Why is there no copying at QNetworCookieJar?



  • Hi,

    I think the title says everything
    there is the "Q_DISABLE_COPY(QNetworkCookieJar)" thing in the sourcecode and I and a friend which is a much better programmer than me were not able to find a reason for that.
    My program would be much better if I could use the copy constructor.
    Hope somebody can tell me.



  • Because QNetworkCookieJar inherits from QObject and QObject cannot be copied because of parenting issues (amongst others).



  • That could not be the reason cause QNetworkAccessManager also inherits QObject and can be copied.



  • No it cannot. There is no explicitly defined copy constructor or assignment operator so it will try to use a default member-wise one created by the compiler which will fail when it tries to recurse to the base class' (QObject's) copy constructor or assignment operator.

    Nothing derived from QObject can be copied because QObject marks its copy constructor and assignment operator as private.



  • But do it work for QNetworkAccessManager wich also inherits QObject and has no copy constructor?



  • If you can copy a QNetworkAccessManager, then I would considder that a Qt bug. Don't (try to) copy QObject or QObject derived objects.



  • Does anybody then know how to get a object inheriting QNetworkCookieJar into a QNetworkObjectJar variable? Currently I am using a <reinterpretate_cast> to do it (this works cause my class has no member varibales just two functions).



  • You could subclass QNetworkObjectJar, and implement a clone function like this:

    @
    MyNetworkCookieJar* MyNetworkCookieJar::getClone() const {
    MyNetworkCookieJar *newJar = new MyNetworkCookieJar;
    newJar->setAllCookies( allCookies() );
    return newJar
    }
    @

    But watch out: you must delete the returned pointer yourself. There is NO way of treating QObject derived classes as value types!



  • That was the idea I also had first but keep in mind that that the CooieJar also can connect some cookies with an URI. This connections will be lost if do like you discribed.



  • Class QNetworkCookie contains the URI the cookie is bound to. According to the API docs the methods allCookies() and setAllCookies() can be used to implement a permanent cookie storage. So I think the returned list of cookies contains all the necessary information. If not, I'd definitely call this a bug :-)



  • Your right the data is being set up in QNetworkCookie. So your funtion will work and I am happy now that I do not have to use this cast. :) Thanks



  • Hi,
    I realized that my problem description was insufficient and the problem has not been solved yet.
    I want to do something like this:
    @void Class::SetCookieList(const QList<QNetworkCookie> &Cookies)
    {
    _CookieJar->setAllCookies(Cookies);
    }

    QList<QNetworkCookie> Class::CookieList() const
    {
    return _CookieJar->allCookies();
    }@

    But since the functions "setAllCookies" and "allCookies" are protected I am unable to do so. What is the proper way to do what I want to do?



  • the docs say:

    bq.
    This function is suitable for derived classes to save cookies to disk, ....

    So should you extend QNetworkCookieJar class and override these methods?



  • [quote author="haithun" date="1287528028"]Hi,
    I realized that my problem description was insufficient and the problem has not been solved yet.
    I want to do something like this:
    @void Class::SetCookieList(const QList<QNetworkCookie> &Cookies)
    {
    _CookieJar->setAllCookies(Cookies);
    }

    QList<QNetworkCookie> Class::CookieList() const
    {
    return _CookieJar->allCookies();
    }@

    But since the functions "setAllCookies" and "allCookies" are protected I am unable to do so. What is the proper way to do what I want to do?[/quote]
    If all you want to do is access those methods from an outside class, then just make a trivial subclass of QNetworkCookieJar and make those two method public. You can then access them from your external class. If what you want to do really belongs to the cookie jar itself, perhaps implementing your functionality in that subclass itself would be a better choice. It may help to keep encapsulation.



  • That was also my idea but I have the problem that I want to write the data of a normal QNetworkCookieJar to my class which is quite difficult without a copyconstructor.
    Anybody knowing a good solution?



  • Why do you need more than one?



  • I dont need more than one but I want to give my function "SetCookieJar" a QNetworkCookie. Thats data should be saved in my extended class. I am doing this because I want to avoid to have two member variables and the different treatment in my functions.



  • So as Andre wrote - subclass QNetworkCookieJar, make the two protected functions public and use them to transfer the cookie data. No need for a copy constructor as far as I see. Left alone that a copy constructor would do quite the same.

    @
    this->setAllCookies(other->allCookies());
    @

    Or implement a cloneFromOther Method to simulate an assignment operator/copy constructor:

    @
    void MyCookieJar::cloneFromOther(const QNetworkCookieJar *other)
    {
    if(other)
    setAllCookies(other->allCookies())
    }
    @

    As mentioned before, QNetworkCookieJar is a QObject and there simply IS NO copy constructor. It does not help to ask again and again - the trolls will not add it :-)



  • I understood that there is no copy cunstructor and also the reason.
    Well your code is very similar to the one that I wrote cause subclassing QNetworkCookieJar was also my idea. But your version wont work, because you try to call the protected "allCookies()" function of the "other" object as if it's public. Would it be public my problem would not exist.



  • If you derive class MyCookieJar from QNetworkCookieJar you have access to the protected methods of the base class QNetworkCookieJar. And of course you have access to the protected methods of the other object pointer in that method.



  • Well I tried to do it in a constructor of my class but it wont work.

    Here the class with constructor:
    @
    class MyClass : public QNetworkCookieJar
    {
    Q_OBJECT
    public:

    CopyFromCookieJar(QNetworkCookieJar *Cookies, QObject *parent = 0) : NetworkCookieJar(parent)
    {
        this->setAllCookies(Cookies->allCookies());
    }
    

    @



  • The code fragment wouldn't even compile:

    Your class is named "MyClass", your Constructor is named "CopyFromCookieJar" - both names must be equal.

    Also the argument of the constructor should be
    @const QNetworkCookieJar *Cookies@

    Additionally, you should check for a null pointer before calling any methods on the pointer.



  • Yeah sry I just changed the name of the function and forgot that it should be a constructor.

    Why do you declare the pointer as const?

    The reason of the compiler is "'QList<QNetworkCookie> QNetworkCookieJar::allCookies() const' is protected".



  • Sorry for the wrong guidance. You cannot access the protected method of the base class in a derived class using a base class pointer.

    You must replace all appearences of QNetworkCookieJar with your derived class in your application and pass around only pointers/references of this derived class. Then you have access to the protected methods.

    @
    class MyCookieJar : public QNetworkCookieJar
    {
    Q_OBJECT
    public:
    explicit MyCookieJar(QObject *parent = 0)
    : QNetworkCookieJar(parent)
    {
    // nothing
    }

    void cloneFromOther(const MyCookieJar *other) {
        setAllCookies(other->allCookies());
    }
    

    };
    @

    For the const pointer:
    the other cookie jar will not be changed by the clone method (or the copy constructor), so it's actually a good idea to make the pointer const. There are good explanations on const correctnes on the "C++ FAQ":http://www.parashift.com/c++-faq-lite/const-correctness.html



  • Yeah but now we are right there where I started cause I did not want to pass my derived class. I want to pass a QNetworkCookieJar to the function and then I want to put the cookies into my derived class. The reason why I want to have just Qt classes is no real reason it's just that I hate it to include the header in every upcoming projects.

    I guess my first version by using a reinterpretate cast is the only solution.



  • Then there's no solution to your problem - you can't have your cake and eat it too :-)

    I would go for the subclass approach. reinterpret_cast is ugly and should be avoided whenever theres a more elegant (and save!) solution - just my 0,02 € - :-)


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