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How to Use Classes derived from QObject to send signals and have slots

  • Hi

    I have been trying to have a class called Tool that derives from QObject and declares several slots.

    I my QMainWindow class (TheClass), I am trying to use that class.

    For instance TheClass has a private member:

    @private: Tool* TheTool;@

    Now when it comes to connect to signals from Tool, I do in the constructor of TheClass:


    Which compiles fine, but when I run it, I get the message

    bq. QObject::connect: Cannot connect (nulll)::theToolSignal to TheClass::myToolSlot()

    Which seems to indicate that TheTool is not instantiated.

    Yet, when I make a call to a member function


    I see from its qDebug() output that it is being called

    I have read the notes on the QObject class and that it has a Q_DISABLE_COPY macro which means that implied or explicit assignments won't work and that one cannot instantiate the QObject?

    My question for you is then:

    How can I modify my code so that I can have a class derived from Q_Object that uses Signals and Slots, and then use that class from another class?

  • First make sure that the connect statement is after the line: TheTool = new Tool(); and also at the connect you are missing a pair of parenthesis: @SIGNAL(theToolSignal () )@

  • Hi

    Yes, I transcribed the signal wrong in this question.

    However, the problem I have is that when I do use new, the linker starts whinging at me:

    bq. in function Tool::Tool()': Tool.h:77: undefined reference tovtable for Tool::Tool'

  • -Did you include the header file where the Tool class is declared?-

    LE: Make sure that Tool class has Q_OBJECT macro, if it doesn't add it and than run qmake and than build your project.

  • Hi

    I finally figured it out, your mentioning the Q_OBJECT macro again made me check and lo, I did not include the "tool.moc" file.

    Interestingly enough, it is not created by my cmake either even though qt4_automoc is fed both the tool as well as the MyClass

    I did a moc ../Tool.h > tool.moc in my build directory after appending the include "tool.moc" line in my tool.cpp file, then make and voila! it is compiling!

    Thank you, you're a star :)

  • FYI; getting the cmake to build the moc for the tool as wel was as simple as adding a line

    @set(myCoolProject_H tool.h myclass.h)@

    In addition to having these lines:

    @QT4_WRAP_UI(myCoolProject_H ${myCoolProject_GUI} )


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