Mailman 3 August 2005 - Cplusplus-sig

Re: [C++-sig] problem chaning object state using a python class that inherits from c++ class
by Alexis H. Rivera-Rios Aug. 16, 2005

Aug. 16, 2005

David, Im sending the email again, since I don't remember if I have send it or not. Please forgive the formatting since I don't know how to control that with yahoo mail. struct Data { int a; double b; }; class B { public: virtual void Update(Data& a, int ellapsedTime) = 0; ~B() {} }; class D : public B { virtual void Update(Data& a, int ellapsedTime) { a.a = 10; a.b = -9; } }; #include <boost/shared_ptr.hpp> class C { public: void SetBptr(boost::shared_ptr<B> newB) { Bptr = newB; } void Update(int ellapsedTime) { if (Bptr.get()) { B *ptr = Bptr.get(); ptr->Update(d,ellapsedTime); } } int GetData_a() const { return d.a; } double GetData_b() const { return d.b; } private: Data d; boost::shared_ptr<B> Bptr; }; Here is the pyste script Class("Data","test.h") B = Class("B","test.h") use_shared_ptr(B) Class("C","test.h") D = Class("D","test.h") final(D.Update) Here is an example of the error: (test 1) works as expected >>> from PysteError import * >>> c = C() >>> d = D() >>> c.SetBptr(d) >>> c.Update(1) >>> print c.GetData_a() 10 >>> print c.GetData_b() -9.0 (test 2) fails when called from C >>> class E(B): ... def Update(self,data,val): ... data.a = 999 ... data.b = 100 ... >>> e = E() >>> cc = C() >>> cc.SetBptr(e) >>> cc.Update(1) >>> print cc.GetData_a() 6488163 >>> print cc.GetData_b() 6.95352557184e-308 (test 3) but it works when called directly >>> data = Data() >>> e.Update(data,1) >>> print data.a 999 >>> print data.b 100.0 >>> Hope this illustrates the problem better, Thanks, Alexis Programming Tutorial: In Python: To do this, do this In Perl: To do this, do this or this or this or this... In C: To do this, do this, but be careful In C++: To do this, do this, but don't do this, be careful of this, watch out for this, and whatever you do, don't do this ____________________________________________________ Start your day with Yahoo! - make it your home page http://www.yahoo.com/r/hs

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Re: [C++-sig] problem chaning object state using a python class that inherits from c++ class
by Alexis H. Rivera-Rios Aug. 16, 2005

Aug. 16, 2005

David, Here is an example (excuse the formatting, because I don't know how to change it from yahoo mail) struct Data { int a; double b; }; class B { public: virtual void Update(Data& a, int ellapsedTime) = 0; ~B() {} }; class D : public B { virtual void Update(Data& a, int ellapsedTime) { a.a = 10; a.b = -9; } }; #include <boost/shared_ptr.hpp> class C { public: void SetBptr(boost::shared_ptr<B> newB) { Bptr = newB; } void Update(int ellapsedTime) { if (Bptr.get()) { B *ptr = Bptr.get(); ptr->Update(d,ellapsedTime); } } int GetData_a() const { return d.a; } double GetData_b() const { return d.b; } private: Data d; boost::shared_ptr<B> Bptr; }; The pyste code: Class("Data","test.h") B = Class("B","test.h") use_shared_ptr(B) Class("C","test.h") D = Class("D","test.h") final(D.Update) An example of the error: >>> from PysteError import * >>> c = C() >>> d = D() >>> c.SetBptr(d) >>> c.Update(1) >>> print c.GetData_a() 10 >>> print c.GetData_b() -9.0 >>> class E(B): ... def Update(self,data,val): ... data.a = -8; ... data.b = -1; ... >>> e = E() >>> c.SetBptr(e) >>> c.Update(1) >>> print c.GetData_a() 10 >>> print c.GetData_b() -9.0 >>> cc = C() >>> cc.SetBptr(e) >>> cc.Update(1) >>> print cc.GetData_a() 6488163 >>> print cc.GetData_b() 6.95352557184e-308 Hope this is clearer now ;) Thanks for you help, Alexis Programming Tutorial: In Python: To do this, do this In Perl: To do this, do this or this or this or this... In C: To do this, do this, but be careful In C++: To do this, do this, but don't do this, be careful of this, watch out for this, and whatever you do, don't do this ____________________________________________________ Start your day with Yahoo! - make it your home page http://www.yahoo.com/r/hs

