Debugging a C++ Example Application

This section 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, place a breakpoint in textfinder.cpp, as follows:

  1. Click in between the line number and the window border on the line where we change the cursor position to set a breakpoint.

  2. Select Debug > Start Debugging > Start Debugging 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,
          QTextDocument::FindWholeWords);
      ui->textEdit->setTextCursor(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,
                  QTextDocument::FindWholeWords);
              ui->textEdit->setTextCursor(cursor);
          } else
              return;
      }
      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).