[C++-sig] Re: New-style polymorphism

David Abrahams dave at boost-consulting.com
Mon Jul 26 18:44:25 CEST 2004

I forgot one thing:

David Abrahams <dave at boost-consulting.com> writes:

> I just checked in changes that improve the way we can wrap classes
> with virtual functions in Boost.Python.  This is essentially what's
> described in
> http://www.boost.org/libs/python/todo.html#base-class-for-virtual-function-callback-wrappers
> The differences are most apparent if you compare
> libs/python/test/polymorphism.cpp and
> libs/python/test/polymorphism2.cpp.
>   * you no longer need an initial "self" argument to yourconstructor.
>     Instead you derive your class X from wrapper<X> (I'm looking for
>     better names for that base class)
>   * You no longer need to define a special "default" member function
>     for the default behavior, and you only have to pass one member
>     function pointer to "def()"
>   * You no longer need to use call_method; you instead ask the
>     wrapper<X> base class for the method override, and then call that
>     instead.  If there's no override, you call the default directly.
>     On some broken compilers you may need to use
>     call<result_type>(...) args for some of the calls (always on
>     vc6/7, for reference return types only on some other compielrs).
>     Most other reasonably conforming compilers will convert the result
>     of calling the override directly into the function's return type.
>   * It should be much faster in the case where you have a default
>     implementation that isn't overridden in Python, because you'll
>     usually not have to enter the Python interpreter.
>   * Many problems with dangling references have been eliminated,
>     because you don't have to take a round-trip through Python.  You
>     can wrap virtual functions that return char const* !

    * A C++ pointer or reference to a polymorphic T created from
      Python (one that is actually also a wrapper<T>) can be
      converted back to Python without creating a new Python object.

Dave Abrahams
Boost Consulting

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