Boost Python properties/getter functions for strings

Shawn McGrath

Hi, I'm trying to expose a C++ class' internals to python via
boost::python. I can do integer/boolean functions fine, but as soon
as I do a string get/set it craps out.

Expand|Select|Wrap|Line Numbers

 boost::python::class_<Entity, std::auto_ptr<pyEntity("Entity")

//publics

.def("isActive", &Entity::isActive) //bool

.def("activate", &Entity::activate) //bool

.def("deactivate", &Entity::deactivate) //bool

//...

.add_property("name", &Entity::getName) //compile error (1)

.def("getName", &Entity::getName,

boost::python::return_internal_reference<>()); //runtime error(2)
 
Compile error (1) shows this: C:/MinGW/include/boost/python/detail/

invoke.hpp: In function `PyObject*

boost::python::detail::invoke(boost::python::detail::invoke_tag_<

false,  true>, const RC&, F&, TC&) [with RC =

boost::python::detail::specify_a_return_value_policy_to_wrap_functions_returning<const

std::string&>, F = const std::string&(rsblsb::Entity::*)() const, TC =

boost::python::arg_from_python<rsblsb::Entity&>]':

C:/MinGW/include/boost/python/detail/caller.hpp:199:   instantiated

from `PyObject* boost::python::detail::caller_arity<1u>::impl<F,

Policies, Sig>::operator()(PyObject*, PyObject*) [with F = const

std::string&(rsblsb::Entity::*)() const, Policies =

boost::python::default_call_policies, Sig = boost::mpl::vector2<const

std::string&, rsblsb::Entity&>]'

C:/MinGW/include/boost/python/object/py_function.hpp:38:

instantiated from `PyObject*

boost::python::objects::caller_py_function_impl<Caller>::operator()

(PyObject*, PyObject*) [with Caller =

boost::python::detail::caller<const std::string&(rsblsb::Entity::*)()

const, boost::python::default_call_policies, boost::mpl::vector2<const

std::string&, rsblsb::Entity&]'

C:\Game\svn\Platform\Framework\Python\PyModuleSetup.cc:58:

instantiated from here

C:/MinGW/include/boost/python/detail/invoke.hpp:88: error: no match

for call to `(const

boost::python::detail::specify_a_return_value_policy_to_wrap_functions_returning<const

std::string&>) (const std::basic_string<char, std::char_traits<char>,

std::allocator<char&)'

Runtime error 2 just crashes whenever I try:
import modulename
I = modulename.Entity()
I.getName()

Anyone have any idea what I can try? thanks a lot!

-Shawn.

Mar 19 '07 #1

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viewed: 3938

4 Replies

Shawn McGrath

I forgot to mention, getname is defined as:
const std::string &Entity::getName() const;

Mar 19 '07 #2

Shawn McGrath

On Mar 19, 12:00 pm, "Shawn McGrath" <shawn.mcgr...@gmail.comwrote:

I forgot to mention, getname is defined as:
const std::string &Entity::getName() const;

After more reading I found the copy_const_reference, and replaced:
boost::python::return_internal_reference<>());
with:

boost::python::return_value_policy<boost::python:: copy_const_reference>());

and it fixed my problem. Is there any downside to using
copy_const_reference over return_internal_reference?

Thanks,
Shawn.

Mar 19 '07 #3

Jon Clements

On 19 Mar, 16:40, "Shawn McGrath" <shawn.mcgr...@gmail.comwrote:

On Mar 19, 12:00 pm, "Shawn McGrath" <shawn.mcgr...@gmail.comwrote:

I forgot to mention, getname is defined as:
const std::string &Entity::getName() const;

After more reading I found the copy_const_reference, and replaced:
boost::python::return_internal_reference<>());
with:

boost::python::return_value_policy<boost::python:: copy_const_reference>());

and it fixed my problem. Is there any downside to using
copy_const_reference over return_internal_reference?

You might get some answers here; if not, can I suggest
http://mail.python.org/mailman/listinfo/c++-sig ? I think a lot of the
Boost.Python developers hang around on that list.
hth,

Jon.

Mar 19 '07 #4

Shawn McGrath

On Mar 19, 12:49 pm, "Jon Clements" <jon...@googlemail.comwrote:

On 19 Mar, 16:40, "Shawn McGrath" <shawn.mcgr...@gmail.comwrote:

On Mar 19, 12:00 pm, "Shawn McGrath" <shawn.mcgr...@gmail.comwrote:

I forgot to mention, getname is defined as:
const std::string &Entity::getName() const;

After more reading I found the copy_const_reference, and replaced:
boost::python::return_internal_reference<>());
with:

boost::python::return_value_policy<boost::python:: copy_const_reference>());

and it fixed my problem. Is there any downside to using
copy_const_reference over return_internal_reference?

You might get some answers here; if not, can I suggesthttp://mail.python.org/mailman/listinfo/c++-sig? I think a lot of the
Boost.Python developers hang around on that list.

hth,

Jon.

Cool thanks a lot.

The problem is actually due to python's strings being immutable (I
knew this, but I thought returning const std::string& would do it).
return_internal_reference<works for other pointers/references, just
not strings.

(I just answered it so if it gets searched later on people will find
the solution)

-Shawn.

Mar 19 '07 #5

Boost Python properties/getter functions for strings

This discussion thread is closed

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