...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
instance_holder
instance_holder
synopsis
instance_holder
destructor
instance_holder
modifier functions
instance_holder
observer functions
<boost/python/instance_holder.hpp>
provides
class instance_holder
, the base class for types
which hold C++ instances of wrapped classes.
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_holder
s. 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; }; }}
instance_holder
destructorvirtual ~instance_holder();
instance_holder
modifiersvoid install(PyObject* inst) throw();
inst
is a Python instance of a
wrapped C++ class type, or is a type derived from a wrapped C++
class type.
instance_holder
observersvirtual void* holds(type_info x) = 0;
x
if *this
contains such an object,
0 otherwise.
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; };
Revised 13 November, 2002
© Copyright Dave Abrahams 2002.