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What’s the difference between a function and a method?

Paul Hudson    @twostraws   

Updated for Xcode 12.5

Swift’s functions let us name a piece of functionality and run it repeatedly, and Swift’s methods do much the same thing, so what’s the difference?

Honestly, the only real difference is that methods belong to a type, such as structs, enums, and classes, whereas functions do not. That’s it – that’s the only difference. Both can accept any number of parameters, including variadic parameters, and both can return values. Heck, they are so similar that Swift still uses the func keyword to define a method.

Of course, being associated with a specific type such as a struct means that methods gain one important super power: they can refer to the other properties and methods inside that type, meaning that you can write a describe() method for a User type that prints the user’s name, age, and city.

There is one more advantage to methods, but it’s quite subtle: methods avoid namespace pollution. Whenever we create a function, the name of that function starts to have meaning in our code – we can write wakeUp() and have it do something. So, if you write 100 functions you end up with 100 reserved names, and if you write 1000 functions you have 1000 reserved names.

That can get messy quickly, but by putting functionality into methods we restrict where those names are available – wakeUp() isn’t a reserved name any more unless we specifically write someUser.wakeUp(). This reduces the so-called pollution, because if most of our code is in methods then we won’t get name clashes by accident.

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