Typelists to be used for mavlink streams in PX4

typelist.cpp

#include <iostream>

template<bool B, class T, class F>
struct conditional { typedef T type; };
 
template<class T, class F>
struct conditional<false, T, F> { typedef F type; };

// from https://stackoverflow.com/a/16803836
template <typename... Ts>
struct largest_type;

template <typename T>
struct largest_type<T>
{
    using type = T;
};

template <typename T, typename U, typename... Ts>
struct largest_type<T, U, Ts...>
{
    using type = typename largest_type<typename conditional<
            (sizeof(U) <= sizeof(T)), T, U
        >::type, Ts...
    >::type;
};

template<typename... Args>
struct pack { };

// Variant 1 - use functions to do stuff with the typelists
// disadvantages: hard to read, have to instantiate the typelist class A to be able to call the functions

template<class... Args>
struct A
{
    using args = pack<Args...>;
};

using x = A<int, char, double>;
template<typename... Args, typename A>
constexpr int f(pack<Args...>, A a) { return sizeof(typename largest_type<Args...>::type); }

template<typename A>
constexpr int f(A a) { return f(typename A::args(), a); }

// Variant 2 - using constexpr, looks more beautiful ;)

template<typename... Args>
struct MavlinkStreamTypeList {
	using streams = pack<Args...>;

	// could have also used an enum, but we do not live in the 2000s anymore
	static constexpr unsigned int largest_size = sizeof(typename largest_type<Args...>::type);
};

int main() {
	std::cout << f<x>(x()) << std::endl;
	std::cout << MavlinkStreamTypeList<int, char, double>::largest_size << std::endl;
	std::cout << sizeof(largest_type<int, char, double>::type) << std::endl;
	return 0;
}