Tutorial: C++ debugging

This tutorial uses the TextFinder example to illustrate how to debug Qt C++ applications in the Debug mode.

TextFinder reads a text file into QString and then displays it with QTextEdit. To look at the TextFinder class and see the stored data:

  1. In textfinder.cpp, click between the line number and the window border on the line where we change the cursor position to set a breakpoint.

  2. Go to Debug > Start Debugging > Start Debugging of Startup Project or press F5.
  3. To view information about the breakpoint, go to the Breakpoints view.

  4. To remove a breakpoint, right-click it and select Delete Breakpoint.
  5. To view the base classes and data members of the TextFinder class, go to the Locals view.

Modify the on_findButton_clicked() function to move back to the start of the document and continue searching once the cursor hits the end of the document. Add the following code snippet:

void TextFinder::on_findButton_clicked()
    QString searchString = ui->lineEdit->text();

    QTextDocument *document = ui->textEdit->document();
    QTextCursor cursor = ui->textEdit->textCursor();
    cursor = document->find(searchString, cursor,

    bool found = cursor.isNull();

    if (!found && previouslyFound) {
        int ret = QMessageBox::question(this, tr("End of Document"),
        tr("I have reached the end of the document. Would you like "
        "me to start searching from the beginning of the document?"),
        QMessageBox::Yes | QMessageBox::No, QMessageBox::Yes);

        if (ret == QMessageBox::Yes) {
            cursor = document->find(searchString,
        } else
    previouslyFound = found;

If you compile and run the above code, however, the application does not work correctly due to a logic error. To locate this logic error, step through the code using the following buttons: (Stop Debugger), (Step Over), (Step Into), and (Step Out).

See also Tutorial: Qt Widgets application, Debugging, Debuggers, and Debugger.

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