] = foo();
(void)a;