Once we create a variable using var, we can mutate it. To create a variable that we can mutate, we need to create it using var: This is a great help when reading through code. , and you know that point is a struct variable, then you also know that it will never, ever, change. For example, if you read a line like let point =. For example, the following code will not work:Įven though we defined x within the struct as a var property, we cannot change it, because origin is defined using let. This means we can now initialize a new variable:īecause structs in Swift have value semantics, we cannot change any of the properties of a struct variable that’s defined using let. This is similar to CGPoint, except that it contains Ints, whereas CGPoint contains CGFloats.įor structs, Swift automatically adds a memberwise initializer. Let’s start with a simple struct that describes a Point. However, in Swift, all structs behave this way, not just scalar types. This is so natural that it seems like stating the obvious. This means that whenever a value is assigned to a new variable, it is copied rather than passed by reference:Īfter the code above executes, the value of b will be 43, but a will still be 42. For example, in almost all programming languages, scalar types are value types. Value types imply that whenever a variable is copied, the value itself - and not just a reference to the value - is copied. Thanks to Ole Begemann for suggestions and improvements. It’ll be a free update for everyone who has bought a digital version of the book. We’re currently updating the book for Swift 3. : This post is a draft-version of a new section in our book Advanced Swift. How I Learned To Stop Worrying and Love mutating Structs and mutation in Swift - Chris Eidhof
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