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Re: [C++-sig] problem chaning object state using a python class that inherits from c++ class
by Alexis H. Rivera-Rios Aug. 16, 2005

Aug. 16, 2005

Abraham, The idea is that I have a class Entity that contains a pointer to a strategy object that implements the Trajectory interface. The Trajectory interface modifies the Entity's state in UpdateState(). The entity class delegates the update of its state to the Trajectory strategy. The problem is that in python, when I implement the Trajectory interface and pass the object to the Entity class, the call to Trajectory.UpdateState doesn't seem to work correctly. It gets executed but the state is not updated. (I guess it has to do with the fact that I'm passing the object as a reference). However, if I call UpdateState directly from the python object that implements Trajectory interface it works fine. (ie. it changes the content of the object) If this is still too much let me know and I'll try to reproduce the error with simpler classes. But here is the code. class Entity { /// specify a trajectory strategy /// which is a shared pointer to a Trajectory object specified below SetTrajectory(const TrajectoryPtr &T) { trajectory = T; } /// delegates to the Trajectory strategy /// the updating the state virtual void UpdateState(double ellapsedTime=1) { t->UpdateState(state,ellapsedTime); } private: EntityState state; ///< object to be modified TrajectoryPtr trajectory; /// this is a shared pointer }; class Trajectory /// interface that i want to inherit in python { public: /// note it takes the state as a reference virtual void UpdateState(EntityState& state, double ellapsedTime) const = 0; }; I exposed the class using pyste and in python I'm derived a class from it. class HackedT(Trajectory): def UpdateState(self, entitystate, ellapsedTime): entitystate.position = gmtl.Point3d(-1,-1,-1) The problem is that when I do this in python: target = Entity() target.SetTrajectory(hackedT) target.UpdateState() # enventually calls the HackedT's UpdateState, which modifies the state print target.GetPosition() I expect the result to be (-1,-1,-1) but instead its coming out to be the original value (10,10,0). However, if I use Trajectory objects created from C++ then the state is modified. Also, if I call the function directly it modifies the state. s = EntityState() ## by default s.position = (0,0,0) hackedT.UpdateState(s,1) print s.position ## returns (-1,-1,-1) Is this a bug in Boost Python or do I have to specify some policy or something to make this work? Let me know if you need additional information. Again, any help will be greatly appreciated, (Hope it's better summarized) Alexis Programming Tutorial: In Python: To do this, do this In Perl: To do this, do this or this or this or this... In C: To do this, do this, but be careful In C++: To do this, do this, but don't do this, be careful of this, watch out for this, and whatever you do, don't do this __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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problem chaning object state using a python class that inherits from c++ class
by Alexis H. Rivera-Rios Aug. 15, 2005

Aug. 15, 2005

Hi, I have the following classes: class Entity { public: /// type of trajectory pointer typedef boost::shared_ptr<StatATA::Trajectory> TrajectoryPtr; /// constructor Entity() : trajectory(), state() {} /// destructor virtual ~Entity() {} /// sets the position void SetPosition(const gmtl::Point3d &pos) { state.position = pos; } /// returns the position gmtl::Point3d GetPosition() const { return state.position; } //.... other methods here... /// delegates to the Trajectory object /// the updating the state virtual void UpdateState(double ellapsedTime=1) { if (trajectory.get()) { Trajectory *t = trajectory.get(); t->UpdateState(state,ellapsedTime); } } private: EntityState state; ///< entity's physical state TrajectoryPtr trajectory; /// copy constructor disabled Entity(const Entity& ); /// assignment disabled Entity& operator=(const Entity &); }; class Trajectory { /// disabled copy constructor Trajectory(const Trajectory &); /// disabled assignment Trajectory& operator=(const Trajectory&); public: /// constructor Trajectory() {} /// destructor virtual ~Trajectory(){} /// updates the state of the entity virtual void UpdateState(EntityState& state, double ellapsedTime) const = 0; }; I exposed the class using pyste and in python I'm derived a class from it. class HackedT(Trajectory): def UpdateState(self, entitystate, ellapsedTime): entitystate.position = gmtl.Point3d(-1,-1,-1) The problem is that when I do this in python: target = Entity() target.SetPosition(gmtl.Point3d(10,10,0)) target.SetTrajectory(hackedT) target.UpdateState(1) result = target.GetPosition() print result[0] print result[1] print result[2] I expect the result to be (-1,-1,-1) but instead its coming out to be the original value. However, if I use trajectories defined in C++ the new values come out correct. Is this a bug in Boost Python or do I have to specify some policy or something to make this work? Any help will be greatly appreciated, Alexis Programming Tutorial: In Python: To do this, do this In Perl: To do this, do this or this or this or this... In C: To do this, do this, but be careful In C++: To do this, do this, but don't do this, be careful of this, watch out for this, and whatever you do, don't do this __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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Is there a good Python C/C++ IDE?
by Scott Kaplan Aug. 15, 2005

