Instance member cannot be used on type

问题:

I have the following class:

class ReportView: NSView {  
    var categoriesPerPage = [[Int]]()
    var numPages: Int = { return categoriesPerPage.count }
}

Compilation fails with the message:

Instance member 'categoriesPerPage' cannot be used on type 'ReportView'

What does this mean?

回答1:

You just have syntax error when saying = {return self.someValue}. The = isn't needed.

Use :

var numPages: Int {
    get{
        return categoriesPerPage.count
    }

}

if you want get only you can write

var numPages: Int {
     return categoriesPerPage.count
}

with the first way you can also add observers as set willSet & didSet

var numPages: Int {
    get{
        return categoriesPerPage.count
    }
    set(v){
       self.categoriesPerPage = v;
    }
}

allowing to use = operator as a setter

myObject.numPages = 5;

回答2:

For anyone else who stumbles on this make sure you're not attempting to modify the class rather than the instance! (unless you've declared the variable as static)

eg.

MyClass.variable = 'Foo' // WRONG! - Instance member 'variable' cannot be used on type 'MyClass'

instanceOfMyClass.variable = 'Foo' // Right!

回答3:

It is saying you have an instance variable (the var is only visible/accessible when you have an instance of that class) and you are trying to use it in the context of a static scope (class method).

You can make your instance variable a class variable by adding static attribute.

You can make your class method an instance method by removing class attribute.

回答4:

Another example is, you have class like :

@obc class Album: NSObject {
    let name:String
    let singer:Singer
    let artwork:URL
    let playingSong:Song


    // ...

    class func getCurrentlyPlayingSongLyric(duration: Int = 0) -> String {
        // ...
       return playingSong.lyric
    }
}

you will also get the same type of error like :

instance member x cannot be used on type x. 

It's because you assign your method with "class" keyword (which makes your method a type method) and using like :

Album.getCurrentlyPlayingSongLyric(duration: 5)

but who set the playingSong variable before? Ok. You shouldn't use class keyword for that case :

 // ...

 func getCurrentlyPlayingSongLyric(duration: Int = 0) -> String {
        // ...
       return playingSong.lyric
 }

 // ...

Now you're free to go.

回答5:

I kept getting the same error inspite of making the variable static. Solution: Clean Build, Clean Derived Data, Restart Xcode. Or shortcut Cmd + Shift+Alt+K

UserNotificationCenterWrapper.delegate = self

public static var delegate: UNUserNotificationCenterDelegate? {
        get {
            return UNUserNotificationCenter.current().delegate
        }
        set {
            UNUserNotificationCenter.current().delegate = newValue
        }
    }

回答6:

Your initial problem was:

class ReportView: NSView {
  var categoriesPerPage = [[Int]]()
  var numPages: Int = { return categoriesPerPage.count }
}

Instance member 'categoriesPerPage' cannot be used on type 'ReportView'

previous posts correctly point out, if you want a computed property, the = sign is errant.

Additional possibility for error:

If your intent was to "Setting a Default Property Value with a Closure or Function", you need only slightly change it as well. (Note: this example was obviously not intended to do that)

class ReportView: NSView {
  var categoriesPerPage = [[Int]]()
  var numPages: Int = { return categoriesPerPage.count }()
}

Instead of removing the =, we add () to denote a default initialization closure. (This can be useful when initializing UI code, to keep it all in one place.)

However, the exact same error occurs:

Instance member 'categoriesPerPage' cannot be used on type 'ReportView'

The problem is trying to initialize one property with the value of another. One solution is to make the initializer lazy. It will not be executed until someone access the value.

class ReportView: NSView {
  var categoriesPerPage = [[Int]]()
  lazy var numPages: Int = { return categoriesPerPage.count }()
}

now the compiler is happy!

回答7:

Just in case someone really needs a closure like that, it can be done in the following way:

var categoriesPerPage = [[Int]]()
var numPagesClosure: ()->Int {
    return {
        return self.categoriesPerPage.count
    }
}