Re: MI and anonymous unions

[Jim Ingham <jingham@apple.com>]

Support for anonymous unions & structures is another of the "things we  
fixed but haven't submitted back"...  We do pretty much what Vladimir  
suggests, though I use #anon#N for the varobj name: the choice doesn't  
much matter.  We still return a blank expression (since that's really  
what the  There's one other tricky bit 'caused by the fact that you  
might have more than one anonymous union or structure inside a  
structure.  This wouldn't be a problem, but the varobj code tends to  
look up structure elements by name, which obviously won't work here.   
So you have to convert all the code that looks up structure elements  
to use the index rather than the name.

Another of the things we do in our varobj code for C++ is that we look  
up the dynamic type of objects (the same mechanism that "set print  
object on" uses, and the value & children we return are based on the  
dynamic type.  As soon as you do that, you really do need a "give me  
the expression that can recreate this varobj" for child varobj's.     
Otherwise the MI client needs to get all these dynamic casts right to  
get down to the proper child, which is a bit of a pain.

But it's a generally useful command.  We called it "var-info-path- 
expression" and given a varobj, it returns the expression that if  
evaluated would result in the same value.  Xcode uses this for "show  
in separate window" actions in the debugger variable view.  You can  
also use the to eliminate the "." for the anonymous structures or  
unions - though of course the MI client could do this too...

Jim


On Nov 15, 2006, at 6:58 AM, Vladimir Prus wrote:

>
> Daniel Jacobowitz wrote:
>
>> On Wed, Nov 15, 2006 at 12:38:16PM +0300, Vladimir Prus wrote:
>>> Traversing it with MI eventually gives:
>>>
>>>   -var-list-children V.public
>>>
> ^ 
> done 
> ,numchild="1",children=[child={name="V.public.",exp="",numchild="1",
>>>                  type="union {...}"}]
>>>     (gdb)
>>>     -var-list-children V.public.
>>>     ^done,numchild="1",children=[
>>>          child={name="V.public..public",exp="public",numchild="2"}]
>>>      (gdb)
>>>      -var-list-children V.public..public
>>>      .....
>>>
>>> Although this kinda works, I'm pretty sure UI won't be happy about  
>>> empty
>>> expression for a variable object, and if you have two anonymous  
>>> unions,
>>> you can't even address them.
>>
>> I'm not sure what to do for the empty expression.  There's nothing we
>> can put there which would act like a named union, since you need one
>> less period - hmm, we were just discussing an MI command to recreate
>> expressions the other day...
>>
>> How do people use the exp="" result?  Should it be "<anonymous>"?
>
> KDevelop uses it to construct the full expression, which will  
> obviously
> break. I'm not sure about Eclipse, I think it does the same.
>
> But given that there's no way to put anything there that can be used  
> to
> recreate expression, I don't see much difference. "<anonymous>"  
> would work
> better if this is to be shown in some UI, I think.
>
>>> How about using some unique identifier for variable objects  
>>> corresponding
>>> for anonymous unions? Say "@N"?
>>
>> That sounds reasonable.
>
> Ok.
>
> - Volodya
>