So there are a couple of ways objects can be bridged:

1) has-a style bridges - RunLoop and CFRunLoop for example. The only 
association is where RunLoop contains a CFRunLoop and there is a method to 
fetch the CFRunLoop out of the RunLoop. Some times this relationship is not 

2) is-a style bridges - NSCFString and CFString for example. These are where 
the structural size and layout of the objects are identical. So when casting 
the only change is a re-interpretation of the type and the underlying data 
structure backing the object is exactly the same.

3) toll-free bridges - NSString and CFString for example. CF can take NSStrings 
because the methods that transact upon CFStrings all check to determine if the 
object being passed is a subclass of NSString or if it is a NSCFString, in the 
NSString subclass cases it will dispatch calls out to the subclass to specific 
funnel points such that it can reasonably transact upon that object.

4) Compiler generated struct to reference bridges - NSString and String. When 
the translation occurs the compiler emits a call to a method on String to 
convert NSString to String. This is just a specialized and automatically 
generated form of style 1

The styles 1, 2 and 3 work for swift-corelibs-foundation, however 4 is disabled 
for linux even though we have both the reference types and the structural 

On Darwin CFReadStream is actually a NSCFInputStream (which is a subclass of 
NSInputStream) but currently in swift-corelibs-foundation CFReadStream is a 
NSCFType (I would consider that discrepancy an incomplete point of 
swift-corelibs-foundation) with an ivar of a CFReadStream. So what would need 
to be done is that InputStream and OutputStream would need to be made 
conditionally abstract and the ivar layout would need to exactly mimic the 
layout of the CF counterpart. Two new subclasses would need to be made to 
dispatch back into CF the functionality for streams created from CF and 
dispatching methods would need to be made. Doing this would make InputStream 
and OutputStream more like NSString (which might be a decent place to take a 
look at how it works).

Let me know if there is anything I can help clear up on more.

> On Sep 20, 2016, at 11:29 PM, Dave Abrahams <> wrote:
> From: Bouke Haarsma via swift-users <>
> Subject: How to bridge between CoreFoundation and SwiftFoundation on Linux?
> Date: September 20, 2016 at 9:38:54 PM PDT
> To:
> Reply-To: Bouke Haarsma <>
> Hi all,
> When working with CoreFoundation objects (e.g. CFReadStream, 
> CFWriteStream) it isn't immediately obvious to me how to bridge them to 
> SwiftFoundation counterparts (InputStream / OutputStream).
> The following works on OSX, but doesn't work on Linux;
>    let readStream: CFReadStream = ...
>    readStream as InputStream
>    // error: cannot convert value of type 'CFReadStream' to expected 
> argument type 'InputStream'
> In some other places I need to bridge a String to a CFString, the 
> following works on OSX, but doesn't work on Linux;
>    let string: String = ...
>    string as CFString
>    // error: error: 'String' is not convertible to 'CFString'; did you 
> mean to use 'as!' to force downcast?
> Thanks.
> -- 
> -Bouke
> _______________________________________________
> swift-users mailing list
> -- 
> -Dave

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