Re: Custom director overrides

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Hi. Thanks for replying.


Rokas Kupstys via Swig-user <[email protected]> writes:

> In case of python, output parameters are returned by function itself.
> If your f function had a void return type then wrapped function would
> return modified string s. If return type was say int, then function
> would return a tuple (result_int, output_s). See
> https://www.swig.org/Doc4.2/Python.html#Python_nn46

These functions work to allow a python program to call a swig-wrapped
function.

But as far as I can tell, they do NOT work to allow a Python director
overload to provide its own flavor of the virtual function. The
documentation of the various functions in

  https://www.swig.org/Doc4.2/Library.html#Library_nn12

makes that clear; it says:

  It is important to emphasize that this function does not mutate the
  string value passed---instead it makes a copy of the input value,
  mutates it, and returns it as a result.

I think I need to %extend or %rename something to change the
director-generated wrapping function to be able to write into the char*
buffer.

I'm attaching a trivial example. This can be built on my Debian box with

  swig \
    -c++ \
    -python \
    -o Tst_wrap.cpp \
    Tst.i && \
  g++ \
    -shared \
    -fPIC \
    -o _Tst.cpython-312-x86_64-linux-gnu.so Tst_wrap.cpp Tst.cc \
    -I/usr/include/python3.12 \
    -I. && \
  python3 tst.py

This has a class Tst with two virtual functions: f0() and f1(). And a
non-virtual function g() that calls f0() and f1(). The non-derived
implementation of f0() and f1() modify the memory pointed to by its
argument.

The sample program tst.py calls Tst::g() to show that indeed f0() and
f1() do modify the input memory.

And the sample program tst.py ALSO creates a derived class Tst2 using a
swig director. It provides its own implentations of f0() and f1(), but
those cannot modify the input memory, without some extra swig stuff that
I cannot figure out.

Help appreciated.

Thanks


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