<boost/python/instance_holder.hpp> provides class instance_holder, the base class for types which hold C++ instances of wrapped classes.

instance_holder is an abstract base class whose concrete derived classes hold C++ class instances within their Python object wrappers. To allow multiple inheritance in Python from C++ class wrappers, each such Python object contains a chain of instance_holders. When an __init__ function for a wrapped C++ class is invoked, a new instance_holder instance is created and installed in the Python object using its install() function. Each concrete class derived from instance_holder must provide a holds() implementation which allows Boost.Python to query it for the type(s) it is holding. In order to support the held type's wrapped constructor(s), the class must also provide constructors that can accept an initial PyObject* argument referring to the owning Python object, and which forward the rest of their arguments to the constructor of the held type. The initial argument is needed to enable virtual function overriding in Python, and may be ignored, depending on the specific instance_holder subclass.

namespace boost { namespace python
{
  class instance_holder : noncopyable
  {
   public:
      // destructor
      virtual ~instance_holder();

      // instance_holder modifiers
      void install(PyObject* inst) throw();

      // instance_holder observers
      virtual void* holds(type_info) = 0;
  };
}}

virtual ~instance_holder();

void install(PyObject* inst) throw();

virtual void *holds(type_info x) = 0;

The following is a simplified version of the instance holder template used by Boost.Python to wrap classes held by smart pointers:

template <class SmartPtr, class Value>
struct pointer_holder : instance_holder
{
   // construct from the SmartPtr type
   pointer_holder(SmartPtr p)
       :m_p(p)

   // Forwarding constructors for the held type
   pointer_holder(PyObject*)
       :m_p(new Value())
   {
   }

   template<class A0>
   pointer_holder(PyObject*,A0 a0)
       :m_p(new Value(a0))
   {
   }

   template<class A0,class A1>
   pointer_holder(PyObject*,A0 a0,A1 a1)
       :m_p(new Value(a0,a1))
   {
   }
   ...

 private: // required holder implementation
   void* holds(type_info dst_t)
   {
       // holds an instance of the SmartPtr type...
       if (dst_t == python::type_id<SmartPtr>())
           return &this->m_p;

       // ...and an instance of the SmartPtr's element_type, if the
       // pointer is non-null
       return python::type_id<Value>() == dst_t ? &*this->m_p : 0;
   }

 private: // data members
   SmartPtr m_p;
};