Strange errors with my code

  • Hi guys,

    Please look at my code.
    This is Hello.h:

    #ifndef HELLO_H
    #define HELLO_H
    #include <QDialog>
    class QCheckBox;
    class QLabel;
    class QLineEdit;
    class QPushButton;
    class Hello : public QDialog
        Hello(QWidget* parent = 0);
        void findNext(const QString& str, Qt::CaseSensitivity cs);
        void findPrevious(const QString& str, Qt::CaseSensitivity cs);
    private slots:
        void findClicked();
        void enableFindButton(const QString& text);
        QLabel* label;
        QLineEdit* lineEdit;
        QCheckBox* caseCheckBox;
        QCheckBox* backwardCheckBox;
        QPushButton* findButton;
        QPushButton* closeButton;
    #endif // HELLO_H

    And this one, Hello.cpp

    #include <QtWidgets>
    #include "hello.h"
    Hello::Hello(QWidget *parent)
        : QDialog(parent)
        label = new QLabel(tr("Find &What"));
        lineEdit = new QLineEdit;
        label -> setBuddy(lineEdit);
        caseCheckBox = new QCheckBox (tr("Match &case"));
        backwardCheckBox = new QCheckBox (tr("Search &backward"));
        findButton = new QPushButton (tr("&Find"));
        findButton -> setDefault(true);
        findButton -> setEnabled(false);
        closeButton = new QPushButton(tr("Close"));
        connect(lineEdit, SIGNAL(textChanged(const QString&)),
                this, SLOT(enableFindButton(const QString&)));
        connect(findButton, SIGNAL(clicked()),
                this, SLOT(findClicked()));
        connect(closeButton, SIGNAL(clicked()),
                this, SLOT(close()));
        QHBoxLayout* topLeftLayout = new QHBoxLayout;
        topLeftLayout -> addWidget(label);
        topLeftLayout -> addWidget(lineEdit);
        QVBoxLayout* leftLayout = new QVBoxLayout;
        leftLayout -> addLayout(topLeftLayout);
        leftLayout -> addWidget(caseCheckBox);
        leftLayout -> addWidget(backwardCheckBox);
        QVBoxLayout* rightLayout = new QVBoxLayout;
        rightLayout -> addWidget(findButton);
        rightLayout -> addWidget(closeButton);
        rightLayout -> addStretch();
        QHBoxLayout* mainLayout = new QHBoxLayout;
        mainLayout -> addLayout(leftLayout);
        mainLayout -> addLayout(rightLayout);
    void Hello::findClicked()
        QString text = lineEdit -> text();
        Qt::CaseSensitivity cs = caseCheckBox -> isChecked() ? Qt::CaseSensitive
                                                             : Qt::CaseInsensitive;
        if(backwardCheckBox -> isChecked()) {
            emit findPrevious(text, cs);
        } else {
            emit findNext(text, cs);
    void Hello::enableFindButton(const QString& text)
        findButton -> setEnabled(!text.isEmpty());

    And this, main.cpp:

    #include <QApplication>
    #include <hello.h>
    int main(int argc, char* argv[])
        QApplication app(argc, argv);
        Hello* dialog = new Hello;
        dialog -> show();
        return app.exec();

    I get some strange errors that I don't know the reason for them, like:
    C:\Users...\Qt\Hello\hello.cpp:5: error: undefined reference to `vtable for Hello'.

    This is for line 5 of Hello.cpp ( : QDialog(parent))

    Would you please help me?

  • You try to assign a QWidget pointer to QDialog.

    Change to

    class Hello : public QDialog
        Hello(QDialog* parent = 0);


    Hello::Hello(QDialog *parent)
        : QDialog(parent)

  • @koahnig :
    Thanks for the answer. I understand your reason but do you have the book C++-GUI-Programming-with-Qt-4-2ndEdition on your hand to have a look at page 29 of it?
    There it mentions these:

    class Hello : public QDialog
        Hello(QWidget* parent = 0);


    Hello::Hello(QWidget *parent)
        : QDialog(parent)

    I also modified my code as you suggested but get the errors yet! :(

  • Lifetime Qt Champion


    • "undefined reference to `vtable "

    From build menu, try
    Clean All
    Build All

  • @mrjj said in Strange errors with my code:


    • "undefined reference to `vtable "

    From build menu, try
    Clean All
    Build All

    Thank you. You solved the issues.

    Could I ask why I faced this kind of problem? What was the reason?

  • Sometimes a fresh rebuild simply helps.

    Especially when you have a lot of different source and header not all dependencies are always covered and the system cannot detect.
    This is just a trial to explain, it is very often a mystery to me why you are ending up in such failures.

    The sequence given by @mrjj is basically some sort of Ctrl+Alt+Del for Qt creator

  • Qt Champions 2017

    @tomy said in Strange errors with my code:

    What was the reason?

    Stale translation units (object files). Probably Creator missed that you changed something, just as @koahnig explained, and didn't recompile the .cpp, which in turn led to the linker error.

  • Lifetime Qt Champion


    To add to my fellows, the vtable error usually comes when adding/removing the Q_OBJECT macro. This one requires to re-run qmake to refresh the build setup with the add/removed information.

  • So whenever I will see such error (vtable) again, I will do the steps mrjj said. :)
    Thanks so much to all, especially to SGaist for his simple language.

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