## boost/python/instance_holder.hpp

### Introduction

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

### Class template `instance_holder`

`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;
      };
    }}

#### Class `intance_holder` destructor

    virtual ~instance_holder();

Effects

  destroys the object

#### Class `intance_holder` modifiers

    void install(PyObject* inst) throw();

Requires

  `inst` is a Python instance of a wrapped C++ class type, or is a type derived from a wrapped C++ class type.

Effects

  installs the new instance at the head of the Python object's chain of held instances.

Throws

  nothing

#### Class `intance_holder` observers

    virtual void *holds(type_info x) = 0;

Returns

  A pointer to an object of the type described by `x` if `*this` contains such an object, 0 otherwise.

### Examples

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;
    };