I would like to know whether major c++ compilers on "major" computer
architectures support namespaces.

This is rather practical:  I would like to use namespaces.  But I can
only do so if for each architecture to be targeted,  there exists at
least one good c++ compiler supporting them.

Architectures should include: Linux, Windows, Sparc, AIX, PowerPC, ARM,..

Hi jacobi-ga, 
   The namespaces feature was added to the C++ language as part of the
ISO C++ 1998 standards. Given that its been six years since then, and
the namespace support is comparatively trivial to add, I will be
surprised if you find any current compiler that does not support them!
For your convenience, I have provided links to two major ISO C++ 1998
compliant compilers for each platform you mentioned.

-> Windows 
       - Microsoft Visual Studio
         (http://msdn.microsoft.com/vstudio/productinfo/default.aspx)
       - Intel C++ Compiler
         (http://www.intel.com/software/products/compilers/cwin/)

-> Linux
       - The GNU Compiler Collection
         (http://gcc.gnu.org/)
       - Intel C++ Compiler for Linux
         (http://www.intel.com/software/products/compilers/clin/)

-> SPARC
       - Sun Studio 8
         (http://wwws.sun.com/software/products/studio/datasheet.html)
       - The GNU Compiler Collection
         (http://gcc.gnu.org/)
 
-> PowerPC
       - IBM XL C/C++ Advanced Edition 
         (http://www-306.ibm.com/software/awdtools/vacpp/features/xlcpp-mac.html)
       - PowerPC Software Development Tools 
         (http://www.tasking.com/products/PPC/)

-> AIX
       - IBM VisualAGE C++ Compiler
         (http://www-306.ibm.com/software/awdtools/vacpp/)
       - The GNU Compiler Collection
         (http://gcc.gnu.org/)

Note that there are literally hundreds of complient free and
commercial compilers available for the Windows  and Linux platforms.
For the others, the GNU compiler collection supports all the platforms
on your list, and then some! For a list of supported platforms
supported by GCC , please see:
          - GCC Installation: Supported Platforms
            (http://gcc.gnu.org/install/specific.html)

-------------------------------------
Hope this helps.
If you need any clarifications, just ask!
Regards,
Theta-ga
:-)


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Google Search Terms used:
 AIX C++ compiler
 SPARC C++ compiler

Thanks.  You saved me lots of time