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Comment: demonstrate usage of shared_ptr as explaint in the Sig-C++ mailinglist
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| give away ownership of objects in C++. The following suggests a way to handle shared_ptr objects in Python, introducing the notation {{{(+ptr).foo()}}} in python to immitate C++'s {{{(*ptr).foo()}}}: |
give away ownership of objects in C++. Theese kind of smart pointer are automatically handled, if you declare their existence, when declaring the class to boost.python. E.g. |
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| class_<A>("A",init<>()) .def("create",&A::create,return_value_policy<return_by_value>()) |
class_<A, shared_ptr<A> >("A",init<>()) .def("create",&A::create ) |
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| ; class_< shared_ptr<A> >("A_ptr", init<const shared_ptr<A>& >()) .def("__pos__",&boost::shared_ptr<A>::get,return_internal_reference<>()) ; |
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| an_A = A_ptr.create() print (+an_A).hello() |
an_A = A.create() print an_A.hello() |
Pointers and smart pointers
Since Python handles memory allocation and garbage collection automatically, the concept "pointer" is not meaningful within Python. However, many C++ API exposes either raw pointers or shared pointers, and to wrap such APIs one would need to deal with pointers.
Pointers (raw C++ pointers)
The life time of C++ objects created by "new A" kan be handled by Pythons garbage collection by using the manage_new_object storage policy:
{{{struct A {
- static A* create () { return new A; } std::string hello () { return "Hello, is there anybody in there?"; }
};
BOOST_PYTHON_MODULE(shared_ptr) {
class_<A>("A",no_init)
def("create",&A::create,return_value_policy<manage_new_object>())
- staticmethod("create")
def("hello",&A::hello) ;
} }}}
A sample python program:
from pointer import * an_A = A.create() print an_A.hello()
Smart pointers
Smart pointers, e.g. boost::shared_ptr<T>, is another common way to give away ownership of objects in C++. Theese kind of smart pointer are automatically handled, if you declare their existence, when declaring the class to boost.python.
E.g.
{{{#include <boost/shared_ptr.h> using namespace boost;
struct A {
shared_ptr<A> create () { return shared_ptr<A>(new A); } std::string hello () { return "Just nod if you can hear me!"; }
};
BOOST_PYTHON_MODULE(shared_ptr) {
class_<A, shared_ptr<A> >("A",init<>())
def("create",&A::create )
- staticmethod("create")
def("hello",&A::hello)
} }}}
A sample python program:
from shared_ptr import * an_A = A.create() print an_A.hello()