Swift/SwiftUI property initializer error in struct

Question

I've included a very basic example that summarizes my issue

struct Greeting {

    var name = "Bob"
  
    var message = "Hi, " + name
}

var a = Test("John")
print(a.message)

I get the following error:

error: cannot use instance member 'name' within property initializer; property initializers run before 'self' is available

I've tried initializing the values, creating my best guess at lazy vars, and making the vars computed values. Any help would be appreciated!

All executable code like print(a.message) needs to go inside a function. Unless you're using Swift Playgrounds. — aheze
– aheze, Commented Aug 6, 2021 at 4:12
You're using SwiftUI right? You can put var a = Test("John"); print(a.message) inside an onAppear. — aheze
– aheze, Commented Aug 6, 2021 at 4:13

George · Accepted Answer · 2021-08-08 02:24:47Z

2

You are using name before the struct is initialized. If you make message a computed properties it should work.

struct Greeting {
    var name = "Bob"

    var message: String {
        "Hi, " + name
    }
}

edited Aug 8, 2021 at 2:24

George

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answered Aug 6, 2021 at 4:20

Ccr

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1 Comment

George Over a year ago

Just a small typo (already edited & fixed) - you including the = so wasn't a computed property. Wouldn't have compiled, but good answer! I think this is better, because if name changes (since it's a var), message also changes / has right value when called, which the accepted answer doesn't.

workingdog support Ukraine · Accepted Answer · 2021-08-06 05:22:21Z

You are getting the error because in Swift you need to give variables some value before you can use them.

   struct Greeting {
        var name = "Bob"      // <-- this variable is initialized with "Bob"
        var message: String   // <-- this var is not, you need to initialize it yourself
                             // which means you cannot use this before you give it a value
        
        // a very common approach is to have your own initializer function init.
        init(name: String, message: String) {
            self.name = name    // <-- this is initialized (again in our case)
            self.message = "Hi " + name  // this is now initialized the way you want it
        }

       // you can have many customized init, like this
       init() {
          // here because name is already initialized with Bob, it's ok
          self.message = "Hi " + name
       }
        
        // if you don't have a init function, swift creates a basic one (or more) for you
        // but it will not be the special one you wanted, like above.
    }

So when Greeting is created the init function is called first. That is the place where you can customize the creation of Greeting. Before that you cannot use the variable "name" as you do in "var message = "Hi, " + name".

You can use it like this:

        let greet = Greeting()
        print(greet.message)

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