...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
<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; }; }}
void 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.
installs the new instance at the head of the Python object's chain of held instances.
nothing
virtual void *holds(type_info x) = 0;
A pointer to an object of the type described by x
if *this
contains such an object, 0 otherwise.
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; };