[C++-sig] 1-D numeric array <-> std::vector<double>

[David Abrahams]

John Hunter <jdhunter at ace.bsd.uchicago.edu> writes:

> I have been experimenting with boost::python v2, and just got tuple
> <-> std::vector<double> working in my extension code using the
> container_conversions in scitbx.  Quite impressive!
>
> Now, I am wondering if someone has implemented a similar functionality
> for transparent passing of Numeric 1D arrays to and from C++ functions
> which expect std::vector<double>.
>
> If this can be done, could someone point me to some example code or
> documentation.


Look at slice_from_python in scitbx/boost_python/slice.cpp and
scitbx/include/scitbx/boost_python/slice.h.  This is an rvalue
from_python converter for a class called "slice".

The protocol for an rvalue converter (which is what you want, since
there's no std::vector<double> in a Numeric array) is that:

1. The "convertible" function must return non-NULL if overload
   resolution should succeed with that converter.  In your case, if
   the source object is a 1D Numeric array.  That pointer can be
   anything, but it should not be a resource that might need to be
   deallocated.

2. The "construct" function should downcast the data to
   boost::python::converter::rvalue_from_python_storage<std::vector<double>
   >* and construct the result vector in its storage.bytes using
   placement new.

3. As soon as construction succeeds, you must set data->convertible to
   point at the storage.

This is all undocumented because it's so hairy.  I plan to come up
with something easier in the next few months so it's fit for users ;-)

-- 
                       David Abrahams
   dave at boost-consulting.com * http://www.boost-consulting.com
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