TypeListOp.cpp

gistfile1.txt

#include <variant>
#include <type_traits>

// =================================================================

template<typename ...TArgs>
struct WhateverTypes
{
	template<typename TOp>
	auto operator |(TOp op)
	{
		return op.Apply(*this);
	}

	template<template<typename ...UArgs> class T>
	using As = T<TArgs...>;
};

// =================================================================

template<typename T>
struct AppendOp
{
	template<typename ...TArgs>
	auto Apply(WhateverTypes<TArgs...>) -> WhateverTypes<TArgs..., T> { return {}; }
};

template<typename T>
AppendOp<T> append() { return {}; }

// =================================================================

struct ReverseOp
{
	auto Apply(WhateverTypes<> ts) { return ts; }

	template<typename T>
	auto Apply(WhateverTypes<T> ts) { return ts; }

	template<typename T, typename ...TArgs>
	auto Apply(WhateverTypes<T, TArgs...>)
	{
		return WhateverTypes<TArgs...>{} | *this | append<T>();
	}
};

inline ReverseOp reverse() { return {}; }

// =================================================================

template<int N>
struct Drop
{
	template<typename ...TArgs>
	auto Apply(WhateverTypes<TArgs...> ts)
	{
		return ts | drop<1>() | drop<N - 1>();
	}
};

template<>
struct Drop<1>
{
	template<typename T, typename ...TArgs>
	auto Apply(WhateverTypes<T, TArgs...> ts) -> WhateverTypes<TArgs...> { return {}; }
};

template<>
struct Drop<0>
{
	template<typename ...TArgs>
	auto Apply(WhateverTypes<TArgs...> ts) { return ts; }
};

template<int N>
Drop<N> drop() { return {}; }

// =================================================================

template<typename T>
struct Whatever;

template<template<typename...> class T, typename ...TArgs>
struct Whatever<T<TArgs...>>
{
	using Extract = WhateverTypes<TArgs...>;
};

// =================================================================

using namespace std;

int main()
{
	static_assert(is_same_v<
		decltype(Whatever<variant<int, double, float>>::Extract{} | reverse() | drop<1>())::As<variant>,
		variant<double, int>
		>);
	return 0;
}