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
    }
};

Contributors

dingyuqi

Changelog

10/11/25, 2:26 AM

View All Changelog

• 941fa-feat(theme): upgrade and use collectionson 10/11/25
• eb6eb-improve(docs): use pangu formatteron 8/5/25
• dd170-feat(docs): add solid principles serieson 3/27/25

Copyright

Copyright Ownership:dingyuqi

License under:Attribution-NonCommercial-NoDerivatives 4.0 International (CC-BY-NC-ND-4.0)

Edit this page