How to apply SWIG OUTPUT typemaps for class types in Python?

Question

I am having some trouble generating a Python wrapper around a C++ library using SWIG (version 3.0.6).

My issue relates to applying the OUTPUT typemap, specifically i

Answer 1

This question has appeared as unresolved for quite some time, so I thought that I better provide a solution to the question. The OUTPUT typemap only applies to simple types, so a solution is given by combining an in and an argout typemap.

Consider the situation, where we have a C++ class SampleImpl implementing a C++ interface SampleBase, which is technically not an interface, since it involves the implementation of a virtual destructor. Suppose we have a static function, which returns an error code and an implementation of the interface. The latter as a reference to a pointer, which is the situation above.

Interface header:

// Sample.hpp
#pragma once
namespace Module {
  class SampleBase {
  public:
#ifndef SWIG
    // Hint to the programmer to implement this function
    static int SampleCreate(SampleBase *&obj);
#endif
    virtual ~SampleBase() = default;
  };
}

Implementation header:

// Sample_impl.hpp
#pragma once
#include "Sample.hpp"

namespace Module {
  class SampleImpl : public SampleBase {
  public:
    static int SampleCreate(Module::SampleBase *&obj);

    SampleImpl();
    virtual ~SampleImpl();
  private:
    float a;
  };
}

Implementation:

// Sample_impl.cpp
#include "Sample_impl.hpp"
#include 

namespace Module {
  int SampleImpl::SampleCreate(Module::SampleBase*& obj) {
    obj = (SampleBase*) new SampleImpl();
    return 0;
  }
  SampleImpl::SampleImpl() {
    printf("SampleImpl::SampleImpl()\n");
  }

  SampleImpl::~SampleImpl() {
    printf("SampleImpl::~SampleImpl()\n");
  }
}

SWIG interface (using argout typemap)

// example.i
%module example
%{
  #define SWIG_FILE_WITH_INIT
  #include "Sample.hpp"
  #include "Sample_impl.hpp"
%}

%include "typemaps.i"

%typemap(in, numinputs=0) Module::SampleBase *&obj (Module::SampleBase *temp) {
  $1 = &temp;
}

%typemap(argout) Module::SampleBase *& {
  PyObject* temp = NULL;
  if (!PyList_Check($result)) {
    temp = $result;
    $result = PyList_New(1);
    PyList_SetItem($result, 0, temp);

    // Create shadow object (do not use SWIG_POINTER_NEW)
    temp = SWIG_NewPointerObj(SWIG_as_voidptr(*$1),
             $descriptor(Module::SampleBase*),
             SWIG_POINTER_OWN | 0);

    PyList_Append($result, temp);
    Py_DECREF(temp);
  }
}

Usage in Python

import example

// Creating specialization
obj = example.SampleImpl()
del obj

// Creation of object using output typemap
errorCode, obj = example.SampleImpl_SampleCreate()
del obj