CODEWITHSUNDEEP
c++templatesc++11metaprogrammingperfect-forwarding
Vincent
Vincent

Reputation: 60381

Forward an element of a tuple

Consider the following code:

template <unsigned int Index, class Tuple, class Type = /* Something1 */>
Type forward(Tuple&& tuple)
{
    return /* Something2 */;
}

I would like to find what is the correct syntax of /* Something1 */ and /* Something2 */ in order to forward the element specified by Index of the input tuple. I know that other conceptions/syntax would be available, but in order to understand correctly how things work in such a context (consider this as an academic exercice), I would like an answer that satisfy the following conditions:

For example, if Tuple = std::tuple<char, int, double>, then forward<2>(tuple) should forward the third element (if the tuple is passed as a const reference, then the function should return a const reference to the double).

Upvotes: 1

Views: 834

Answers (1)

nosid
nosid

Reputation: 50044

The answer depends on what you want to achieve with that function. In general it is a bad idea to propagate rvalue-ness to members. However, there is already an implementation in the standard library. You can use std::get in the following way:

template <
    std::size_t Index,
    typename Tuple,
    typename Type = decltype(std::get<Index>(std::declval<Tuple>()))>
auto forward(Tuple&& tuple) -> Type
{
    return std::get<Index>(std::forward<Tuple>(tuple));
}

Upvotes: 1

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