[C++-sig] Boost.Python at runtime

David Abrahams dave at boost-consulting.com
Thu Aug 18 01:07:13 CEST 2005

Allen Bierbaum <allenb at vrsource.org> writes:

> David Abrahams wrote:
>>Allen Bierbaum <allenb at vrsource.org> writes:
> Is this an area that anyone else has ever asked about or have you ever 
> toyed with the idea of having run-time capabilities?

We have about as many run-time capabilities as are possible in

>>and what you're asking isn't possible to do in portable C++ without
>>knowing all of the classes and member function signatures that you
>>might want to wrap at compile-time anyway.

> I could be wrong, but I think I will have all this anyway (or be able to 
> get it) through a reflective interface in the library.  Basically each 
> run-time class 

Too late.  The information is needed at compile-time, as I said.

> can return a type descriptor object that can return as 
> much information as necessary.  I don't pretend to know C++ as well as 
> you though, so maybe I am missing something.  Is there some information 
> or knowledge that would be needed that could not be stored and then 
> looked up later at run-time when registering a wrapper?

It doesn't matter if it can be looked up at run-time, because it needs
to be available at compile-time.

>>However, see
> I may be missing something here. I see how this is related, but I
> think what I want to do is quite different.  

Yes, what you want to do includes generating function call wrappers.

> I effectively want to do exactly what I could do at compile time
> with the current code but instead delay the creation so I do it on
> demand

I understand that.

>>You can of course generate Boost.Python wrapping code dynamically and
>>compile it into extension modules that are then loaded into your
>>running Python program :)
> That would be fun but I think my users would wonder a little why the CPU 
> and disk are churning so much at startup. :)

You would cache the results, so you only have to pay once.  There's a
successful system that does it that way: it allows you to embed C++
code directly in your Python and have it compiled on the fly, so I
know some people can tolerate it.  Of course, if you're going to
generate code anyway I'd probably generate traditional Python/C API
code rather than Boost.Python stuff, since it will compile faster.

Dave Abrahams
Boost Consulting

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