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Boost.PythonHeader <boost/python/instance_holder.hpp> |
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_holderinstance_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;
};
}}
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.