C++ Paramater pack expansion over chained function calls

Question

It is common for libraries to have types which return instances of themselves from member functions to encourage chaining calls. For example, nlohmann json:

auto my_data = my_json_object["first key"]["second key"];

Is there some way to call a member function using the contents of a parameter pack? e.g.:

template<class... Keys>
auto get_elem(json json, Keys... keys)
{
   return json([keys]...); // -> json[keys[0]][keys[1]]...[keys[N]]
}

auto my_data = get_elem(my_json_object, "first key", "second key");

Answer 1

The simplest there is:

template<typename A, class First>
auto get_elem(A json, First first) {
   return json[first];
}


template<typename A, class First, class... Keys>
auto get_elem(A json, First first, Keys... keys) {
   return get_elem(json[first], keys...);
}