A way for retrieve type from complex object inside a tuple.

main.cpp

#include <iostream>
#include <tuple>
#include <unordered_map>

template<typename ...TKeyTypes>
struct A {
    std::tuple<std::unordered_map<TKeyTypes, size_t>...> tupled_maps;
};

int main()
{
    A<int, double> a;
    using mapType = std::tuple_element<0, decltype(a.tupled_maps)>::type;
    using mapKeyType = mapType::key_type;
    
    static_assert(std::is_same<mapType, std::unordered_map<int, size_t>>::value, "!");
    static_assert(std::is_same<mapKeyType, int>::value, "!");
}