PythonのデコレーターをC++で再現しようと思ったらよくわからないバケモンが完成した

decorator.hpp

// copyright (c) 2021 M.K (a.k.a plasma-effect)
// Distributed under the Boost Software License, Version 1.0.
// (See http://www.boost.org/LICENSE_1_0.txt)

#pragma once

#include <utility>
#include <tuple>

namespace cpp_decorator
{
	namespace detail
	{
		template<class FuncData, class Base, class IndexSequence, class Tuple>struct decorator;
		template<class Return, class... FuncArgs, class Base, std::size_t... Is, class Tuple>
		struct decorator<Return(FuncArgs...), Base, std::index_sequence<Is...>, Tuple>
		{
			Base base;
			Tuple args;
			template<class Func>constexpr auto operator<<=(Func func)const
			{
				return [base=base, args=args, func=func](FuncArgs... fargs)->Return
				{
					return base(func, std::get<Is>(args)...)(fargs...);
				};
			}
		};
		
		template<class FuncData, class Base>struct decorator_t;
		template<class Return, class... FuncArgs, class Base>
		struct decorator_t<Return(FuncArgs...), Base>
		{
			Base base;
			template<class... Args>constexpr auto operator()(Args... args)const
			{
				auto t = std::make_tuple(args...);
				return decorator<Return(FuncArgs...), Base, std::make_index_sequence<sizeof...(Args)>, decltype(t)>{base, t};
			}
		};

		template<class FuncData>struct decorator_t2;
		template<class Return, class... FuncArgs>
		struct decorator_t2<Return(FuncArgs...)>
		{
			template<class Base>constexpr auto operator()(Base base)const
			{
				return decorator_t<Return(FuncArgs...), Base>{base};
			}
		};
	}
	template<class FuncData>constexpr detail::decorator_t2<FuncData> decorator;
}

main.cpp

#include "decorator.hpp"
#include <functional>
#include <iostream>
using namespace cpp_decorator;

auto add_i(std::function<int(int)> func, int v)
{
	return [=](int u)
	{
		return func(u) + v;
	};
}

auto mul_i(std::function<int(int)> func, int v)
{
	return [=](int u)
	{
		return func(u) * v;
	};
}

constexpr auto add = decorator<int(int)>(add_i);
constexpr auto mul = decorator<int(int)>(mul_i);

auto func = mul(2) <<= add(1) <<= [&](int v)
{
	return 2 * v;
};

int main()
{
	std::cout << func(5) << std::endl;
}