Re: Bug in TObjArray?

It's worth noting that some STL containers have similar behaviour, for similar reasons. For example std::map.

Regards,
James.

On 17 Sep 2010, at 08:06, Fons Rademakers wrote:

> The problem with TObject *&operator[] is that it can be used as an lvalue:
>
> a[0] = new TObject
>
> and hence when using it we assume it will be set and GetEntriesFast() will return it as set. In your case if you loop over the array then GetEntriesFast() will serve as a safe upper-bound. If you use At(), as Axel suggests to access values there is no problem.
>
> Cheers, Fons.
>
>
>
> On 17/09/2010 14:50, Axel Naumann wrote:

>> Hi John,
>> 
>> as I said, operator[] is intentionally not const: it's used to adds
>> elements via placement new(). I think you're looking for the member
>> function At().
>> 
>> Cheers, Axel
>> 
>> John Idarraga wrote on 09/17/2010 02:44 PM:
>>> But then why Arr.GetEntries() gives 0 ?  I thought the Fast version of
>>> GetEntries was precisely a Fast access to the number of entries, only
>>> safe when you have not performed any manipulation.  Available for
>>> certain cases.  I agree with Vassili that it is somewhat annoying and
>>> may be an error prone.  But I may be misunderstanding.
>>> 
>>> thanks,
>>> 
>>> John
>>> 
>>> Axel Naumann wrote:
>>>> Hi Vassili,
>>>> 
>>>> yes, that's on purpose, think of
>>>> 
>>>>   new (Arr[0]) TObject;
>>>> 
>>>> Cheers, Axel.
>>>> 
>>>> 
>>>> Vassili Maroussov wrote on 09/17/2010 02:31 PM:
>>>> 
>>>>> Dear ROOTers,
>>>>> 
>>>>> I'm wondering why TObjArray is designed in such a way that an attempt to
>>>>> access the [fLowerBound] element of an empty TObjArray doesn't cause any
>>>>> error (please see the example attached). Moreover, it has a rather
>>>>> unpleasant side effect: after the access GetEntriesFast() reports that
>>>>> the array isn't empty any more. Is it done purposely?
>>>>> 
>>>>> Regards,
>>>>> 
>>>>> Vassili
>>>>> 
>>>>> 
>>>> 
>>>> 
>>> 
>>> 
>>