Re: Default argument for C++ shared_ptr in Python

On Thu, 13 Feb 2020 at 14:13, Michael Riesch <[email protected]> wrote:

> Hello all,
>
> First of all, thanks for your work on SWIG! Very useful tool!
>
> There is one issue, though: I am trying to build a Python interface for
> a C++ library with different classes. Now I would like to write a
> function with an optional argument of std::shared_ptr<device>, where
> device is one of those C++ classes. SWIG generates the interface code
> which compiles readily. And as long as I call the function with an
> argument (also if I provide "None") everything works. However, if I
> don't provide an argument, Python does not find the correct signature.
>
> SWIG 2.0.12 works fine, by the way. Apparently, the pointer support was
> revised in SWIG 3, which is where my troubles start.
>
> This behavior can be reproduced with the minimal (not) working example:
>
> test.hpp:
>
>      #include <iostream>
>      #include <memory>
>
>      class device {
>      public:
>       double number;
>      };
>
>      void my_func(std::shared_ptr<device> ptr = NULL) {
>
>        if (ptr != NULL) {
>
Above should be ptr == NULL.

         std::cout << "Pointer is NULL!" << std::endl;
>        } else {
>          std::cout << "Pointer is not NULL!" << std::endl;
>        }
>      }
>
> test.i:
>
>      %module test
>      %{
>      #include "test.hpp"
>      %}
>
>      %include "stl.i"
>      %include "std_shared_ptr.i"
>
>      %shared_ptr(device)
>
>      %include "test.hpp"
>
> Now SWIG >= 3.0.2 generates the interface code (by calling swig -c++
> -python test.i), where the generated test.py contains the crucial lines
>
>      def my_func(ptr=0):
>        return _test.my_func(ptr)
>      my_func = _test.my_func
>
> If I replace 0 with None, everything works fine. But in the original
> version it complains that there is no matching signature.
>
> This is a bug due to a change in optional argument handling where they are
by default obtained from Python rather than C++ code. Unfortunately this
won't be so easy to fix. Can you please raise a bug on Github? The
workaround is documented at
http://www.swig.org/Doc4.0/Python.html#Python_default_args to restore
obtaining the default argument from C++ code. You would add this before
SWIG parses my_func:

%feature("python:cdefaultargs") my_func;

William

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