Emulation of properties for composite objects

[Sergey]

Hi, Florian!

I'm working on a scripting interface for the object representation of XML documents. In this regard, there is a need to emulate the properties of objects, whose members are other objects (composite objects), for example:

class MyElem {
public:
    QString attr2;
    MyElem ();
};
class MyDoc {
public:
    QString attr1;
    MyElem elem;
    MyDoc ();
};

In a Python script to access a member of a nested object should be as follows:

py> q = MyDoc ()
py> q.elem.attr2 = "value"
py> print q.elem.attr2

Class decorators, obviously, must be determined as follows:

class MyElemWrapper: public QObject {
    Q_OBJECT
public slots:
    / / Constructor & destructor
    MyElem * new_MyElem () {return new MyElem ();}
    void delete_MyElem (MyElem * o) {delete o;}
    QString py_get_attr2 (MyElem * o) {return o-> attr2;}
    void py_set_attr2 (MyElem * o, QString value) {o-> attr2 = value;}
};
class MyDocWrapper: public QObject {
    Q_OBJECT
public slots:
    / / Constructor & destructor
    MyDoc * new_MyDoc () {return new MyDoc ();}
    void delete_MyDoc (MyDoc * o) {delete o;}
    QString py_get_attr1 (MyDoc * o) {return o-> attr1;}
    void py_set_attr1 (MyDoc * o, QString value) {o-> attr1 = value;}
    MyElem py_get_elem (MyDoc * o) {return o-> elem;}
    void py_set_elem (MyDoc * o, MyElem value) {o-> elem = value;}
};

However, since the method MyDocWrapper::py_get_elem () returns a value (not a reference), the assignment is performed for a temporary object of MyElem. Therefore, the result of the print statement in the above script example is an empty string.
Reading of attr2 member is performed correctly (understand why).

Question: is there a way to ensure the desired behavior of the properties for the member elem?

Sincerely,
Sergey.

[Florian Link]

Sounds like you should do:

MyElem* py_get_elem (MyDoc * o) {return &(o-> elem);}

instead, so that you return the exact MyElem object to Python, instead of a copy.

[Sergey]

I am ashamed to admit, I tried only the reference and got an error during script execution.
Of course, one could assume that in such a wonderful library there should not be similar drawbacks. Thanks and successes!

Sincerely,
Sergey.

[Florian Link]

Thanks!

The problem with references is that Qt's moc compiler doesn't like them as return values on slots, neither const & nor &,
so one has to use pointers instead.

Regards,
Florian