12.1. The Adapter pattern

Design patterns provide a reliable and easy way to follow proven design principles and to write well-structured and maintainable code. One of the popular and often used patterns in object-oriented software development is the adapter pattern. It follows Robert C. Martin’s Dependency Inversion Principle and enables you to reuse an existing class even so it doesn’t implement an expected interface.

If you do some research on the adapter pattern, you will find two different versions of it:

  1. The class adapter pattern that implements the adapter using inheritance.

  2. The object adapter pattern that uses composition to reference an instance of the wrapped class within the adapter.

The object adapter pattern is generally more popular and the one used within the STL to implement stack and queue.

The general idea of an adapter in software development is identical to the one in the physical world. If you have travelled to foreign countries, you probably noticed that electrical outlet vary from country to country. Outlet shapes vary such that the plug of your electrical device doesn’t fit. How do you connect the charger of your mobile phone or laptop to these power outlets?

The answer is simple. You get an adapter which you can put into the power outlet and then you put your plug into the other end of the adapter. The adapter changes the form of your plug so that you can use it with the power outlet. In that example and in most other situations, the adapter doesn’t provide any additional features. It just enables you to connect your plug to a different outlet.

classDiagram client --> adapter : uses legacy_component --o adapter class adapter { +do_this() int } class legacy_component { +do_that() void }

Often when programming you have a class that does almost what you need it to, or it contains a lot of capability you would like to reuse, but the class interface is not a good fit. A class adapter is a simple design pattern that helps solve problems like this. We can use an adapter when we want to convert the interface of a class into another interface clients expect. An adapter lets classes work together that couldn’t otherwise because of incompatible interfaces. An adapter also allows us to wrap an existing class with a new interface without making any changes to the original class - the class being adapted.

Socket wrenches provide an example of the Adapter. A socket attaches to a ratchet, provided that the size of the drive is the same. Typical drive sizes in the United States are 1/2” and 1/4”. Obviously, a 1/2” drive ratchet will not fit into a 1/4” drive socket unless an adapter is used. A 1/2” to 1/4” adapter has a 1/2” female connection to fit on the 1/2” drive ratchet, and a 1/4” male connection to fit in the 1/4” drive socket.

classDiagram ratchet --> adapter : uses socket --o adapter class ratchet { +drive_6mm (male) } class adapter { +drive_6mm (female) +drive_3mm (male) } class socket { +drive_3mm (female) }

The data structures in this chapter stack and queue both use the adapter patter to achieve their design goals.

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