Problem wrapping a C++ class with a mutex in a member, using Swig

Question

I have a C++ class containing a (Poco) mutex:

class ClassWithMutex
{
    public:
                                ClassWithMutex();
        virtual                 ~ClassWithMutex();

        Poco::Mutex             mMyMutex;

    private:
        ClassWithMutex(const ClassWithMutex& );
        ClassWithMutex& operator=(const ClassWithMutex& other);

};

And another class, using it:

class ClassHavingAClassWithMutex
{
    public:
        ClassHavingAClassWithMutex();
        ~ClassHavingAClassWithMutex();
        ClassWithMutex      A;

    private:
        ClassHavingAClassWithMutex(const ClassHavingAClassWithMutex&);
        ClassHavingAClassWithMutex& ClassHavingAClassWithMutex::operator=(const ClassHavingAClassWithMutex& other);

};

When trying to create a wrapper for ClassHavingAClassWithMutex, I get an error:

Error   C2248   'ClassWithMutex::operator =': cannot access private member declared in class 'ClassWithMutex'   mvr C:\builds\vs2015\mvr\python_package\src\mvr\mvrPYTHON_wrap.cxx  6493    

where the Swig generated code looks like this:

SWIGINTERN PyObject *_wrap_ClassWithMutex_mMyMutex_get(PyObject *SWIGUNUSEDPARM(self), PyObject *args) {
  PyObject *resultobj = 0;
  ClassWithMutex *arg1 = (ClassWithMutex *) 0 ;
  void *argp1 = 0 ;
  int res1 = 0 ;
  PyObject *swig_obj[1] ;
  Poco::Mutex result;

  if (!args) SWIG_fail;
  swig_obj[0] = args;
  res1 = SWIG_ConvertPtr(swig_obj[0], &argp1,SWIGTYPE_p_ClassWithMutex, 0 |  0 );
  if (!SWIG_IsOK(res1)) {
    SWIG_exception_fail(SWIG_ArgError(res1), "in method '" "ClassWithMutex_mMyMutex_get" "', argument " "1"" of type '" "ClassWithMutex *""'"); 
  }
  arg1 = reinterpret_cast< ClassWithMutex * >(argp1);
  {
    SWIG_PYTHON_THREAD_BEGIN_ALLOW;
    result =  ((arg1)->mMyMutex);
    SWIG_PYTHON_THREAD_END_ALLOW;
  }
  resultobj = SWIG_NewPointerObj((new Poco::Mutex(static_cast< const Poco::Mutex& >(result))), SWIGTYPE_p_Poco__Mutex, SWIG_POINTER_OWN |  0 );
  return resultobj;
fail:
  return NULL;
}

and errors emitted from this line:

if (arg1) (arg1)->A = *arg2;
and
resultobj = SWIG_NewPointerObj((new Poco::Mutex(static_cast< const Poco::Mutex& >(result))), SWIGTYPE_p_Poco__Mutex, SWIG_POINTER_OWN |  0 );

Swig interface file:

%immutable
%include "aiClassWithMutex.h"
%include "ClassHavingAClassWithMutex.h"
%mutable

Any suggestions on how to properly wrap the classes above with Swig? I made the copy and assignment ctor's private in the classes above, in order to prevent any copying, but it seems swig insist on it?

Answer 1

That's not caused by the copy constructor but the property setter (b/c A is public). As discussed in the comments, one option is to hide it, by adding:

%ignore ClassWithMutex::mMyMutex;

before the %include "aiClassWithMutex.h" in the .i file.

Also as discussed, SWIG resorts to copying when making mMyMutex immutable. AFAICT, there's no way around that other than writing a typemap and misusing the optimal setting (to prevent the construction of a temporary in the first place). With that, a functional .i file looks like:

%module mut

%{  
#include "aiClassWithMutex.h"
#include "ClassHavingAClassWithMutex.h"
%}      

%feature("immutable", 1) ClassWithMutex::mMyMutex;
%typemap(out, optimal="1") Poco::Mutex %{
  $result = SWIG_NewPointerObj(($1_ltype*)&$1, $&1_descriptor, 0);
%}      
%include "aiClassWithMutex.h"
%include "ClassHavingAClassWithMutex.h"