Re: [Boost-users] [boost.intrusive] Assertion failure with SunCC

From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2007-06-05 12:29:41

Zeljko Vrba wrote:
> On Tue, Jun 05, 2007 at 04:35:16PM +0200, Ion Gaztañaga wrote:
>> I just realized about this just after sending my previous mail. SunCC
>> seems to use the classical cfront approach encoding a pointer to member
>> as offset + 1. So it seems that parent_from_member detection is correct
>> for SunCC.
>>
> Slightly off-topic, but: do you know the background WHY did they decide
> to implement ptr-to-member as offset +1?

Because you need to represent a null pointer to data member. So they 
choose to represent a null pointer to member as 0 (which is a valid 
offset inside the object) and add 1 to the valid offset to represent a 
valid offset. The offset + 1 approach is shared by more implementations 
(CW for windows and Digital Mars, for example). This was the historical 
presentation C-front chose, so I think many compilers implement it like 
that because of this historical heritage.

Other implementations used another encoding. For example, the Itanium 
C++ ABI says (http://www.codesourcery.com/cxx-abi/abi.html#member-pointers):

"A pointer to data member is an offset from the base address of the 
class object containing it, represented as a ptrdiff_t. It has the size 
and alignment attributes of a ptrdiff_t. A NULL pointer is represented 
as -1."

>>
> I have tracked the conversion to here myself.  I wondered whether this was
> a compiler bug, but the following simple program works, ie. it does NOT
> raise an assertion failure.  I hope I managed to reproduce correctly the
> usage pattern of ptr-to-member..
> 
> [snip]
> 
> I'm kinda clueless as how to proceed from here.  If it _is_ a genuine compiler
> bug, we need a more complex test-case, which I'm unable to produce myself
> (as I don't know how many intermediate instantiation steps there are in
> the intrusive library).

This seems a strange issue. I'll try to investigate it when I install 
the Sun compiler. Thanks for your effort!

Ion