How to return nil in swift in a custom initializer?

Question

I am trying to write a custom initializer in swift. I have something like this:

convenience init(datasource:SomeProtocol) {
    assert(datasource != nil, "Invalid datasource")
    if(datasource == nil) {
        return nil
    }
    self.init()
}

The "return nil" line gives me this error: "could not find an overload for '__conversion' that accepts the supplied arguments"

So, all I am trying to accomplish is to have this convenience initializer to return nil if the caller does not provide a valid datasource.

What am I doing wrong here?

Thanks

Answer 1

It's because initializers actually don't have return any value. You can't return nil if a method don't expect a return type.

To achieve what you are trying to do (i.e. force your class to have a datasource), just make your datasource not an optional. Variables that are not optional must have a value (so cannot be nil) and must be initialized.

Answer 2

If the superclass of this class is an Objective-C class, and you know that there's some input that will cause its initializer to return nil, you can simply pass that input to superclass's initializer, and that will cause your initializer to return nil:

import Foundation
class Foo : NSURL {
  convenience init(x: Int) {
    if x == 42 {
      self.init(string: "http://www.google.com/")
    } else {
      self.init(string: "not a url") // returns nil
    }
  }
}

let a: Foo? = Foo(x: 42)
println(a) // prints "Optional(http://www.google.com/)"
let b: Foo? = Foo(x: 17)
println(b) // prints "nil"

Of course, this only works if you're lucky enough to have a superclass whose initializer will return nil. So this is not a general solution.

Answer 3

I think this would work, though it's not clear from your post where you are working with dataSource:

class func instOrNil(datasource:A) -> Self? {
    // do something with datasource
    if datasource.isValid() {
        return self()
    } else {
        return nil
    }
}

Update Swift 3

You can, of course, now return nil from an init?(...):

init?(datasource: A){
    if datasource.isValid() {
        // initialise your properties from datasource
    } else {
        return nil
    }
}

Answer 4

Update: Starting in Xcode 6.1 beta 1 (available in the Mac Dev Center), Swift initializers can be declared to return an optional:

convenience init?(datasource:SomeProtocol) {
    if datasource == nil {
        return nil
    }
    self.init()
}

or an implicitly-unwrapped optional:

convenience init!(datasource:SomeProtocol) {
    if datasource == nil {
        return nil
    }
    self.init()
}