Instance member cannot be used on type

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?

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

Sometimes Xcode when overrides methods adds class func instead of just func. Then in static method you can't see instance properties. It is very easy to overlook it. That was my case.

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!

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/class attribute.

You instantiate an instance of your class and call the instance method on that variable.

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.