llvm dev - May 2008 - [LLVMdev] Iterator issue in BranchFolder::RemoveBlocksWithHash?

On May 23, 2008, at 10:19 AM, Dale Johannesen wrote:
> On May 23, 2008, at 4:10 AM, Nicolas Capens wrote:
>
>> I updated from 2.2 to the latest SVN head and I now get a debug  
>> assert in BranchFolder::RemoveBlocksWithHash: “vector iterators  
>> incompatible”. I’m using Visual C++ 2005. I think this is the  
>> culprit code:
>>
>>     MergePotentials.erase(CurMPIter);
>>     if (CurMPIter==B)
>>       break;
>>
>> The erase clears the _Mycont field (i.e. the iterator’s container),  
>> while the == expects CurMPIter and B to have the same container.  
>> I’m no STL guru but it seems wrong to first erase an element and  
>> then try to compare it. I traced it back to revision 50921 made on  
>> May 10’th. I rewrote it like following, which I’m not entirely sure  
>> is the intended behavior but it ‘works for me’:
>>
>>     CurMPIter = MergePotentials.erase(CurMPIter);
>>     if (CurMPIter==B)
>>       break;
>
> Thanks for analyzing the problem.
>
> I also am not a STL guru; the standard says erase
> "Invalidates all the iterators and references after the point of the  
> erase"
> which is not wonderfully worded, but I take it to mean an iterator  
> referring to the point of the erase remains valid....
 From n2461:
> 8 The insert members shall not affect the validity of iterators and  
> references to the container, and the erase members shall invalidate  
> only iterators and references to the erased elements.

Pretty clear.

If you play with --i, be careful of iterators that point to the  
beginning.

On May 27, 2008, at 1:40 PM, Mike Stump wrote:
> On May 23, 2008, at 10:19 AM, Dale Johannesen wrote:
>> On May 23, 2008, at 4:10 AM, Nicolas Capens wrote:
>> I also am not a STL guru; the standard says erase
>> "Invalidates all the iterators and references after the point of
the
>> erase"
>> which is not wonderfully worded, but I take it to mean an iterator
>> referring to the point of the erase remains valid....
>
> From n2461:
>
>> 8 The insert members shall not affect the validity of iterators and
>> references to the container, and the erase members shall invalidate
>> only iterators and references to the erased elements.
>
> Pretty clear.

In the current standard that language appears under "associative  
containers", but this is a vector.  Has it been moved?
The language I quoted applies specifically to vectors.

In any event, I'm not arguing that this ought to work, just that the  
(current) standard is badly worded.
> If you play with --i, be careful of iterators that point to the
> beginning.
Thanks, I've already hit that one.

On May 27, 2008, at 2:01 PM, Dale Johannesen wrote:
> On May 27, 2008, at 1:40 PM, Mike Stump wrote:
>>
>> From n2461:
>>
>>> 8 The insert members shall not affect the validity of iterators and
>>> references to the container, and the erase members shall invalidate
>>> only iterators and references to the erased elements.
>>
>> Pretty clear.
>
>
> In the current standard that language appears under "associative
> containers", but this is a vector.
Ah, and from vector:

23.2.5.4 vector modifiers [vector.modifiers]

iterator erase(const_iterator position);
iterator erase(const_iterator first, const_iterator last);
4 Effects: Invalidates iterators and references at or after the point  
of the erase.

:-)  Sorry, didn't realize this was vector and the page this was on  
was marked container requirements and I didn't realize I was reading  
just the associative container requirements.
> In any event, I'm not arguing that this ought to work, just that the
> (current) standard is badly worded.
I think it was a bug in the original language standard.  You can ask  
Howard, if you're curious.