- Qt Creator Documentation
- Tutorial: C++ debugging
- Qt Creator 15.0.1

Tutorials ◦ How To

# 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,
             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).

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