Signals and Slots and vtable, oh my!!

[radar]

Using Qt5 with my C++ (open source) project, under both VS Code/makefile and Qt Creator dev environments, execution of the connect() method yields:

QObject::connect: No such slot QComboBox::MyIndexChanged() in BashQtHelper.cpp:118

Or, when I include Q_OBJECT, the linker error:

undefined reference to `vtable for MyComboBox'

The class is:

class MyComboBox: public QComboBox
{
public:
    MyComboBox(QWidget *parent = nullptr): QComboBox(parent) {
        connect(this, SIGNAL(activated(int)), this, SLOT(MyIndexChanged()));
        }
    ~MyComboBox(){}

public slots:
    void MyIndexChanged() {
        if (currentIndex()) {std::cout << currentText().toStdString() << "\n"; exit(0);}}
};

When I try the alternative call, namely with pointers to functions:

connect(this, &activated, this, &MyComboBox::MyIndexChanged);

The compiler complains:

error: no matching function for call

The minimalist program by ChatGPT is:

#include <QtWidgets/QApplication>
#include <QtWidgets/QComboBox>
#include <QtWidgets/QVBoxLayout>
#include <QtWidgets/QMessageBox>

class ComboBoxApp : public QWidget {
public:
    ComboBoxApp(QWidget *parent = nullptr) : QWidget(parent) {
        // Create a combo box
        comboBox = new QComboBox(this);

        // Add items to the combo box
        comboBox->addItem("Option 1");
        comboBox->addItem("Option 2");
        comboBox->addItem("Option 3");

        // Connect the signal for item selection to the custom slot
        connect(comboBox, SIGNAL(currentIndexChanged(int)), this, SLOT(onComboBoxSelectionChanged(int)));

        // Create a layout and set it for the main window
        QVBoxLayout *layout = new QVBoxLayout(this);
        layout->addWidget(comboBox);

        setLayout(layout);
    }

public slots:
    // Custom slot to handle combo box selection changes
    void onComboBoxSelectionChanged(int index) {
        QString selectedOption = comboBox->itemText(index);
        QMessageBox::information(this, "Selection Changed", "Selected: " + selectedOption);
    }

private:
    QComboBox *comboBox;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    ComboBoxApp window;
    window.setWindowTitle("ComboBox Application");
    window.resize(300, 150);
    window.show();

    return app.exec();
}

How do I make this code work, or do I need to scrap the whole thing and start with a better SIGNAL/SLOT template?

[Pl45m4]

@radar said in Signals and Slots and vtable, oh my!!:

The minimalist program by ChatGPT is:

Rule No. 1: Dont trust any AI/Bot, when you are not able to confirm the result's correctness ;-)

class MyComboBox: public QComboBox
{
public:

You are missing the Q_OBJECT macro. Without this, no signals ands slots ;-)
Add the macro, include QComboBox and then clean your build, and build again.

connect(this, &activated, this, &MyComboBox::MyIndexChanged);

You need the complete, fully qualified function pointer there &activated is not enough, unless it's a global function which doesn't belong to any class.

Apart from all this, connecting this to this doesn't make too much sense. Then you just can call the function when you need it

[radar]

@Pl45m4 said in Signals and Slots and vtable, oh my!!:

Q_OBJECT

I tried that, that's when I got the vtable not found on linking.

[radar]

@Pl45m4 said in Signals and Slots and vtable, oh my!!:

Rule No. 1: Dont trust any AI/Bot, when you are not able to confirm the result's correctness ;-)

Okay then, where's the official template/example provided by Qt?

It's easy to say "don't do this, don't use that", the useful answers are, "here's an example of how to do it right".

[Pl45m4]

@radar said in Signals and Slots and vtable, oh my!!:

Okay then, where's the official template/example provided by Qt?

Have you tried the official documentation instead of ChatGPT? :)

• https://doc.qt.io/qt-6/signalsandslots.html
  • https://doc.qt.io/qt-6/signalsandslots.html#a-small-example

[Pl45m4]

@radar said in Signals and Slots and vtable, oh my!!:

VS Code/makefile

I checked your GitHub... and there is no moc in your makefile... Maybe I've missed something but AFAICS you are not running the meta-object-compiler, which would explain why none of your QObject / metaobject stuff, including signals, is working.

• https://doc.qt.io/qt-6/moc.html#writing-make-rules-for-invoking-moc

Edit:
Also, why do you have everything (two QObject classes and even the main) in one cpp file?!
MOC can only run *.h files and you need to include your moc'ed output in your *.cppfile.
[ Edit: can also run cpp files, but it's unusual as stated by @Christian-Ehrlicher ]

Start to learn how Qt and everything that comes with it works, otherwise you will run into more trouble sooner or later

[Christian Ehrlicher]

@Pl45m4 said in Signals and Slots and vtable, oh my!!:

MOC can only run *.h files and you need to include your moc'ed output in your *.cppfile.

Thats not true

But why create a custom Makefile instead using CMake or qmake to let do all this work?

[Pl45m4]

@Christian-Ehrlicher said in Signals and Slots and vtable, oh my!!:

Thats not true

What part? The h-only or that you have to include your moc in your cpp?
I know that you dont have to, but you have to run moc on the files where you declare your Q_OBJECT, right?! So usually you would declare your class in some header file and moc outputs something like moc_myclass.cpp

Found this, because I wasnt 100% sure:

• https://forum.qt.io/topic/83854/class-with-q_object-macro-declared-and-defined-in-cpp

Now I'm also confused :D

[Christian Ehrlicher]

@Pl45m4 You can also moc a .cpp file even though it's unusual.
e.g. when you've a small private, object-derived class in your cpp file (as some Qt source code is doing it) instead moving the class definition into an own foo_p.h.
I mostly use it for small testcases only containing a main.cpp - then you need to include <main.moc> and cmake is doing the rest (instead fiddling around with a Makefile).