Interface Isolation Principle

About 399 wordsAbout 1 min

2025-03-27

Definition

Clients should not be forced to depend upon interfaces that they do not use.

Example

Suppose there is a door that can be locked and unlocked. In addition, an alarm can be installed. Customers can choose to use a normal door or a door with an alarm.

Counter Example

Door.h

class Door {
public:
	virtual void unlock() = 0; // Unlock
	virtual void lock() = 0; // Lock
	virtual void installAlarm() = 0; // Install alarm
	virtual void ringAlarm() = 0; // Alarm rings
};

SimpleDoor.h

class SimpleDoor : public Door {
public:
	void unlock() override {
		// Unlock
	}

	void lock() override {
		// Lock
	}

	void installAlarm() override {
	// Alarm function is not needed
	// The implementation here may throw an exception or make some kind of mark to indicate that this function is not applicable
	throw std::logic_error("Alarm not supported");
	}

	void ringAlarm() override {
	// Alarm function is not needed
	throw std::logic_error("Alarm not supported");
	}
};

DoorWithAlarm.h

class DoorWithAlarm : public Door {
public:
	void unlock() override {
		// Unlock
	}

	void lock() override {
		// Lock
	}

	void installAlarm() override {
		// Install alarm
	}

	void ringAlarm() override {
		// Ring alarm
	}
};

Positive Example

Interface.cpp

class Lockable {
public:
	virtual void unlock() = 0; // Unlock
	virtual void lock() = 0; // Lock
};

class Alarmable {
public:
	virtual void installAlarm() = 0; // Install alarm
	virtual void ringAlarm() = 0; // Ring alarm
};

SimpleDoor.h

class SimpleDoor : public Lockable {
public:
	void unlock() override {
		// unlock
	}

	void lock() override {
		// lock
	}
};

DoorWithAlarm.h

class DoorWithAlarm : public Lockable, public Alarmable {
public:
	void unlock() override {
		// unlock
	}

	void lock() override {
		// lock
	}

	void installAlarm() override {
		// install alarm
	}

	void ringAlarm() override {
		// ring alarm
	}
};