Aug. 15, 2005

Being new to Python, I was wondering if anyone knows of a good IDE that would let me develop Python and add extensions in C/C++, with the capability of debugging those extensions. This will be on a Mac. It looks like the standard download comes with PyObjC, however I want to extend Python to handle my C/C++ extensions not the other way around + I don't want to have to learn any ObjectiveC / ObjC syntax. Thanks,

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Transfer of ownership
by Eric Jardim Aug. 15, 2005

Aug. 15, 2005

Hi, all... This is a simple question. I was reading the docs where it says about the "transfer of ownership" of a C++ object pointer. I am dealing exacly this situation, where the "with_custodian_and_ward" policy is not enough to prevent Python objects to delete C++ couterparts. The only hack is to hold references, but it is not a complete solution. So, I read the FAQ entry where it explain how to deal with this issue: http://www.boost.org/libs/python/doc/v2/faq.html#ownership But there, it speaks about using an "std::auto_ptr". On the other side, I am using "boost::shared_ptr" to hold my objects. I really do not know the difference between them, but I imagine they provide similar funcionality. So I will split my question in 3 parts: - Is it possible to take the ownership with "boost::shared_ptr", or do I have to change my held types to "std::auto_ptr"? - Is there any difference in practice, using boost's or std's smart pointers? I just use them for holding Boost.Python extensions. - As those functions with smart pointers do not exists, I suppose that I will have to write wrappers like: void b_insert(B& b, std::auto_ptr<A> a) { b.insert(a.get()); a.release(); } Is there any special reason for this to be a non-const reference to "b"? Could it be a const reference or a const/non-const pointer to "b"? Is there any implications? Thanks, [Eric Jardim]

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Re: [C++-sig] Mapping an entire data structure over to a single Python instance. (Newbie question)
by g.j.murphy＠sageserpent.com Aug. 15, 2005

