QDialog Error: mingw32-make: *** [Makefile:45: debug] Error 2

[ElfMoon]

I'm new in QT, this is code I copy from another source (which I forget).
Please help me to explain what is Error 2

/* Dialog Example */
#include "mainwindow.h"
#include <QAction>
#include <QApplication>
#include <QComboBox>
#include <QDialog>
#include <QDialogButtonBox>
#include <QFormLayout>
#include <QGroupBox>
#include <QLabel>
#include <QLineEdit>
#include <QMainWindow>
#include <QMenu>
#include <QMenuBar>
#include <QPushButton>
#include <QSpinBox>
#include <QTextEdit>
#include <QVBoxLayout>
#include <QWidget>

class Dialog : public QDialog {
    Q_OBJECT

    public:
    Dialog();

    private:
    void createMenu();
    void createHorizontalGroupBox();
    void createGridGroupBox();
    void createFormGroupBox();

    enum { NumGridRows = 3, NumButtons = 4 };

    QMenuBar *menuBar;
    QGroupBox *horizontalGroupBox;
    QGroupBox *gridGroupBox;
    QGroupBox *formGroupBox;
    QTextEdit *smallEditor;
    QTextEdit *bigEditor;
    QLabel *labels[NumGridRows];
    QLineEdit *lineEdits[NumGridRows];
    QPushButton *buttons[NumButtons];
    QDialogButtonBox *buttonBox;

    QMenu *fileMenu;
    QAction *exitAction;
};

Dialog::Dialog() {
    createMenu();
    createHorizontalGroupBox();
    createGridGroupBox();
    createFormGroupBox();

    bigEditor = new QTextEdit;
    bigEditor->setPlainText(
      tr( "This widget takes up all the remaining space "
         "in the top-level layout." ) );

    buttonBox = new QDialogButtonBox( QDialogButtonBox::Ok |
                         QDialogButtonBox::Cancel );

    connect( buttonBox, &QDialogButtonBox::accepted, this,
        &QDialog::accept );
    connect( buttonBox, &QDialogButtonBox::rejected, this,
        &QDialog::reject );

    QVBoxLayout *mainLayout = new QVBoxLayout;
    mainLayout->setMenuBar( menuBar );
    mainLayout->addWidget( horizontalGroupBox );
    mainLayout->addWidget( gridGroupBox );
    mainLayout->addWidget( formGroupBox );
    mainLayout->addWidget( bigEditor );
    mainLayout->addWidget( buttonBox );
    setLayout( mainLayout );

    setWindowTitle( tr( "Basic Layouts" ) );
}

void Dialog::createMenu() {
    menuBar = new QMenuBar;

    fileMenu = new QMenu( tr( "&File" ), this );
    exitAction = fileMenu->addAction( tr( "E&xit" ) );
    menuBar->addMenu( fileMenu );

    connect( exitAction, &QAction::triggered, this, &QDialog::accept );
}
void Dialog::createHorizontalGroupBox() {
    horizontalGroupBox = new QGroupBox( tr( "Horizontal layout" ) );
    QHBoxLayout *layout = new QHBoxLayout;

    for ( int i = 0; i < NumButtons; ++i ) {
        buttons[i] = new QPushButton( tr( "Button %1" ).arg( i + 1 ) );
        layout->addWidget( buttons[i] );
    }
    horizontalGroupBox->setLayout( layout );
}
void Dialog::createGridGroupBox() {
    gridGroupBox = new QGroupBox( tr( "Grid layout" ) );
    QGridLayout *layout = new QGridLayout;

    for ( int i = 0; i < NumGridRows; ++i ) {
        labels[i] = new QLabel( tr( "Line %1:" ).arg( i + 1 ) );
        lineEdits[i] = new QLineEdit;
        layout->addWidget( labels[i], i + 1, 0 );
        layout->addWidget( lineEdits[i], i + 1, 1 );
    }

    smallEditor = new QTextEdit;
    smallEditor->setPlainText(
      tr( "This widget takes up about two thirds of the "
         "grid layout." ) );
    layout->addWidget( smallEditor, 0, 2, 4, 1 );
    layout->setColumnStretch( 1, 10 );
    layout->setColumnStretch( 2, 20 );
    gridGroupBox->setLayout( layout );
}
void Dialog::createFormGroupBox() {
    formGroupBox = new QGroupBox( tr( "Form layout" ) );
    QFormLayout *layout = new QFormLayout;
    layout->addRow( new QLabel( tr( "Line 1:" ) ), new QLineEdit );
    layout->addRow( new QLabel( tr( "Line 2, long text:" ) ),
            new QComboBox );
    layout->addRow( new QLabel( tr( "Line 3:" ) ), new QSpinBox );
    formGroupBox->setLayout( layout );
}

