[C++-sig] Arrays and Boost.Python

[Patrick Hartling]

David Abrahams wrote:
> Patrick Hartling <patrick at vrac.iastate.edu> writes:
> 
> 
>>Does Boost.Python (from Boost 1.30.0 or the Boost CVS trunk) provide
>>handling of arrays as public data members in a C++ class?  Using
>>Boost.Python from Boost 1.30.0, I get an error from GCC 3.2 about line
>>68 of boost/python/data_members.hpp stating that "ISO C++ forbids
>>assignment of arrays."  If I change the array definition in my
>>Boost.Python code so that the data member is read-only instead of
>>read/write, that gets rid of the error.  
> 
> 
> But I doubt it will do anything other than raise an exception when
> you try to use it from Python.

That is true.  I hadn't actually tested it prior to reading your 
response.  Oh well.

>>I don't mind doing that in this case--really I am just curious about
>>how Boost.Python handles C++ arrays.
> 
> 
> So far, it doesn't.  If you can write down precisely what semantics
> you'd like to see, considering what would constitute an argument match
> for overloaded functions, we might be able to do something about it.

I hadn't really thought about it much so far.  It was one of those 
things I was hoping would "just work."  The fact that it is not handled 
right now is not a big problem for me personally.

What I see as a problem is that arrays in C/C++ are just syntactic sugar 
for accessing a block of memory.  In languages such as Java and C# (and 
even Managed C++ from what I have seen so far), their handling is more 
formalized.  In other words, there is no question that a function with a 
signature such as 'void f(int[] intList)' takes an array of integers. 
In the land of C/C++, we have to deal with the uncertainty of 'void 
f(int* intList)'.  Is it a pointer to a single integer, an array, or a 
pointer into the middle of an array?  All that is certain is that it 
points to four bytes of memory (for ILP32, anyway).

Honestly, I don't feel too strongly about seeing support for arrays in 
Boost.Python.  Considering that neither lists nor tuples in Python 
provide quite the right semantics for a C-style array, the concept may 
just not map into the world of Python very well.  Of course, I could be 
wrong about all of this--I'm just a user, not an expert.  :)

  -Patrick


-- 
Patrick L. Hartling                     | Research Assistant, VRAC
patrick at vrac.iastate.edu                | 2624 Howe Hall: 1.515.294.4916
http://www.137.org/patrick/             | http://www.vrac.iastate.edu/