Aug. 15, 2005

"Dave Abrahams" <dave at boost-consulting.com> writes: > "Gerard Murphy" <g.j.murphy(a)sageserpent.com> writes: > > > 3. I'd like to expose these conceptual classes to Python, mapping over > > these instances of 'DataStructure' mentioned above to Python objects - > > but with the Python class changing depending on what instance of > > 'DataStructure' is being mapped. > > Is that an important feature of the system? It's a 'very nice to have' - but I could fallback to just treating all maps as being, well, dictionaries in Python. :-) Something I didn't mention in the original post is that I would like to add certain methods onto certain of these classes, and that the client code on the Python side would know in advance what set of methods the instances would be expected to possess: these objects would be passed over to Python in sets where each member of the set would respond to the same set of method calls. In effect I'm saying that if objects 'A' and 'B' both possess the attributes 'foo' and 'bar', then they would acquire a method 'doSomethingImportantInvolvingFooAndBar'. Object 'C' on the other hand that possesses property 'baz' acquires another method, say 'shutdownReactorAfterInternalDialogueWithBaz'. ;-) 'A' and 'B' would be passed together to one Python function, 'C' would be passed to another. All three of these objects might have other attributes that could be optionally accessible on the Python side, but are not vital for the correct operation of the system. > > > The classes of the Python objects would be dynamically created by > > construction of a 'class_' object:- > > > > > > > > > > // Somewhere this line will be repeatedly executed... > > > > boost::python::class_<DataStructure> classObject(name); > > No, that won't work. > > What you can do is use Python's metaclass API. > http://gnosis.cx/publish/programming/metaclass_1.html > > Export your C++ DataStructure class once. > > > object data_structure > = class_<DataStructure>( ... ) > ... > ; > > > Then get its metaclass: > > object meta = object( > handle<>( > borrowed(data_structure.ptr()->ob_type) > ) > ); > > Now you can manufacture distinct subclasses on-the-fly: > > object new_subclass = meta( > "name-of-subclass", python::make_tuple(data_structure), > dict()); > Excellent! > > > I'm assuming that although the class_ object will be destroyed at each > > repetition, the corresponding Python class lives on afterwards. It isn't a > > problem if this is not the case, as I could always manage these objects in a > > long-lived container or on the heap or whatever. > > > > > > What I intend to do is to dynamically add properties to 'classObject' > > that use the name of the property to fetch the appropriate associated > > value from the map. This would be done once per repetition, so that each > > repetition would create a brand new Python class. > > I suggest you don't do that either. It would be better to implement a > __getattr__ method in your exported DataStructure. > > object getattr(DataStructure& ds, std::string name) > { > return object( ...lookup name in ds... ); > } > > object data_structure > = class_<DataStructure>( ... ) > .def("__getattr__", getattr) > ; > > For this you'll need to export class_<AbstractValue> and > class_<ConcreteValue<T> > for each ConcreteValue<T> you are using. Ah - yes, that's much better. > > > To be a bit more specific, I want to call 'classObject.add_property()' with > > an application of boost::bind to a template function instantiation that > > takes a name, looks up the associated value in the map, dynamically > > casts to the template type parameter and returns the value. > > > > I can ask an exemplar map to walk itself so that on behalf of a set of maps > > sharing the same common structure, it will populate a single class > > object with the appropriate properties. > > Doing it with add_property is going to be a bear. Why would you want > to do that? As a Python novice, I had automatically assumed that properties were the only form of abstraction available on instance variable access (I had C# in mind). Now that you've introduced me to '__getattr__' and the like, I've seen the light. :-) > > > *** Now for the questions *** > > > > OK, that's what I want to do ... but can I do the following first:- > > > > i) Can I construct more than one instance of 'class_<DataStructure>' > > using lots of different names as arguments at each construction > > call? > > Nope > No problem, that's now obsolete in the light of using the metaclass object to produce more classes. Many thanks, Gerard

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Post-constructor action
by Eric Jardim Aug. 15, 2005

Aug. 15, 2005

Hi, Anybody knows how do I get the Python object of a newly created object of a Boost.Python extension class object, just after the call of the constructor, before the pointer return to Python? I was thinking how to do it, but I am a bit confused. I suppose those I could use customized call policies, but I am not sure neither know how to do it. Another thing that I saw interesting is this "make_constructor" function. I could use factory functions, but I'll loose my Python constructor. Does anybody have any idea of how to deal with it? Thanks, [Eric Jardim]

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Mapping an entire data structure over to a single Python instance. (Newbie question)
by Gerard Murphy Aug. 14, 2005

