3.8 — Using an integrated debugger: Watching variables

In the previous lessons (3.6 -- Using an integrated debugger: Stepping and 3.7 -- Using an integrated debugger: Running and breakpoints), you learned how to use the debugger to watch the path of execution through your program. However, stepping through a program is only half of what makes the debugger useful. The debugger also lets you examine the value of variables as you step through your code, all without having to modify your code.

As per previous lessons, our examples here will use Visual Studio -- if you are using a different IDE/debugger, the commands may have slightly different names or be located in different locations.

Watching variables

Watching a variable is the process of inspecting the value of a variable while the program is executing in debug mode. Most debuggers provide several ways to do this.

Let’s take a look at a sample program:

#include <iostream>

int main()
{
	int x{ 1 };
	std::cout << x << ' ';

	x = x + 2;
	std::cout << x << ' ';

	x = x + 3;
	std::cout << x << ' ';

	return 0;
}

This is a pretty straightforward sample program -- it prints the numbers 1, 3, and 6.

First, run to cursor to line 6.

At this point, the variable x has already been created and initialized with the value 1, so when we examine the value of x, we should expect to see the value 1.

The easiest way to examine the value of a simple variable like x is to hover your mouse over the variable x. Some modern debuggers support this method of inspecting simple variables, and it is the most straightforward way to do so.

For Code::Blocks users

If you’re using Code::Blocks, this option is (inexplicably) off by default. Let’s turn it on. First, go to Settings menu > Debugger…. Then under the GDB/CDB debugger node, select the Default profile. Finally, check the box labeled Evaluate expression under cursor.

Hover your mouse cursor over variable x on line 6, and you should see something like this:

Note that you can hover over any variable x, not just the one on the current line. For example, if we hover over the x on line 12, we’ll see the same value:

If you’re using Visual Studio, you can also use QuickWatch. Highlight the variable name x with your mouse, and then choose “QuickWatch” from the right-click menu.

This will pull up a subwindow containing the current value of the variable:

Go ahead and close QuickWatch if you opened it.

Now let’s watch this variable change as we step through the program. Either choose step over twice, or run to cursor to line 9. The variable x should now have value 3. Inspect it and make sure that it does!

The watch window

Using the mouse hover or QuickWatch methods to inspect variables is fine if you want to know the value of a variable at a particular point in time, but it’s not particularly well suited to watching the value of a variable change as you run the code because you continually have to rehover/reselect the variable.

In order to address this issue, all modern integrated debuggers provide another feature, called a watch window. The watch window is a window where you can add variables you would like to continually inspect, and these variables will be updated as you step through your program. The watch window may already be on your screen when you enter debug mode, but if it is not, you can bring it up through your IDE’s window commands (these are typically found in a View or Debug menu).

You should now see something like this:

The watches window may or may not contain anything in it already.

There are typically two different ways to add variables to the watch window:

1. Pull up the watch window, and type in the name of the variable you would like to watch in the leftmost column of the watch window.
2. In the code window, right click on the variable you’d like to watch, and choose Add Watch (Visual Studio) or Watch x (replace x with the variable’s name) (Code::Blocks).

If you’re not already in a debugging session with the execution marker on line 9 of your program, start a new debugging session and run to cursor to line 9.

Now, go ahead and add the variable “x” to your watch list. You should now see this:

Now step over twice, or run to cursor to line 12, and you should see the value of x change from 3 to 6.

Variables that go out of scope (e.g. a local variable inside a function that has already returned to the caller) will stay in your watch window, but will generally either be marked as “not available”, or may show the last known value but grayed out. If the variable returns to scope (e.g. the function is called again), its value will begin showing again. Therefore, it’s fine to leave variables in the watch window, even if they’re out of scope.

Using watches is the best way to watch the value of a variable change over time as you step through your program.

Setting a breakpoint on watched variables

Some debuggers will allow you to set a breakpoint on a watched variable rather than a line. This will cause the program to stop execution whenever the value of that variable changes.

For example, setting such a breakpoint on variable x in the above program will cause the debugger to stop after executing lines 8 and 11 (which is where the value of x is changed).

The watch window can evaluate expressions too

The watch window will also allow you to evaluate simple expressions. If you haven’t already, run to cursor to line 12. Then try entering x + 2 into the watch window and see what happens (it should evaluate to 8).

You can also highlight an expression in your code and then inspect the value of that expression via hover or by adding it to the watch window via the right-click context menu.

Local watches

Because inspecting the value of local variables inside a function is common while debugging, many debuggers will offer some way to quickly watch the value of all local variables in scope.

If you’re just looking to watch the value of a local variable, check the locals window first. It should already be there.