Iterating over a parameter pack

Question

I have a parameter pack args... of vectors of arbitrary types, corresponding to which there is a vector of indices say v = {3,0,5...} having the same size, and order as the number of members of args.... I would like get_tuple to return a tuple of the elements of args... at the indices given by v.

Here's what I have so far, but I'm stuck trying to iterate over the members of the parameter pack.

template<typename... Args> auto get_tuple(const std::vector<size_t>& vector, const Args &... args) { return std::make_tuple(args[v[0]]...); }

For example:

std::vector<std::string> v1 = {"a", "b"}; std::vector<int> v2 = {1,2}; std::vector<size_t> v = {0,1}; auto result = get_tuple(v, v1, v2); // ("a",2) expected

Evg · Accepted Answer · 2022-03-04 06:53:19Z

In C++17, you need additional level of indirection to get a pack of indices to get a pack of elements at those indices:

template<typename... Args, std::size_t... Is> auto get_tuple_impl(const std::vector<std::size_t>& indices, std::index_sequence<Is...>, const Args&... args) { return std::make_tuple(args[indices[Is]]...); } template<typename... Args> auto get_tuple(const std::vector<std::size_t>& indices, const Args&... args) { return get_tuple_impl(indices, std::index_sequence_for<Args...>(), args...); }

In C++20, we could you a lambda function with template parameters invoked in-place:

template<typename... Args> auto get_tuple(const std::vector<std::size_t>& indices, const Args&... args) { return [&]<std::size_t... Is>(std::index_sequence<Is...>) { return std::make_tuple(args[indices[Is]]...); }(std::index_sequence_for<Args...>()); }

You might also want to add an assertion assert(indices.size() == sizeof...(Args)); or use std::array<std::size_t, N> type instead.

Small note, std::make_index_sequence<sizeof...(Args)> is equivalent to std::index_sequence_for<Args...>. I might as well also mention that C++20 makes this technique a bit less ugly with templated lambdas.
@chris would you care to provide the C++20 solution that you have in mind? Thanks in advance.
@marital_weeping, It is indeed almost character-for-character what's in the answer now.

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