Rather than providing an overload of std::apply, I think it would make more
sense to provide implementations of std::get and std::tuple_size for
std::integer_sequence, considering an integer sequence is also an
array-like "collection" that can be iterated through at compile-time.

This has the advantage of not only ending up with the same syntax as you
mentioned, but also it "future-proofs" std::integer_sequence and its
relatives, in case a new function is added to the STL.

On Sat, 13 Jul 2019, 22:55 Jordi Vilar via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

> Hi all,
>
> I would like to propose a very simple addition to the standard library in
> order to simplify variadic parameter pack expansion, and before writing a
> formal proposal I just wanted to share it in order to get feedback.
>
> Typically, expanding indices sequences require writing a dedicated helper
> wrapper. This is tedious and offers no value at all. A Search through the
> Internet shows that similar approaches have been informally proposed, for
> instance, in [stack overflow](
> https://stackoverflow.com/questions/47210956/c17-multiple-parameter-pack-expansion
> )
>
> The proposal is about having a generic, reusable wrapper for expanding
> integer sequences similar to the std::apply facility for expanding tuples,
> considering the following:
> - simplify the common case of expanding an index sequence with a provided
> lambda
> - expansion must be usable as literal values to enable template
> instantiation (for instance, can be used to call std::get<index>(x)...)
> - support for non-sequential index sequences, and integral types other
> than std::size_t
> - syntax similar to std::apply and std::visit
> - the name could be just std::apply as a new overload, as this is
> conceptually the same as expanding a tuple but for integral constants
>
> The proposal is then:
>
> /// Invoke the Callable object function with a sequence of indices
> /// \param[in] function A callable object that will be invoked with
> std::integral_constant arguments instantiated for each index in the \a
> sequence integer_sequence
> /// \param[in] sequence A std::integer_sequence to be expanded
> /// \return The \a apply helper function returns whatever the provided
> callable object returns
> namespace std
> {
> template<typename F, typename T, T... indices>
> constexpr decltype(auto) apply(F&& function, [[maybe_unused]]
> std::integer_sequence<T, indices...> sequence)
> {
> return function(std::integral_constant<T, indices>{}...);
> }
> }
>
> The idea of having arguments encoded as std::integral_constant allows for
> using them not only as constexpr but also as template arguments.
>
> This simple helper function could allow us to write, for instance:
>
> /// performs the addition of two arrays of arithmetic types
> template<typename T, std::size_t N>
> auto operator +(const std::array<T, N>& lhs, const std::array<T, N>& rhs)
> -> std::enable_if_t<std::is_arithmetic_v<T>, std::array<T, N>>
> {
> return std::apply([&](auto... indices) { return std::array<T, N>{
> (std::get<indices>(lhs) + std::get<indices>(rhs))... }; },
> std::make_index_sequence<N>{});
> }
>
> int main(int argc, char** argv)
> {
> auto result = std::array{1, 2, 3} + std::array{5, 4, 3};
> apply([&](auto... indices) { (std::cout << ... << result[indices]); },
> std::make_index_sequence<3>{});
> }
>
> Even the current std::apply function for tuples could be rewriten on top
> of this proposal.
>
> No language changes are required. It can be implemented on top of C++17
> and C++14.
>
> Waiting for your feedback,
>
> Jordi
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> http://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>