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

#ifdef Q_OS_ANDROID
    dialog.showMaximized();
#else
    dialog.show();
#endif
    return app.exec();
}

This is error output:
09:16:22: Running steps for project QTProject03...
09:16:22: Configuration unchanged, skipping qmake step.
09:16:22: Starting: "H:\QTCreator\Tools\mingw810_64\bin\mingw32-make.exe" -j4
H:/QTCreator/Tools/mingw810_64/bin/mingw32-make -f Makefile.Debug
mingw32-make[1]: Entering directory 'D:/1A1 o Disk D/Program Files 01/Cpp Project/build-QTProject03-Desktop_QT-Debug'
g++ -c -fno-keep-inline-dllexport -g -std=gnu++11 -Wall -Wextra -Wextra -fexceptions -mthreads -DUNICODE -D_UNICODE -DWIN32 -DMINGW_HAS_SECURE_API=1 -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -DQT_NEEDS_QMAIN -I..\QTProject03 -I. -IH:\QTCreator\5.15.2\mingw81_64\include -IH:\QTCreator\5.15.2\mingw81_64\include\QtWidgets -IH:\QTCreator\5.15.2\mingw81_64\include\QtGui -IH:\QTCreator\5.15.2\mingw81_64\include\QtANGLE -IH:\QTCreator\5.15.2\mingw81_64\include\QtCore -Idebug -I. -I/include -IH:\QTCreator\5.15.2\mingw81_64\mkspecs\win32-g++ -o debug\main.o ..\QTProject03\main.cpp
g++ -Wl,-subsystem,windows -mthreads -o debug\QTProject03.exe debug/main.o debug/mainwindow.o debug/moc_mainwindow.o H:\QTCreator\5.15.2\mingw81_64\lib\libQt5Widgets.a H:\QTCreator\5.15.2\mingw81_64\lib\libQt5Gui.a H:\QTCreator\5.15.2\mingw81_64\lib\libQt5Core.a -lmingw32 H:\QTCreator\5.15.2\mingw81_64\lib\libqtmain.a -LC:\openssl\lib -LC:\Utils\my_sql\mysql-5.7.25-winx64\lib -LC:\Utils\postgresql\pgsql\lib -lshell32
debug/main.o:main.cpp:(.rdata$.refptr._ZTV6Dialog[.refptr._ZTV6Dialog]+0x0): undefined reference to vtable for Dialog' debug/main.o:main.cpp:(.rdata$.refptr._ZN6Dialog16staticMetaObjectE[.refptr._ZN6Dialog16staticMetaObjectE]+0x0): undefined reference to Dialog::staticMetaObject'
collect2.exe: error: ld returned 1 exit status
mingw32-make[1]: *** [Makefile.Debug:72: debug/QTProject03.exe] Error 1
mingw32-make: *** [Makefile:45: debug] Error 2
mingw32-make[1]: Leaving directory 'D:/1A1 o Disk D/Program Files 01/Cpp Project/build-QTProject03-Desktop_QT-Debug'
09:16:28: The process "H:\QTCreator\Tools\mingw810_64\bin\mingw32-make.exe" exited with code 2.
Error while building/deploying project QTProject03 (kit: Desktop QT)
When executing step "Make"
09:16:28: Elapsed time: 00:06.

I'm using QT 5.15.2 because the latest version of QT Creator cannot run on Windows 8.1
Please keep support for old Windows 7 and Windows 8.1 until poor student can save enough money to buy new Laptop to run Windows 11.

[Christian Ehrlicher]

Since you're using qmake as build system you have to run qmake afterwards so it sees that a main.moc is needed.
Or even better - do what should be done with a separate class - move it out into an own source and header file and you won't need to include main.moc.

[Christian Ehrlicher]

Since you did not put your class in a separate header file, moc is not run automatically to create the needed stuff for the Q_OBJECT macro and you have to add #include "main.moc" at the end of your main.cpp
See also https://doc.qt.io/qt-6/moc.html

[ElfMoon]

@Christian-Ehrlicher said in QDialog Error: mingw32-make: *** [Makefile:45: debug] Error 2:

Since you did not put your class in a separate header file, moc is not run automatically to create the needed stuff for the Q_OBJECT macro and you have to add #include "main.moc" at the end of your main.cpp
See also https://doc.qt.io/qt-6/moc.html

If I paste this code #include "main.moc" at the end of your main.cpp, QT Creator will give error file not found. I dont have file main.moc and I also dont know how to create it.
Could you please post content of file main.moc that need to be created.

[Christian Ehrlicher]

Since you're using qmake as build system you have to run qmake afterwards so it sees that a main.moc is needed.
Or even better - do what should be done with a separate class - move it out into an own source and header file and you won't need to include main.moc.