CSCI 201 Lab 8 -- Debugging in C++

Getting ready

We're going to give you 2 Visual C++ project folders (i.e., workspaces) to start with this time. First download a ZIP'ed copy of all Lab 8 workspaces and store it in the C:\files directory.

Use Powerzip to extract the files in it to the c:\files directory.

This should create a C:\files\Lab08 directory with two subdirectories: weekdays and buggy.

Open the weekday workspace and make sure it compiles. We will uses this workspace to learn about the debugger.

Using the Visual C++ Debugger

1. Executes your program line by line, instead of at full speed.

2. Allows for examination of any variable at any point in your program.

Starting a Simple Trace

This should bring up a code window with a yellow arrow placed after the first executable instruction. The yellow arrow to the left of a statement means that execution is suspended at this line. This will be the next statement executed.

Click somewhere lower in the code and again choose Run to cursor (Ctrl+F10) from the Debug menu (or floating toolbar). This will cause the program to run until it reaches the line with the code you clicked on. Use this feature when you want the debugger to jump to a specific line of code.

Continue the Trace

Repeatedly press F10 or the Step Over button. Each click causes one statement to be executed. When you get to the part of the code that needs keyboard input, you will need to type input into the console window. To switch to the execution window, look on the START menu bar and click on the MSDOS tab with the program name.

Press F11 or the Step Into button when the yellow arrow marker is at a function call. This causes the debugger to step into a function as opposed to stepping over, which traces all the code inside the function at once. Remember that when you get to the part of the code that needs keyboard input, you will need to type input into the console window.

If you "Step Into" a function that you do not need to examine, then get out, Press F11 or the Step Out button. This allows the user to step out of the current function and return control back to the line that called the function.

NOTE: As well as Stepping Into programmer-defined function, the debugger also allows for stepping into C++ library functions.

Stop the Trace

Choose Stop Debugging from the Debug menu or hit Shift+F5

Variable Examination

Examination Methods

• If you pause the mouse over a variable name, it's current value is shown in a popup box. Try this on a few variables.

• The "Variables" window displays variables and their values from the the current expression (the auto tab), local to the current function (the locals tab), or in relation to the this pointer (the this tab).

• The "Watch" window allows variables & expressions to be constantly evaluated while single-stepping through the program. Variables can be typed directly into this window or dragged into the window and dropped with the mouse from either the "Variables" window or the source code window. An entry in the "Watch window" can be deleted by selected the entry and pressing the Delete key. The four tabs at the bottom of the window are used to group varibles.

NOTE: The variable values listed in the "Watch" & "Variables" windows can be modified by the user for testing purposes. To modify a value in either of these windows, click the "Value" field and enter a new value. All modified values are colored red to indicate that changes were made during debugging.

Lab Checkoff 1

Importance of Breakpoints

Breakpoints are markers that are placed at specified lines of code that cause the debugger to suspend execution of the program upon reaching that line of code. Breakpoints are important because they give the programmer a means to stop at a particular line of code and verify that a program is executing correctly.

Setting Unconditional Breakpoints

To set an unconditional breakpoint, click the line in the program where the breakpoint is to be places and select the breakpoint toggle icon or press F9. A big red dot should appear to the left of the line, if a breakpoint has been set. (To remove breakpoints, this same method is used on lines that have breakpoints.)

Start Debugging Process

Now since your breakpoints have been set, start the debugging process by pressing the Go icon or press F5. This will cause the program to run until it reaches the next breakpoint.

Set another breakpoint further along in the code. Then repeat this procedure to get to the next breakpoint.

Conditional Breakpoints

Conditional breakpoints only stop/pause program execution if a specified condition is true. They are commonly used to halt the program during loop execution. To make a conditional breakpoint, first make an unconditional breakpoint. Then select Breakpoints from the Edit menu (Alt + F9).

Click on one of the breakpoint you created or select the cursor line from the popup menu to choose your breakpoint. Its easier to select your breakpoint by clicking on it in the window entitled "Breakpoints:"

Now click the Condition button and type the Boolean expression that you wish to check, as shown in the following example. Now close this window by pressing the "OK" button and then close the "Breakpoint" Window by pressing "OK".

Now you need to enter the debugger to execute your conditional breakpoint. Use the Go command to execute the program until your conditional breakpoint is reached (if, indeed, it is).

NOTE: Breakpoints are persistent. When a project is closed and reopened, any breakpoints set during a debugging session will still be set.

Lab Checkoff 2

Expression Evaluation

Quickwatch

Quickwatch allows you to see the value of given expressions without having to code diagnostic output statements into your program.

You can put a quickwatch statement into a watch window as well.

Activating Quickwatch

To activate quickwatch, click on the quickwatch button ( ) or hit (Shift + F9).

When you click the "Recalculate" button, the value of the expression is recalculated.

If you hit the "Add Watch" button, the expression is included in the watch window and is continuously updated there.

Lab Checkoff 3

Fixing a program

Lab Checkoff 4