[C++-sig] Re: Re: Re: map key

[David Abrahams]

"Ralf W. Grosse-Kunstleve" <rwgk at yahoo.com> writes:

> --- Mike Rovner <mike at bindkey.com> wrote:

> P.S.: I attach two relevant sections from an article I am currently
> working on (for a non-CS audience). Comments are very welcome.

It's a nice article!

<snip>
>
> Considerations of the type discussed in the previous section directly
> lead to the following situation::
>
>   >>> a = flex.int((1,2,3))
>   >>> b = flex.double((2,3,4))
>   >>> a * b
>   TypeError: unsupported operand type(s) for *:
>   'scitbx_boost.array_family.flex_scitbx_ext.int' and
>   'scitbx_boost.array_family.flex_scitbx_ext.double'
>
> In passing we note that there is a simple solution which will produce
> the desired result::
>
>   a.as_double() * b
>
> However, for the purpose of this discussion let's pretend that this
> solution does not exist. Of course the first question is: what is
> the reason for the apparently stupid limitation?
>
> As mentioned before, the Python ``flex`` types are implemented as
> instantiations of C++ class templates. This ensures that all array
> operations are very fast. However, from the discussion in the previous
> section it follows that exposing the full class with its many member
> functions to Python **for each element type** (``int``, ``double``,
> ``miller::index<>``, etc.) creates very sizable object files. If only
> *homogeneous* operators (``int * int``, ``double * double``, etc.) are
> used the combined size of the object files scales linearly with the
> number of element types involved. However, if the library is expanded
> to support *heterogeneous* operators (``int * double``, ``double *
> int``, etc.) the combined object files grow proportional to the square
> of the number of array element types involved! With current technology
> this is simply prohibitive.

You might consider the impact of using something like weave for a case
like this.  It generates and compiles different instantiations of the
same code on demand.

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