Bridge
- Decouple an abstraction from its implementation so that the two can vary
independently.

- Abstraction : Defines the abstraction's interface.
- RefinedAbstraction : Extends the interface defined by Abstraction.
- Implementor : Defines the interface for implementation classes.
- ConcreteImplementor : Implements the Implementor interface and
defines its concrete implementation.
- you want to avoid a permanent binding between an abstraction and its
implementation. This might be the case, for example, when the implementation
must be selected or switched at run-time.
- both the abstractions and their implementations should be extensible by
subclassing. In this case, the Bridge pattern lets you combine the different
abstractions and implementations and extend them independently.
- changes in the implementation of an abstraction should have no impact on
clients; that is, their code should not have to be recompiled.
- (C++) you want to hide the implementation of an abstraction completely
from clients. In C++ the representation of a class is visible in the class
interface.
- you have a proliferation of classes as shown earlier in the first
Motivation diagram. Such a class hierarchy indicates the need for splitting
an object into two parts. Rumbaugh uses the term "nested generalizations"
[RBP+91] to refer to such class hierarchies.
- you want to share an implementation among multiple objects (perhaps using
reference counting), and this fact should be hidden from the client.
- Decoupling interface and implementation. An implementation is
not bound permanently to an interface. The implementation of an abstraction
can be configured at run-time. It's even possible for an object to change
its implementation at run-time. Decoupling Abstraction and Implementor also
eliminates compile-time dependencies on the implementation. Changing an
implementation class doesn't require recompiling the Abstraction class and
its clients. This property is essential when you must ensure binary
compatibility between different versions of aclass library.
- Improved extensibility. You can extend the Abstraction and
Implementor hierarchies independently.
- Hiding implementation details from clients . You can shield clients
from implementation details, like the sharing of implementor objects and the
accompanying reference count mechanism (if any).