Re: Swift description
Re: Swift description
- Subject: Re: Swift description
- From: Greg Parker <email@hidden>
- Date: Sun, 12 Jul 2015 13:37:24 -0700
> On Jul 11, 2015, at 6:29 PM, Roland King <email@hidden> wrote:
>
>>
>> However, Roland got it slightly wrong, according to the documentation. This:
>>
>> print (“\(myObj)”)
>>
>> never uses debugDescription, only description. To use debugDescription (if it exists, or description instead):
>>
>> debugPrint (“\(myObj)”)
>
> Dunno about the documentation - I’m remembering a WWDC video which said one falls back to the other. So I tested it in a playground. \(variable) appears to do as suggested, uses description if that exists, falls back to debugDescription if it doesn’t and prints the class name if neither do.
>
> Surprisingly all debugPrint() does is not, as you might expect, use the debugDescription in preference, but just puts quotes around the whole thing and still uses description if it exists. That feels wrong.
>
> class IHaveNothing{}
>
> class OnlyDebugPrintable : IHaveNothing, CustomDebugStringConvertible
> {
> var debugDescription : String { return "I am CustomDebugStringConvertible" }
> }
>
> class HasTheLot : OnlyDebugPrintable, CustomStringConvertible
> {
> var description : String { return "I have the lot" }
> }
>
>
> let a = IHaveNothing()
> let b = OnlyDebugPrintable()
> let c = HasTheLot()
>
> print( "\(a)” ) // IHaveNothing
> print( "\(b)” ) // I am CustomDebugStringConvertible
> print( "\(c)” ) // I have the lot
> debugPrint( "\(a)” ) // “IHaveNothing"
> debugPrint( "\(b)” ) // “I am CustomDebugStringConvertible"
> debugPrint( "\(c)” ) // “I have the lot"
Note that there is a distinction between "print an object" and "perform string interpolation on an object and print that string".
print(c) // I have the lot
print("\(c)") // I have the lot
debugPrint(c) // I am CustomDebugStringConvertible
debugPrint("\(c)") // "I have the lot"
Note that (1) string interpolation prefers the non-debug description when available, and (2) debugPrint() of a String prints the quotes but print() of a String does not.
--
Greg Parker email@hidden Runtime Wrangler
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