CallbackFunctor.h

namespace detail
{
	template <typename Res = void, typename ... Args>
	class Lambda {
	public:
		virtual Res operator()(Args ... args) = 0;
		virtual ~Lambda() noexcept = default;
	};

	template <typename T, typename Res, typename ... Args>
	class LambdaImpl : public Lambda<Res, Args...> {
	public:
		LambdaImpl(T t) : _t(t) {}
		Res operator()(Args ... args) override
		{
			return _t(std::forward<Args>(args)...);
		}

	private:
		T _t;
	};
};

template <typename Res, typename ... Args, typename T>
std::unique_ptr<detail::Lambda<Res, Args...>> MakeLambda(T t)
{
	return std::make_unique<detail::LambdaImpl<T, Res, Args...>>(t);
}


template <typename Res = void, typename ... Args>
class CallbackFunctor {
public:
	CallbackFunctor() {}

	template <typename T>
	CallbackFunctor(T t)
	{
		_lambda = MakeLambda<Res, Args...>(t);
	}

	template <typename T>
	void operator=(T t)
	{
		_lambda = MakeLambda<Res, Args...>(t);
	}

	void operator=(std::nullptr_t)
	{
		_lambda = nullptr;
	}
	template<typename ...Args>
	auto operator()(Args &&... args) const
	{
		if(!_lambda)
			return Res(0);
		else
			return (*_lambda)(std::forward<Args>(args)...);
	}

	operator bool() const
	{
		return (bool)_lambda;
	}

private:
	using LambdaType = detail::Lambda<Res, Args...>;
	std::unique_ptr<LambdaType> _lambda;
};