Why do we need to mark some methods as mutating?

Paul Hudson    @twostraws    May 28th 2020

Updated for Xcode 12.5

It’s possible to modify the properties of a struct, but only if that struct is created as a variable. Of course, inside your struct there’s no way of telling whether you’ll be working with a variable struct or a constant struct, so Swift has a simple solution: any time a struct’s method tries to change any properties, you must mark it as mutating.

You don’t need to do anything else other than mark the method as mutating, but doing that gives Swift enough information to stop that method from being used with constant struct instances.

There are two important details you’ll find useful:

• Marking methods as mutating will stop the method from being called on constant structs, even if the method itself doesn’t actually change any properties. If you say it changes stuff, Swift believes you!
• A method that is not marked as mutating cannot call a mutating function – you must mark them both as mutating.

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