Why does Int(false) work but Int(booleanVariable) does not?

Question

The code shown below:

Int(false) // = 1, it's okay

//but when I try this
let emptyString = true //or let emptyString : Bool = true
Int(emptyString) //error - Cannot invoke initializer with an argument list of type '(Bool)'

Can anyone explain this fact? It's confusing. What happens inside?

I don't think that it's duplicated, the question is "why", not "how". The title of the question should be edited. — Ahmad F, Nov 29 '16 at 14:43

vacawama · Accepted Answer · 2016-11-29T14:29:32.280

5

To find out what is going on with Int(false), change it to:

Int.init(false)

and then option-click on init. You will see that it is calling this initializer:

init(_ number: NSNumber)

Since false is a valid NSNumber and NSNumber conforms to the protocol ExpressibleByBooleanLiteral, Swift finds this initializer.

So why doesn't this work?:

let emptyString = false
Int(emptyString)

Because now you are passing a Bool typed variable and Int doesn't have an initializer that takes a Bool.

In Swift 2 this would have worked because Bool was automatically bridged to NSNumber, but that has been removed.

You can force it like this:

import Foundation // or import UIKit or import Cocoa
Int(emtpyString as NSNumber)

This only works if Foundation is imported. In Pure Swift there is no NSNumber, of course.

edited Nov 29 '16 at 14:29

answered Nov 29 '16 at 14:17

vacawama

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Perhaps add that this requires Foundation to be included, `Int(false)` won't compile in pure Swift environment. – Martin R Nov 29 '16 at 14:21
@MartinR, about a 'Foundation' question - I can compile this in swift playground with 'import UIKit' – G. Veronika Nov 29 '16 at 14:24
1

Yes, `import UIKit` also imports Foundation. – vacawama Nov 29 '16 at 14:25
1

And for the sake of completeness: `NSNumber` conforms to `ExpressibleByBooleanLiteral`, that's why you can pass the `false` literal as an argument. – Martin R Nov 29 '16 at 14:27
Thanks for the suggestions, @MartinR. – vacawama Nov 29 '16 at 14:31

Dileep · Answer 2 · 2016-11-29T14:06:05.907

0

Try this

let intValue = emptyString ? 1 : 0

Update

You want to use Int(), use this

Int(NSNumber(value:emptyString))

edited Nov 29 '16 at 14:06

answered Nov 29 '16 at 14:01

Dileep

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yes, I know about this, thanks. But my interest is to understand the fact in my question :) – G. Veronika Nov 29 '16 at 14:03
Cool. You cannot convert int to bool like that, if you need convert to NSNumber hope this helps – Dileep Nov 29 '16 at 14:07
But why can I convert 'false' or 'true' by Int(false)? – G. Veronika Nov 29 '16 at 14:09

score 0 · Answer 3 · answered Nov 29 '16 at 14:09

0

Int doesn't have initialiser with Bool as argument.

answered Nov 29 '16 at 14:09

Artem Novichkov

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score 0 · Answer 4 · answered Nov 29 '16 at 14:13

Are you sure you do not have something like this somewhere in your codebase:

extension Int : ExpressibleByBooleanLiteral {
    public init(booleanLiteral: BooleanLiteralType) {
        self = booleanLiteral ? 1 : 0
    }
}

Because otherwise, the line like let foo = Int(false) should not compile.

Why does Int(false) work but Int(booleanVariable) does not?

4 Answers4