Literal String vs. `String` in swift

Question

Playing with swift I found this surprising:

"123".integerValue // <= returns 123

var x = "123"
x.integerValue     // <= Error: String does not have a member named integerValue

Can someone explain?

It looks like "123".integerValue is bridging it over to NSString. var x = "123"; (x as NSString).integerValue // <= returns 123 — Malcolm Jarvis
– Malcolm Jarvis, Commented Jun 3, 2014 at 15:51
@MalcolmJarvis Or the more verbose: x.bridgeToObjectiveC().integerValue — Brian Nickel
– Brian Nickel, Commented Jun 3, 2014 at 16:01

lassej · Accepted Answer · 2014-06-03 15:51:40Z

6

My guess is that in the first example the compiler uses the call to integerValue as additional information to infer the type (choosing between NSString and a Swift String).

In the second example it probably defaults to a Swift String because it doesn't evaluate multiple lines.

answered Jun 3, 2014 at 15:51

lassej

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2 Comments

Austin Over a year ago

To add to this, you can explicitly type the variable var x:NSString = "123" to resolve the issue

gwcoffey Over a year ago

I came to the same conclusion. Didn't realize things like method calls would factor into inference, but it makes sense. Thanks!

Jiaaro · Accepted Answer · 2014-06-03 15:53:52Z

1

I believe this is an example of type inference in action. When you do: "123".integerValue the compiler detects that in this case you want to use an NSString (which it also does when you use string literals as arguments to objective-c functions.

A similar example is:

// x is an Int
let x = 12

// here we tell the compiler that y is a Double explicitly
let y: Double  = 12

// z is a Double because 2.5 is a literal Double and so "12" is also
// parsed as a literal Double
let z = 12 * 2.5

answered Jun 3, 2014 at 15:53

Jiaaro

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Comments

Unheilig · Accepted Answer · 2014-06-03 15:59:11Z

1

Use .toInt()

var x = "123"

var num: Int = x.toInt()

answered Jun 3, 2014 at 15:59

Unheilig

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1 Comment

gwcoffey Over a year ago

Yeah I know that. I was just surprised by the result I was seeing.

John Riselvato · Accepted Answer · 2014-06-03 15:55:58Z

0

If you do:

var x = "123" as NSString
x.integerValue

var x : NSString = "123" // or this as Sulthan suggests

it won't show that error.

I think your first example is automatically picking up that you want to use a NSString as NSString only has this .integerValue call.

The second example is probably failing because it's not being told what it is and deciding to use a swift string instead.

answered Jun 3, 2014 at 15:55

John Riselvato

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1 Comment

Sulthan Over a year ago

var x : NSString = "123" would probably be better

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