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How is protocol-oriented programming different from object-oriented programming?

Paul Hudson    @twostraws   

Updated for Xcode 13.2

Many Swift developers describe the language as being protocol-oriented, but what does that really mean? In particular, how is it different from the older object-oriented paradigm?

Getting down to the raw facts, there is no practical difference between the two: both can place functionality into objects, use access control to limit where that functionality can be called, make one class inherit from another, and more.

Some might say that the only real difference between the two is that in protocol-oriented programming (POP) we prefer to build functionality by composing protocols (“this new struct conforms to protocols X, Y, and Z”), whereas in object-oriented programming (OOP) we prefer to build functionality through class inheritance. However, even that is dubious because OOP developers also usually prefer composing functionality to inheriting it – it’s just easier to maintain.

In fact, the only important difference between the two is one of mindset: POP developers lean heavily on the protocol extension functionality of Swift to build types that get a lot of their behavior from protocols. This makes it easier to share functionality across many types, which in turn lets us build bigger, more powerful software without having to write so much code.

For more information about protocol-oriented programming, you should watch Dave Abraham’s talk at WWDC that introduced the topic: https://developer.apple.com/videos/play/wwdc2015/408/

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