Re: SIGSERV termination on return statement

[Aurelian Melinte <ame01@gmx.net>]

On 10/10/2012 12:00 PM, Arthur Schwarz wrote:
> The object is deleted and not used within this context. The error occurs before
> the assignment is complete.
>
> I  might have a (very strong) misunderstanding of how C++ references work,  but,
> I don't think that gdb should just quit and not allow me to debug.
>
>
> In SlipSublist::replace if 'SlipCell&  cell' is changed to  'SlipCell* cell',
> with appropriate changes to the remaining code, then  no error occurs. It is
> only when a reference is used do I get into  difficulty. I get a SIGSERV error
> from gdb  in this context and in others when an assignment is made to a
> reference variable, 'cell' in this case. Whenever 'variable = somereference' is
> executed, gdb faults. 'somereference' can be a function or a variable.
>
> The questions I have are (1) can reference variables not be reassigned?, and (2)
> is gdb's SIGSERV fault a legitimate fault or a bug?
>
> Thanks for taking the time;
> art
>    

Well... A C++ reference has different semantics than a Java reference 
(and my Java is rusty).  Under the hood (I mean what the compiler uses 
when dealing with a reference) a reference is a pointer. But at the 
surface where the programmer is, a reference will not allow you certain 
types of operations permitted on pointers, in particular, you can only 
initialize a reference; but you cannot re-assign it. I do not have the 
patience to get through the standardese but check this: 
http://en.wikipedia.org/wiki/Reference_%28C%2B%2B%29: "Once a reference 
is created, it cannot be later made to reference another object; it 
cannot be /reseated/. This is often done with pointers."

Which is why this code makes me think of a compiler bug and I suspect 
that assignment is simply dropped and cell still points to the deleted 
*this:

        SlipCell&   cell = *this;  // Ref initialization
        cell = assignData(X);     // Ref re-assignment verboten. Compiler should have croaked.

You can check with the debugger where that cell ref points before and 
after the re-assignment in the delete case but before returning from 
replace().

Regards,
Aurelian

>
>
> ----- Original Message ----
> From: Aurelian Melinte<ame01@gmx.net>
> To: Arthur Schwarz<aschwarz1309@att.net>
> Cc:gdb@sourceware.org
> Sent: Tue, October 9, 2012 4:32:20 PM
> Subject: Re: SIGSERV termination on return statement
>
> On 09/10/2012 5:48 PM, Arthur Schwarz wrote:
>    
>>      SlipCell&   SlipSublist::replace(SlipCell&   X) {               // Replace a
>> cell
>> on a list with a new cell
>>         SlipCell&   cell = *this;
>>         if (X.isData()) {
>>            cell = assignData(X); // failure before return to this statement
>>      
> from
>    
>> method
>> ***         delete this;
>>         } else {
>>            *this = X;
>>         }
>> ***      return cell;
>>      }; // SlipCell&   SlipSublist::replace(SlipCell&   X)
>>
>>      
> Are you returning from a deleted object then using it afterward?
>
> Regards,
> Aurelian
>
>