Re: [boost] GIL Image Processing Library - Undefined Behavior

From: Ullrich Koethe (koethe_at_[hidden])
Date: 2006-10-19 14:34:07

Hi again,

since there was no response to my earlier questions about undefined 
behavior, I'd like to bring this up again:

GIL makes extensive use of reinterpret_cast and gil_reinterpret_cast. The 
former is, for example, used in code similar to the following:

short * imagedata = new short[3*image_size];
rgb16s_t * rgbdata = (rgb16s_t *)imagedata;

gil_reinterpret_cast is defined as

template <class OutPtr, class In>
OutPtr gil_reinterpret_cast(In * p)
{
     return static_cast<OutPtr>(static_cast<void *>(p));
}

I suspect that undefined behavior results in both cases. IIRC, there is no 
guarantee that the memory layout of an rgb array is equal to that of a 
corresponding scalar array with three times as many elements (although 
this will probably work in practice). Likewise, a static_cast from void * 
is only allowed back to the original pointer type, not to a different type.

Can someone more familiar with the fine points of C++ confirm these 
suspicions? If they are true, I think the issue should be resolved before 
inclusion into boost.

Regards
Ulli
-- 
  ________________________________________________________________
|                                                                |
| Ullrich Koethe  Universitaet Hamburg / University of Hamburg   |
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