Iterating over a parameter pack

March 4, 2022

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

>Solution :

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.