Chcking for onEnter method

onEnter checker

template<typename T>
struct has_onEnter
{
private:
	template<typename U>
	static auto test(int) -> decltype(std::declval<U>().onEnter(), std::true_type
			{});

	template<typename >
	static std::false_type test(...);

public:
	static const bool value = std::is_same<decltype(test<T>(0)), std::true_type>::value;
};

template<typename T>
typename std::enable_if<has_onEnter<T>::value, std::function<void()>>::type check_onEnter(T* value)
{
	return [=]()
	{
		value->onEnter();
	};
}

template<typename T>
typename std::enable_if<has_onEnter<T>::value == false, std::function<void()>>::type check_onEnter(T* value)
{
	return
	{};
}


// example
/*
	void addListener(const int tag, T* listener)
	{
		if (has_onEnter<T>::value || has_onExit<T>::value)
		{
			_listeners.emplace_back( { tag, check_onEnter<T>(*listener), check_onExit<T>(listener) });
		}
	}
*/