Question

Can anybody elaborate the Bridge design pattern and the Decorator pattern for me. I found it similar in some way. I don’t know how to distinguish it?

My understanding is that in Bridge, it separate the implementation from the interface, generally you can only apply one implementation. Decorator is kind of wrapper, you can wrap as many as you can.

For example,

Bridge pattern

class Cellphone {
private:
Impl* m_OS;         // a cellphone can have different OS

}

Decorator pattern

class Shirt {
private:
Person * m_p;           //put a shirt on the person;

}

Answer 1

The Decorator should match the interface of the object you’re decorating. i.e. it has the same methods, and permits interception of the arguments on the way in, and of the result on the way out. You can use this to provide additional behaviour to the decorated object whilst maintaining the same interface/contract. Note that the interface of the Decorator can provide additional functionality to create a more useful object.

The Bridge has no such constraint. The client-facing interface can be different from the underlying component providing the implementation, and thus it bridges between the client’s interface, and the actual implementation (which may not be client-friendly, subject to change etc.)