Query Method

~cpp 
class Switch
{
private:
	string status;
	/* ... */
public:
	void makeOn() {
		status = "on";
	}
	void makeOff() {
		status = "off";
	}
	string& getStatus() {
		return status;
	}
	/* ... */
};

class Light
{
/* ... */
	void makeOn() {
		/* ... */
	}
};

class WallPlate
{
private:
	Switch* switch;
	Light* light;
/* ... */
public:
/* ... */
	void update() {
		if( switch->getStatus() == "on" )
			light->makeOn();
		else if( switch->getStatus() == "off" )
			light->makeOff();
	}
};

~cpp 
class Switch
{
private:
	bool status;
	/* ... */
public:
	bool isOn() {
		return status;
	}
	/* ... */
};

class Light
{
/* ... */
	void makeOn() {
		/* ... */
	}
};

class WallPlate
{
private:
	Switch* switch;
	Light* light;
/* ... */
public:
/* ... */
	void update() {
		if ( switch->isOn() )
			light->makeOn();
		else
			light->makeOff();
	}
};