Unexpected behavior of operator= in Qml/JavaScript

  • I have defined a couple of context variables from C++, something like this:
    @class MyClass : public QObject
    Q_PROPERTY(QString name ...)
    MyClass myVar1, myVar2;
    QQmlContext *ctxt = m_pMainView->rootContext();
    ctxt->setContextProperty("myVar1", &myVar1);
    ctxt->setContextProperty("myVar2", &myVar2);

    In JavaScript in Qml, I can assign anything I want to myVar1, I don't get any errors or warnings, and the value of myVar1 never changes:
    myVar1 = myVar2;
    myVar1 = 3.0;
    myVar1 = "Fred";

    Having written my own code to inject variables into JavaScript (in the past, not related to Qt) I can certainly imagine why, but it seems like a bad idea that I don't get any sort of warning or error.

    Are there any macros I can insert into MyClass, or anything clever I can do that will make the operator= actually do something?


  • Hmm this can't work because your registred variables are a class and you want to set the value of a property of this class. So i think this is the right way
    myvar1.name = "name"
    The problem with the warning and errors is that qml is JIT so when you compile your project only the c++ side will be compiled the qml and javascript would be interpreted i think so the first time you get errors is when your programm is running and the qml is interpreted.

  • Yes, it would work to specify the values one at a time, the problem is that I would like to copy all of the values from myVar2 to myVar1, which means I need to write
    @myVar1.name = myVar2.name;
    myVar1.target = myVar2.target;
    myVar1.size = myVar2.size;
    etc. etc. etc.

    which is not only annoying, but is error-prone and violates several object-oriented design principles.

    And yes, Qml and JavaCcript are interpreted and C++ is compiled, the problem is that the interpreter doesn't give me warnings or errors ever - including when the program is running and the Qml is interpreted and executed.

    I realize that JavaScript doesn't really support copying of objects by value, I'm working around this limitation in the language, my concern I guess is that by JavaScript rules, if I say
    @myVar1 = myVar2;

    it should make myVar1 refer to the same object as myVar2 but that isn't happening, and no errors or warnings are being generated.

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