[boost] How to make a soft copy of a temporary Python object inside C++

Subject: [boost] How to make a soft copy of a temporary Python object inside C++
From: Raul Durand (raul.durand_at_[hidden])
Date: 2010-12-07 09:06:38

Hello everybody,

Maybe it is obvious but I could not figure out how to solve it yet..

Consider the A and B classes:

Class B
{
public:
   int func() {return 1; }
};

class A
{
public:
  void setB(B & b_instance)
  {
     b = &b_instance;
     // if in Python setB is called as:  a=A(); a.setB(*B()*)   where B()
creates a temporary,
     // the b pointer becomes invalid after setB() function gets out of
scope.
  }

  B * b;

};

BOOST_PYTHON_MODULE(my_module)
  {
    _class<B>("B")
    .def("func",&B::func);

    _class<A>("A")
    .def("setB", &A::setB);

  }

The function A::setB sets the b data member of class A.
b is a pointer to an object of class B.

In Python I can use:

  a=A()
  b=B()
  a.setB(b)

that's ok since 'b' exists as long as 'a', which could be dangerous.

Things get worse when in Python I write:

  a=A()
  a.setB(B())

because the pointer b in class A gets invalid when the setB function gets
out of scope.

So, how can I avoid Python for destroying the temporary and keep the object
alive in C++?
Does it have anything to do with the object ownership? Is there a way to
change the ownership?

Can I make a soft copy of the b_instance object inside the setB function? so
it would increase the object counter and keep it alive and consequently the
b pointer will not get invalid.
I want to avoid making a deep copy.

Thanks in advance.

Raul Durand