Important: Please read the Qt Code of Conduct -

setup my own abstract class functions.. (about connect and slots)

  • Hi!

    I have created a function that manages and returns a QPushButton pointer.

    QPushButton* setup_button(QString _txt, QString _style, QRect _geo, UnknownParam _param){
         QPushButton* o = new QPushButton(_txt);
         connect(o, SIGNAL(clicked()), this, _param);
         return o;

    I want that "UnknownParam" parameter to receive a function for a slot so i can setup my button as following example:

    QPushButton* myButton = setup_button("Click me", btnStyle, QRect(10, 10, 20, 20), SLOT(action4myButton()));

    what is the parameter type i need to do this?.

    Thanks in advance.

  • const char *

  • I do not quite understand what you mean,does it mean that a certain button can be generated when the program is running,and then the slot function corresponding to the clicked button is dynamically determined when the program is running,right?In other words, in connect,how to pass a function pointer and use it as the content in SLOT,is that right?


    QMetaObject::Connection QObject::connect(
        const QObject *sender, 
        const char *signal,
        const QObject *receiver,
        const char *method,
        Qt::ConnectionType type = Qt::AutoConnection

  • @U7Development
    Maybe now is the time to move over from your SIGNAL/SLOT() macros to New Signal Slot Syntax ?

  • Lifetime Qt Champion

    @JonB said in setup my own abstract class functions.. (about connect and slots):

    Maybe now is the time to move over from your SIGNAL/SLOT() macros to New Signal Slot Syntax ?

    But this makes it even harder for him because then it's a PMF which has to be passed to the function. And as you can see there are currently problems passing a simple const char * ... :)

  • @Christian-Ehrlicher
    I don't care! It's a function pointer. If you & he want to keep working on managing your slot connections via an untyped, unparsed arbitrary string that's up to you :) In this forum I have seen people pass an utterly rubbish string for the SLOT() (or SIGNAL()) parameter, and then wonder why it doesn't work... I'm surprised at you! ;-)

  • @JonB
    After reading your advice on the problem,I checked the new usage of the signal slot you mentioned on the Internet and tried to learn it,then I used 【connect(btn,&QPushButton::XXX, xx, &XXX::XXX)】this from replaced the previous【SIGNAL】usage,and the program is still robust as before。Although I don't know the working principle of the signal slot and the specific differences between the two,the new usage is more convenient from the writing point of view,so I will follow the guidance you provide and try to use 【&】not【SIGNAL】,thank you!

  • Lifetime Qt Champion


    The main difference between the old and new syntax is runtime lookup of the given slot/signal name versus
    compile-time verification of the given class method pointer.

    the SIGNAL and SLOTS macros will accept any string at compile-time and first fail at runtime

    The "new" & syntax is evaluated at compile-time and is type-safe, meaning
    it will complain about missing signals or slot or even parameter mismatch.

    Poster could do like

    (in .h)
    class MainWindow;
    using CALL = void (MainWindow::*)(void); // function pointer to (named)  class memeber
    (in .cpp) 
    QPushButton *MainWindow::setup_button(QString _txt, QString _style, QRect _geo, CALL _param)
        QPushButton *o = new QPushButton(_txt);
        connect(o, &QPushButton::clicked, this, _param);
        return o;
      auto b = setup_button( "Test", "", QRect(0, 10, 100, 100), &MainWindow::DoSomething );

    To use new syntax, BUT this case is easy as he wants to use just clicked()
    With another type of widgets and with different parameters, it becomes quite the template show.

  • @mrjj
    Oh yeah。。。thank you!!
    you have deepened my understanding of the new usage of signal slot,finding errors at compile time is better than finding errors at runtime!it seens it is time to say goodbye with【SIGNAL】。。。I meet Qt from a book,in that book,it used all 【SIGNAL】,now it seems that the book should be quite old。

  • Lifetime Qt Champion

    Yes for bigger apps and more than one developer, catching type errors and at compile time has a huge benefit
    as the app might have not so often used dialogs and some parameter change in other part of the app might silently fail
    with the MACROS and first be discovered at end users.
    With the new syntax, the first developer to recompile will see it.
    That said, connect with MACROS does return true or false so its possible to catch connect failures, making
    it less of a burden.

    But the new syntax also offers new features, such as lambdas )in place, nameless function) and those can be very handy
    for keep together compact code. So yes, you will not really look back once getting used to the new syntax.

  • I have tried const char pointer with no success but i guess i was in a mistake using connect argument

    const chart _param* used as:

    connect(btn, SIGNAL(clicked()), this, SLOT(_param));

    now i will change to :

    connect(btn, SIGNAL(clicked()), this, _param);

    Regarding to new connect parameter format, its time to check it out.

  • @U7Development
    Yes, to match your

    connect(o, SIGNAL(clicked()), this, _param);

    you wanted to know what UnknownParam _param should be in setup_button() definition, and that should indeed be const char *_param to do things this way. The caller will pass SLOT(whatever), you won't use SLOT() in your code in your setup_button() code here.

Log in to reply