Re: Encode and Decode an NSMapTableKeyCallBacks: expected struct and got unsigned int

[Richard Frith-Macdonald]

On 13 Apr 2004, at 10:12, Philip Mötteli wrote:

> Hi
>
>
> I encode some information about a MapTable:
>
>         - (void)ecnodeWithCoder:(NSCoder *)aCoder
>         {
>                 …
> 		[aCoder encodeValueOfObjCType:@encode(NSMapTableKeyCallBacks) 
> at:&keyCallBack];
>                 …
>         }
>
>
> I decode it like this:
>
>         - initWithCoder:(NSCoder *)aCoder
>         {
>                 …
> 		[aCoder decodeValueOfObjCType:@encode(NSMapTableKeyCallBacks) 
> at:&keyCallBack];
>                 …
>         }
>
>
> This decoding translates into the following call of the unarchiver's 
> decodeValueOfObjCType:
>
> 	- 
> (void)decodeValueOfObjCType:"{_NSMapTableKeyCallBacks=^?^?^?^?^?^v}" 
> at:0x…
>
>
> During decoding, the typeCheck function is called:
>
>         typeCheck(t1='{', t2=6 '\006')
>
>
> Which in turn looks up its parameters in the type map:
>
>         type_map[('\006' & _GSC_MASK)] != '{'
>
>
> Unfortunately this map doesn't give a satisfying value back:
>
>         Exception: NSInternalInconsistencyException
>         REASON: expected struct and got unsigned int
>
>
> Is this a bug? I mean, I can't change a lot here. Everything is 
> handled by the system.

I don't understand how you are managing to get an exception decoding 
... because you should have got an exception when trying to encode the 
structure!

You can't encode pointers to functions, and a map table callbacks 
structure contains mostly pointers to functions.  These appear in the 
encoding specification as '^?' sequences ... meaning pointers to 
unknown items.

Coding/decoding is supposed to store the thing pointed to by a pointer 
rather than the value of the pointer itsself (because it generally it 
makes no sense to try to store actual pointers in an archive since when 
they were restored in another process the things they pointed to would 
not exist) ... but where the size of the thing pointed to (in this case 
a function) is unknown, it cannot be stored in the archive, and an 
exception should be raised.

So ... not a bug, you are just trying to do something that can't 
reasonably be done.
Probably you need to write some custom codein your application to 
archive a string
(or some other object) which describes the information you actually 
want to store/retrieve,
rather than trying to archive the map table directly.