runtime callback, C++ (vc++)

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#include <iostream>
#include <string>
#include <tuple>

using namespace std;

using CTMuint = unsigned;
struct CTMcontext{};

typedef CTMuint(__stdcall *CTMwritefn)(const void *aBuf, CTMuint aCount, void *aUserData);

void __stdcall ctmSaveCustom(CTMcontext aContext, CTMwritefn aWriteFn, void *aUserData, int *newvertexindex)
{
	aWriteFn("foo", 3, aUserData);
}

namespace detail
{

template<typename Func, typename T, size_t... Is>
decltype(auto) apply_impl(Func f, T&& t, index_sequence<Is...>)
{
	return f(get<Is>(t)...);
}

template<typename Func, typename... Ts>
decltype(auto) apply(Func f, tuple<Ts...>&& tup)
{
	return apply_impl(f, move(tup), index_sequence_for<Ts...>{});
}

template<typename T, size_t N>
struct callback_maker;

template<typename R, typename... Params, size_t N>
struct callback_maker<R(*)(Params...), N>
{
	using function_type = R(Params...);

static R __stdcall call(Params... ps)
	{
		auto params_tuple = forward_as_tuple(ps...);
		void const* userData = get<N>(params_tuple);
		auto p = static_cast<pair<function_type*, void*> const*>(userData);
		get<N>(params_tuple) = p->second;
		return apply(p->first, move(params_tuple));
	}
};

}

template<typename Callback, size_t N>
auto make_callback()
{
	using callback_maker = detail::callback_maker<Callback, N>;
	return &callback_maker::call;
}

CTMuint not_stdcall_func(const void *aBuf, CTMuint aCount, void *aUserData)
{
	cout << string((char const*)aBuf, aCount) << endl;
	cout << "Actual user data: " << (char const*)aUserData << endl;
	return 0;
}

int main()
{
	using callback_t = CTMuint(*)(const void *aBuf, CTMuint aCount, void *aUserData);
	auto runtime_ptr = &not_stdcall_func;

char actual_user_data[] = "Actual user data! Smiley face :)";

pair<callback_t, void*> data;
	data.first = runtime_ptr;
	data.second = actual_user_data;

auto callback = make_callback<callback_t, 2>();

ctmSaveCustom({}, callback, &data, nullptr);
}

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Compilation time: 1,66 sec, absolute running time: 0,33 sec, absolute service time: 2 sec