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Thread: Qt Query

  1. #1
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    Question Qt Query

    Hello
    I am quite new to Qt I have some Question which am not got satisfactory answer till now.

    1.Why Q_OBJECT macro is in the private section of code? What will happen if I provide it in Public section of code?
    2.What is return type of connect() function and can we return anything from signal and slot,if yes then how?

  2. #2
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    Default Re: Qt Query

    Quote Originally Posted by anh5kor View Post
    1.Why Q_OBJECT macro is in the private section of code? What will happen if I provide it in Public section of code?
    It is just customary to put it right after the opening brace of the class declaration but it doesn't really matter.
    It expands to code that has access modifiers as needed.

    Quote Originally Posted by anh5kor View Post
    2.What is return type of connect() function
    You could have answered that yourself by looking at the documentation: an object of QMetaObject::Connection, basically a handle to the connection.

    Quote Originally Posted by anh5kor View Post
    and can we return anything from signal and slot,if yes then how?
    Since a signal can be connected to multiple slots and a slot can be connected to multiple signals, the return value of a slot is not available at the place of signal emit.

    For the same reason it is usually also a bad idea to use an argument type that is writable, e.g. a reference or pointer, but if you really, really, need that, then that's the way to get values from a slot to the place of emit.

    Cheers,
    _

  3. The following user says thank you to anda_skoa for this useful post:

    anh5kor (23rd December 2015)

  4. #3
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    Default Re: Qt Query

    Quote Originally Posted by anh5kor View Post
    1.Why Q_OBJECT macro is in the private section of code?
    Presumably because it expands to definitions that may need a visibility specification (like "public:") and there is no way in C++ to restore the current visibility as it was before the macro. By specifying that the current visibility must be private, the macro can restore it by a simple "private:" at the end.
    Quote Originally Posted by anh5kor View Post
    What will happen if I provide it in Public section of code?
    Who knows? This is not documented. In the above scenario you will make private all declarations following the macro until the next visibility specification.
    Quote Originally Posted by anh5kor View Post
    2.What is return type of connect() function and can we return anything from signal and slot,if yes then how?
    QMetaObject::Connection. As explained in the documentation, it is a handle to the connection itself. Signals and slots do not return anything, because they may be called asynchronously.

  5. The following user says thank you to yeye_olive for this useful post:

    anh5kor (23rd December 2015)

  6. #4
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    Default Re: Qt Query

    Then how does the emitter know if the signal was received or not?

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    Default Re: Qt Query

    Quote Originally Posted by skypuppy View Post
    Then how does the emitter know if the signal was received or not?
    The emitter usually doesn't care, but you can check if a signal is connected at all.

    Cheers,
    _

  8. #6
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    Default Re: Qt Query

    Then how does the emitter know if the signal was received or not?
    The emitter doesn't care, as anda_skoa says. A signal is just an ordinary method in the class that declares it. Qt's MOC compiler automatically generates the code for the method, and it is hidden in the moc_xxx.cpp file that is created during the build. You declare and implement slots yourself. A connect() basically adds a pointer to your slot function to the table of connections maintained by the signal method. When you (or a Qt class) "emits" a signal, that is nothing more than invoking the signal function which in turn calls all of the slot functions for the receivers it is connected to.

    It only seems like magic.

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