Pulling a single item from a C++ parameter pack by its index

Raymond Chen

Suppose you have a C++ parameter pack and you want to pluck out an item from it by index.

template<int index, typename...Args>
void example(Args&&... args)
{
    // how do I access the index'th args parameter?
}

One solution is to use std::tie:

template<int index, typename...Args>
void example(Args&&... args)
{
    auto& arg = std::get<index>(
        std::tie(args...));
}

We expand the parameter pack into the parameters of std::tie, which returns a tuple of lvalue references. We can then pull out the element at the specified index via std::get<0>().

The catch here is that std::tie always captures the references as lvalue references. If you want to preserve the reference category of the original parameters, you can use std::forward_as_tuple:

template<int index, typename...Args>
void example(Args&&... args)
{
    auto&& arg = std::get<index>(
        std::forward_as_tuple(
            std::forward<Args>(args)...));
}

This time, we apply std::forward to each of the elements of the parameter pack (to preserve the original reference category), then pass them all as parameters to std::forward_as_tuple. Unlike std::tie, std::forward_as_tuple preserves the reference categories of its arguments, so it produces a tuple of lvalue and rvalue references that match the original parameters. Again, we pull out the element via std::get<index>(), and capture it as an auto&& reference, which again preserves the reference category.

For future reference, you can capture the Args at a specific index by extracting it from the tuple, or more easily, getting it from the variable.

template<int index, typename...Args>
void example(Args&&... args)
{
    auto&& arg = std::get<index>(
        std::forward_as_tuple(
            std::forward<Args>(args)...));

    // The hard way
    using Arg = std::tuple_element_t<index,
        std::tuple<Args&&...>>;

    // The lazy way
    using Arg = decltype(arg);
}

Bonus chatter: There is a C++26 proposal to add core language support for pulling elements out of a parameter pack by index: Pack Indexing. Under the proposed syntax, the above code would be simplified to

template<int index, typename...Args>
void example(Args&&... args)
{
    // Extract as lvalue
    auto& arg = args...[index];
    using Arg = Args...[index];

    // Preserve reference category
    auto&& arg = (Args...[index]&&)args...[index];
    using Arg = Args...[index]&&;
}

1 comment

Discussion is closed. Login to edit/delete existing comments.

  • Markus Werle 0

    Could you please explain why the new language rules are designed in such a way that the cast is required to preserve reference category in

    auto&& arg = (Args...[index]&&)args...[index]; 

    I would prefer

    auto&& arg = args...[index];

    to have the same effect. See my godbolt experiments

Feedback usabilla icon