[C++-sig] Exposing Friend Functions in C++ Python Boost
Kv Gopalkrishnan
kvgopalkrishnan1 at gmail.com
Mon Jul 21 09:27:16 CEST 2014
Hi Michael
Yes this works. Thank you for your answer and help :)
Kind Regards
KV
On Fri, Jul 18, 2014 at 3:41 PM, Michael Rybakov <opilar at ya.ru> wrote:
> When I said “outside” I meant standalone function.
> Moreover I see you’ve added property with the same functions. This won’t
> work. I would recommend you to look at docs -
> http://www.boost.org/doc/libs/1_55_0/libs/python/doc/tutorial/doc/html/python/exposing.html#python.constructors
>
> Valid wrap with untouched class declaration.
>
> class Turtle
> {
> friend const PV& GetHeading(const Turtle& t);
> friend const PV& GetLeft(const Turtle& t);
> friend const P& GetPoint(const Turtle& t);
> friend void SetPoint(Turtle& t, const P& p);
> public:
>
> ...
>
> private:
> PV h;
> PV l;
> };
>
>
> BOOST_PYTHON_MODULE(TurtleWrapper)
> {
> class_<Turtle>("Turtle")
> ;
>
> def("GetHeading",&Turtle::GetHeading)
> def("GetLeft",&Turtle::GetLeft)
> }
>
> Anyway I would recommend you not to use friend functions to get something
> from private section of class. If you’ll make methods your code will be
> cleaner. Working code:
>
> #include <boost/python.hpp>
>
> struct PV{};
> struct P{};
>
> class Turtle
> {
> public:
> const PV& GetHeading()
> {
> return mHeading;
> }
>
> const PV& GetLeft()
> {
> return mLeft;
> }
>
> const P& GetPoint()
> {
> return mPoint;
> }
>
> void SetPoint(const P& p)
> {
> mPoint = p;
> }
>
> private:
> PV mHeading, mLeft;
> P mPoint;
> };
>
> using namespace boost::python;
>
> BOOST_PYTHON_MODULE(TurtleWrapper)
> {
> class_<Turtle>("Turtle")
> .add_property("Heading", make_function(&Turtle::GetHeading,
> return_internal_reference<>()))
> .add_property("Left", make_function(&Turtle::GetLeft,
> return_internal_reference<>()))
> .add_property("Point", make_function(&Turtle::GetPoint,
> return_internal_reference<>()), &Turtle::SetPoint)
> ;
> }
>
> On 18 Jul 2014, at 15:37, Kv Gopalkrishnan <kvgopalkrishnan1 at gmail.com>
> wrote:
>
> Hi
> Thanks Michael For the answer however I am a bit confused about the
> solution you gave. Do you mean to remove the GetHeading function and paste
> it outside class Turtle ?
>
> Jim here is a concrete example.
>
>
> #include <iostream>
> #include <mathsym.h>
> #include <P.h>
> #include <PV.h>
>
> using namespace std;
> using namespace cxxadt;
>
>
> class Turtle{
> friend const PV& GetHeading(const Turtle& t);
> public:
>
> Turtle();
> private:
>
> PV h;
>
> };
>
> now corresponding to this the wrapper i wrote for this
>
> #include <iostream>
>
> #include <boost/python.hpp>
>
> #include <boost/python/module.hpp>
>
> #include <boost/python/def.hpp>
>
> #include <boost/python/scope.hpp>
>
> #include <boost/python/class.hpp>
>
> #include"Turtle.h"
>
> #include <mathsym.h>
>
> #include <P.h>
>
> #include <PV.h>
>
>
> using namespace std;
>
> using namespace boost::python;
>
> using namespace cxxadt;
>
>
>
> BOOST_PYTHON_MODULE(TurtleWrapper)
>
> {
>
>
> class_<Turtle>("Turtle")
>
> .def("GetHeading",&GetHeading) ;
>
> }
>
>
> class_<PV>("PV",init<const PV>());
>
> class_<P>("P",init<const P>());
>
>
> }
>
>
>
>
>
> Kind Regards
>
> KV
>
>
>
>
>
>
>
>
>
>
>
>
>
> On Fri, Jul 18, 2014 at 1:50 PM, Jim Bosch <talljimbo at gmail.com> wrote:
>
>> On Jul 18, 2014 6:32 AM, "Kv Gopalkrishnan" <kvgopalkrishnan1 at gmail.com>
>> wrote:
>> >
>> > Hi Jim
>> > Thank you for the answer. When I tried this solution
>> > i.e. .def("GetHeading",&GetHeading)
>> > error: ‘GetHeading’ was not declared in this scope
>> >
>> >
>> > I get the above error.
>> > Stating the obvious the error says function was not described in the
>> scope. Seems a bit nasty business is it so that I would have to write a get
>> function in someway in to public domain to get the function wrapped in to
>> boost?
>> >
>>
>> I'm pretty sure that should have worked. Could you post a more complete
>> example? I suspect the problem is elsewhere.
>>
>> Jim
>>
>> >
>> > On Thu, Jul 17, 2014 at 3:12 PM, Jim Bosch <talljimbo at gmail.com> wrote:
>> >>
>> >> The problem here is actually a C++ issue, not a Boost.Python one.
>> Friend functions aren't considered to be part of the scope of the class
>> that they're friends with, so when referring to them, just use e.g.
>> "&GetHeading", not "&Turtle::GetHeading".
>> >>
>> >> Jim
>> >>
>> >>
>> >> On Wed, Jul 16, 2014 at 7:52 AM, Kv Gopalkrishnan <
>> kvgopalkrishnan1 at gmail.com> wrote:
>> >>>
>> >>> I want to expose a C++ friend functions to python using Python boost.
>> >>> class Turtle{
>> >>> friend const PV& GetHeading(const Turtle& t);
>> >>> friend const PV& GetLeft(const Turtle& t);
>> >>> friend const P& GetPoint(const Turtle& t);
>> >>> friend void SetPoint(Turtle& t, const P& p);
>> >>> public:
>> >>>
>> >>> ...
>> >>>
>> >>> private:
>> >>> PV h;
>> >>> PV l;
>> >>>
>> >>> };
>> >>>
>> >>>
>> >>> Here I have wrapped the classes PV and P so no problem there. I tried
>> to wrap the friend functions like regular functions. Like
>> >>>
>> >>> BOOST_PYTHON_MODULE(TurtleWrapper)
>> >>> {
>> >>> class_<Turtle>("Turtle")
>> >>> .def("GetHeading",&Turtle::GetHeading)
>> >>> .def("GetLeft",&Turtle::GetLeft)
>> >>> .add_property("h",&Turtle::GetHeading)
>> >>> .add_property("l",&Turtle::GetLeft);
>> >>> }
>> >>> When i run the code i get error messages.
>> >>>
>> >>> error: ‘GetHeading’ is not a member of ‘Turtle’
>> >>> error: ‘GetLeft’ is not a member of ‘Turtle’
>> >>>
>> >>> This error is seen for add_property and also .def.
>> >>> So I assume that this is not the way to declare a friend function and
>> the documentation of python boost does not seem (or at least I did not see
>> a note about friend functions). Any help is much appreciated.
>> >>>
>> >>>
>> >>>
>> >>> Kind Regards
>> >>> K.V.
>> >>>
>> >>> _______________________________________________
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>> >>
>> >>
>> >>
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>> >
>> >
>> >
>> >
>> > --
>> > K.V.
>> >
>> > _______________________________________________
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>
>
>
> --
> K.V.
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--
K.V.
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