Change Type

#include <iostream>
#include <typeinfo>
#include <cxxabi.h>
#include <cstdlib>
#include <string>

std::string getReadableTypeinfo(const char* info)
{
    int status;
    char* s=abi::__cxa_demangle(info, /*nullptr*/NULL, /*nullptr*/NULL, &status);
    if (status != 0)
        return "Invalid argument";
    std::string result = s;
    free(s);
    return result;
}

using namespace std;

// using size_t = decltype(sizeof(0));

template <typename OldType, size_t index, typename NewType>
struct changeType;

template <template <typename... Args> class OldType, size_t index, typename NewType, typename ArgType, typename... Others>
struct changeType<OldType<ArgType, Others...>, index, NewType>
{
    template <typename... Types>
    using type = typename changeType<OldType<Others...>, index-1, NewType>::template type<Types..., ArgType>;
};

template <template <typename... Args> class OldType, typename NewType, typename ArgType, typename... Others>
struct changeType<OldType<ArgType, Others...>, 0, NewType>
{
    template <typename... Types>
    using type = typename changeType<OldType<Others...>, size_t(-1), NewType>::template type<Types..., NewType>;
};

template <template <typename... Args> class OldType, size_t index, typename NewType>
struct changeType<OldType<>, index, NewType>
{
    template <typename... Types>
    using type = OldType<Types...>;
};

template <typename... T>
struct fn
{

};

int main() {
    cout << getReadableTypeinfo(typeid(fn<int, int, int, int, int>).name()) << endl;
    cout << getReadableTypeinfo(typeid(changeType<fn<int, int, int, int, int>, 2, float>::type<>).name()) << endl << endl;

    cout << getReadableTypeinfo(typeid(fn<int*, int*, int*>).name()) << endl;
    cout << getReadableTypeinfo(typeid(changeType<fn<int*, int*, int*>, 1, int const*>::type<>).name()) << endl;
}