[Insight-developers] itkDefaultConvertPixelTraits consistency issues

[Zachary Pincus]

OK, I filed a bug on this: 2725. I'll fix it in the next day or two  
barring objections.

Zach

On Jan 14, 2006, at 4:44 PM, Peter Cech wrote:

> On Sat, Jan 14, 2006 at 10:59:47 -0800, Zachary Pincus wrote:
>>>> Note especially that traits are created for types 'char' and
>>>> 'unsigned char'. Here's the rub -- some compilers treat 'char' as a
>>>> synonym for 'signed char' and others treat it as synonymous with
>>>> 'unsigned char'. I believe that the C++ spec is silent on this  
>>>> point.
>>>>
>>>> Anyhow, this means (I think) that for compilers of the latter type,
>>>> no traits are ever set up for 'signed char' types.
>>>
>>> In case of GCC (tested versions 2.95.3, 3.2.3, 3.3.5, 3.4.3 and
>>> 4.0.2),
>>> char is treated as signed char for the purpose of arithmetic
>>> calculations. However, in the case of template specialization, char,
>>> signed char and unsigned char are three distinct types. If the
>>> specialization for signed char is missing, non-specialized  
>>> template is
>>> used. This is not the case for short or int types, compiler treats
>>> short/int and signed short/int as the same type. It will be
>>> interesting
>>> to check how other compilers are behaving (there is a little program
>>> attached, just compare the behavior with and without (un)signed char
>>> specialization present).
>
> Just for the record, GCC on Sparc/Solaris shows the same behavior
> (the above paragraph is for x86/Linux, but I'm pretty sure that GCC
> recognizes three distinct char types on all supported platforms).
>
>> Thanks for doing this testing. It's interesting to note that GCC on
>> PowerPC and ARM platforms (except under OS X) treats char as
>> unsigned. (http://www.network-theory.co.uk/docs/gccintro/
>> gccintro_71.html ) I'm not sure if this really represents any ITK
>> users... maybe some Yellow Dog Linux folks.
>>
>> I think that MS VC++ also treats char as signed also, but I don't
>> have a windows box.
>
>> From what I found on internet, both GCC and MS VC++ treat 'char' as
> signed by default, but both have a command line switch to change it to
> unsigned.
>
>>>> I would therefore suggest either adding a traits class for 'signed
>>>> char' to this file, or changing the ambiguous 'char' trait to  
>>>> 'signed
>>>> char'.
>>>
>>> I'm for adding 'signed char' specializations, with 'char' ones
>>> #ifdef-ed
>>> as needed.
>>
>> What sort of #ifdef's do you think might be needed? Should there just
>> be a 'char' traits that specifies whatever it is that the compiler
>> does on that platform, since those are the traits that the IO classes
>> will be compiled with?
>
> What I meant is that some compilers might treat '(un)signed char' and
> 'char' as the same type also for template specializations. In that  
> case,
> presence of both specializations would likely cause a compiler  
> error (or
> at least a warning). In the meantime I found a notice of three
> distinct char types in Bjarne Stroustrup's C++ Glossary
> (http://public.research.att.com/~bs/glossary.html), therefore such
> behavior seems unlikely.
>
> Regards,
> Peter Cech
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