SwFormat::DerviedFrom()

[Chris Sherlock]

On 3 Jan 2016, at 8:43 PM, Markus Mohrhard <markus.mohrhard at googlemail.com> wrote:
> 
> Hey
> 
> On Sun, Jan 3, 2016 at 9:20 AM, Chris Sherlock <chris.sherlock79 at gmail.com> wrote:
> Hi all, 
> 
> I’ve been getting an OSL_ENSURE failure when I run LibreOffice on my Ubuntu Linux VM. 
> 
> The error is:
> 
> warn:legacy.osl:25546:1:sw/source/core/attr/format.cxx:227: SwFormat::~SwFormat: Def dependents!
> 
> This appears to be occuring because the OSL_ENSURE is calling on DerivedFrom(), which actually returns the following:
> 
> return const_cast<SwFormat*>(static_cast<const SwFormat*>(GetRegisteredIn()));
> 
> GetRegisteredIn() is just an event source of type SwModify, the odd thing is that it calls on GetRegistedIn() which is NOT a member of SwModify (or any child classes). 
> 
>> In fact, I’m surprised that this even compiles, as I was under the impression that a static_cast was a compile time check...
>> 
>> I can only help you with the C++ part but not explain why it makes sense to call that code.
>> 
>> The code calls SwFormat::GetRegisteredIn which is actually SwClient::GetRegisteredIn (through the inheritance hierarchy SwFormat->SwModify->SwClient). SwClient::GetRegisteredIn returns a pointer to a SwModify where the code actually assumes that it always returns a SwFormat.

Ah - I appear to have misread the inheritance on that class. Thanks :-)

> http://opengrok.libreoffice.org/xref/core/sw/inc/format.hxx#110
> 
> Anyway, what *exactly* is this attempting to do? The name looks like it’s doing a debugging check to ensure that the SwFormat class was registered in a derived class, but I can’t see how this would ever work!
> 
> 
>> It looks a bit like it is using some generic writer code to store the inheritance of the styles.
>  
> 
> In fact, wouldn’t a dynamic_cast be better, as this most specifically down or upcasts a pointer, and returns NULL if it fails?
> 
>> That depends on all the code around it. Here the code just assumes that all returned SwModify objects are actually SwFormat objects.

I did some more reading of static_cast - my understanding is that static_cast will up-cast or down-cast the object without any runtime safety checks. What isn’t clear is what happens when you try to do a static_cast on two entirely unrelated classes. Would DerivedFrom() work by just doing a static_cast? 

Regardless, the OSL_ENSURE is expecting a true, but is getting a false so the original writers of the code think that something is going wrong - I suppose that ultimately it matters more whether there is an issue in the check, or literally something is going badly wrong in the code and the object is *not* derived correctly and there’s a bug there somewhere…

Sorry, not experienced at all in this module, and casting in c++ with the different static, dynamic and reinterpret casts has always been a bit tricky for me (well, not so much now that I’ve had a good read). 

If these questions are a bit obvious, please let me know and I’ll code up some scenarios to get a better understanding of how casting objects in C++ works (in fact, I’ll probably do this anyway, I’m knee-deep in refactoring VCL code). 

Chris

> 
> Any advise on this would be greatly appreciated, got me thoroughly tricked...
> 
> Chris
> 
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