Walkthrough: Testing a Project (C++)

When you run a program in Debug mode, you can use breakpoints to pause the program to examine the state of variables and objects.

In this walkthrough, you watch the value of a variable as the program runs and deduce why the value isn't what you expect.


To run a program in Debug mode

  1. Open Games.cpp for editing.

  2. Select this line of code:

    Cardgame solitaire(1);

  3. To set a breakpoint on that line, on the menu bar, choose Debug > Toggle Breakpoint, or choose the F9 key. A red circle appears to the left of the line; it indicates that a breakpoint is set. To remove a breakpoint, you can choose the menu command or the F9 key again.

    If you're using a mouse, you can also set or remove a breakpoint by clicking in the left margin.

  4. On the menu bar, choose Debug > Start Debugging, or choose the F5 key.

    When the program reaches the line that has the breakpoint, execution stops temporarily, because your program is in Break mode. A yellow arrow to the left of a line of code indicates that it's the next line to be executed.

  5. To examine the value of the Cardgame::totalParticipants variable, move the pointer over Cardgame and then move it over the expansion control at the left of the tooltip window. The variable name totalParticipants and its value of 12 are displayed.

    Open the shortcut menu for the Cardgame::totalParticipants variable and then choose Add Watch to display that variable in the Watch 1 window. You can also highlight a variable and drag it to the Watch 1 window.

  6. To step to the next line of code, on the menu bar, choose Debug > Step Over, or choose the F10 key.

    The value of Cardgame::totalParticipants in the Watch 1 window is now displayed as 13.

  7. Open the shortcut menu for the return 0; statement and then choose Run to Cursor. The yellow arrow to the left of the code points to the next statement to be executed.

  8. The Cardgame::totalParticipants number should decrease when a Cardgame ends. At this point, Cardgame::totalParticipants should equal 0 because all Cardgame instances have been deleted, but the Watch 1 window indicates that Cardgame::totalparticipants equals 18. The difference indicates that there's a bug in the code, which you can detect and fix by completing the next walkthrough, Walkthrough: Debugging a Project (C++).

  9. To stop the program, on the menu bar, choose Debug > Stop Debugging, or choose the Shift+F5 keyboard shortcut.

Next Steps

Previous: Walkthrough: Building a Project (C++)
Next: Walkthrough: Debugging a Project (C++)

See also

C++ Language Reference
Projects and build systems