class ClassB;
class ClassA
{
private:

	void(ClassB::*clickFunc)();
	ClassB* parent = nullptr;

public:
	ClassA() = delete;
	ClassA(ClassB* parent)
		: parent(parent)
	{
	}
	void SetCallback(void(ClassB::*clickFunc)())
	{
		this->clickFunc = clickFunc;
	}

	void ExecuteCallback()
	{
		if (parent)
			(parent->*clickFunc)();
	}
};

class ClassB
{
 
private:

	const char* up = "UP";
	const char* down = "DOWN";
        
public:

	ClassA button1;
	ClassA button2;

	ClassB()
		: button1{ this }, button2{ this }
	{
		button1.SetCallback(&ClassB::UpClick);
		button2.SetCallback(&ClassB::DownClick);
	}

	void UpClick() 
	{
		Serial.println(this->up);
	}

	void DownClick()
	{
		Serial.println(this->down);
	}
};

void setup() 
{
	Serial.begin(115200);
	ClassB b;
	b.button1.ExecuteCallback();
	b.button2.ExecuteCallback();
}

void loop() 
{
}