Aug. 14, 2005

Hello, I'm evaluating the potential use of the Boost.Python facilities. I've looked though the tutorial and *some* of the detailed API documentation available from www.boost.org, and so far it all looks very promising. However... I have a rather strange use planned for the C++ to Python interface that isn't covered by the tutorial. The detailed API documentation doesn't seem to answer my question either; so I'd like to put it to the mailing list, as I'm sure that this idea has already occurred to somebody in the past. *** Background *** What I'd like to do is map an entire data structure over to a single Python instance, so that individual properties of the Python object would correspond to parts of the data structure. To add spice to this, the data structure is in a sense weakly typed: different parts of it are composed out of very different C++ types. The data structure is dynamically built (otherwise there wouldn't be in issue as I could map a C-style structure or getter/setter class over to a Python class using the techniques described in the tutorial). I'll make this a little bit more concrete:- 1. I have what is effectively a weakly typed wrapper around a set of different C++ types using external polymorphism:- class AbstractValue { public: ~virtual AbstractValue(); }; template <typename StrongType> class ConcreteValue: public AbstractValue { public: // Use compiler-generated destructor. ConcreteValue(const StrongType &valueRepresentation): _valueRepresentation(valueRepresentation) StrongType _valueRepresentation; }; Instances of these make their way into a map:- typedef std::map<std::string, boost::shared_ptr<AbstractValue> > DataStructure; 2. Various things in the system populate these maps in different ways, so that from one map instance to another, not only do the associated values change, but so do the number of associations, the keys and the actual types of the associated values. However, lots of these maps share the same common structure in terms of numbers of associations, keys and associated types. These could be thought of as instances of several classes, where each class captures the commonality of structure. These classes effectively come into being as the system runs: they cannot be defined at compile time as conventional C++ classes. No, I can't just write the entire system in Smalltalk (or Python for that matter) ... :-) 3. I'd like to expose these conceptual classes to Python, mapping over these instances of 'DataStructure' mentioned above to Python objects - but with the Python class changing depending on what instance of 'DataStructure' is being mapped. The classes of the Python objects would be dynamically created by construction of a 'class_' object:- // Somewhere this line will be repeatedly executed... boost::python::class_<DataStructure> classObject(name); I'm assuming that although the class_ object will be destroyed at each repetition, the corresponding Python class lives on afterwards. It isn't a problem if this is not the case, as I could always manage these objects in a long-lived container or on the heap or whatever. What I intend to do is to dynamically add properties to 'classObject' that use the name of the property to fetch the appropriate associated value from the map. This would be done once per repetition, so that each repetition would create a brand new Python class. To be a bit more specific, I want to call 'classObject.add_property()' with an application of boost::bind to a template function instantiation that takes a name, looks up the associated value in the map, dynamically casts to the template type parameter and returns the value. I can ask an exemplar map to walk itself so that on behalf of a set of maps sharing the same common structure, it will populate a single class object with the appropriate properties. *** Now for the questions *** OK, that's what I want to do ... but can I do the following first:- i) Can I construct more than one instance of 'class_<DataStructure>' using lots of different names as arguments at each construction call? I get the impression from the documentation that each template instantiation of 'class_ <X>' for a given X might well be a singleton class in terms of usage - only one constructed instance per class template instantiation, if you know what I mean! ii) If I *can* have lots of instances of 'class_<DataStructure>', how does the framework know how to map instances of 'DataStructure' to the appropriate Python class - this seems to be implicitly done rather than explictly done. What I'm driving at here is taking an *existing* instance of DataStructure and wrapping it up as a 'boost::python::object' whose Python type is one of these dynamically constructed classes. This is in contrast to using the 'class_<DataStructure>' object to create the instance of 'boost::python::object' - I would definitely like to avoid doing this if possible, although I appreciate that it is usually the obvious way to go. NOTE: I'm assuming that the properties can be defined using *any* function object that would take an instance of 'DataStructure' and return an appropriately typed value - I am planning on using 'boost::bind' and an appropriate trampoline template function to do this. In the meantime, I'll have a longer trawl through the API documentation and perhaps the implementation source, but I'd appreciate any help from old hands to shorten the process. Many thanks in advance, Gerard

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[Announce] py++ first public release
by Roman Yakovenko Aug. 14, 2005

Aug. 14, 2005

Hi, I would like to introduce a new code generator for the boost.python library, called py++ . py++ is a framework of components for creating a C++ code generator for boost.python . Code generation with the py++ framework is a very flexible and highly configurable process. You can find an introduction to py++ at: 1. http://tinyurl.com/bpe2o 2. http://cvs.sourceforge.net/viewcvs.py/*checkout*/pygccxml/source/pyplusplus… Status: First of all, this project is under active development. py++ can export the following declarations: enums ( named and unnamed ) global variables operators ( partially ) free functions member functions classes py++ does not allow the user to override virtual functions from Python for exported classes, i.e. py++ does not create a wrapper class for exported classes. Currently, py++ does not have its own homepage. You can download py++ from http://sourceforge.net/projects/pygccxml. Please note that pyplusplus-0.0.1 requires pygccxml-0.6.4 . Installation instructions: for pygccxml there is a setup.py file, so installation should be a simple task. for py++ there is no setup yet. py++ is a regular python package, so place it somewhere and add its location to sys.path . Ideas, comments, suggestions or help are welcomed. Roman Yakovenko

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