Entering Data in a Code::Blocks Window

Sometimes it isn’t very obvious how to enter data into a Code::Blocks window. One of the windows that seems to be causing problems for a number of readers is the Watches window. You open this window by choosing Debug | Debugging Windows | Watches. The purpose of this window is to let you view the content of variables in your application, which is an essential part of the debugging process. In order to view the variable (or other expression) content, you must enter it in the Watches window. Book III of C++ All-In-One Desk Reference For Dummies tells you all about debugging.

One technique for entering the variable is to select it in the editing window and the drag it to the Watches window. The variable will appear in the Watches window along with its value. However, this approach only works for variables and expressions that actually appear in your code. You might want to enter some other expression (or manually enter the variable, rather than drag and drop it). The Watches window consists of rows and columns as shown here.

WatchEntry01

The name of the variable or the expression you want to view appears in the first column. To enter a new value into the Watches window, click directly in the first empty left column cell. The row will turn blue and you’ll see a red insertion point appear in the cell as shown in the screenshot. Now you can type the variable name or expression you want to work with and press Enter. Let’s say your variable is named i. It might look like this:

WatchEntry02

Notice that the row is now white because it isn’t selected. However, you can see the name of the variable, i, it’s value 1983844706, and it’s type int. The row is in red because the value of i has just changed (unchanged values appear in black so you can see them easier). As you debug your application, you can now watch the value of i for changes.

Sometimes it isn’t obvious how to enter information into Code::Blocks (or any other application for that matter). When that happens, the focus turns to the application, rather than the work you need to do, and the experience becomes frustrating. Let me know about your book-related Code::Blocks questions at [email protected] and I’ll do my best to answer them. Because I don’t have a direct connection to the vendor, my ability to answer other sorts of questions is limited.

 

Passing By Value Versus By Reference

A number of readers have written me about the examples that use pointers in the book. One of the most common questions comes from the example on page 149 of C++ All-In-One Desk Reference For Dummies. In this example, the application contains a function that accepts an argument by reference. The use of a reference seems to have many people confused and this post will hopefully clear the issue up.

The first thing you need to understand is that there isn’t any magic going on here. The process for working with functions remains constant in C++. When you call any function with an argument, the application creates memory to hold the argument. The memory is local to that function. When the function ends, the memory is released. What the memory holds, now that changes.

Think of the memory as a box. This box has an address that points to it. When you work with a pointer, you’re working with the address. On the other hand, when you work with the value, you’re working with the content pointed to by the pointer, or the contents of the box. Whether the box the function receives contains a pointer or the value depends on how you write the function and what you pass to it from the caller.

Now, let’s look at an example so that you can better understand what’s going on.  This example will do the same thing to the number each time, but using different techniques: by value, using a pointer, and by reference.

#include <iostream>
 
using namespace std;
 
void MessMeUpA(int myparam)
{
    myparam = myparam * 2 + 10;
}
 
void MessMeUpB(int* myparam)
{
    // Obtain the current value.
    int currentValue = *myparam;
 
    // Perform the required math.
    currentValue = currentValue * 2 + 10;
 
    // Save the result.
    *myparam = currentValue;
}
 
void MessMeUpC(int &myparam)
{
    myparam = myparam * 2 + 10;
}
 
int main()
{
    // Call by value.
    int mynumber = 30;
    MessMeUpA(mynumber);
    cout << "By Value: " << mynumber << endl;
 
    // Call using a pointer.
    mynumber = 30;
    MessMeUpB(&mynumber);
    cout << "By Pointer: " << mynumber << endl;
 
    // Call using a reference.
    mynumber = 30;
    MessMeUpC(mynumber);
    cout << "By Reference: " << mynumber << endl;
 
    return 0;
}

You may notice that part of this example comes directly from page 149. If you run this example, you’ll see this output:

PassingData01

When you pass the data by value, you’re passing the information, not a pointer to the information. As a result, MessMeUpA() receives a value of 30. It doesn’t receive a pointer to the initial variable, nor does it obtain a reference to the initial variable. As a result, when the application performs the calculation, the result is thrown away.

When the application calls MessMeUpB(), it provides a pointer to the variable. However, the pointer isn’t a value. As a result, you move the value pointed to by the pointer into a local variable, perform the required math, and then move the value back into the original pointer. As a consequence, the original value is altered to 70.

Finally, when the application calls MessMeUpC(), the function obtains a reference to the original memory location. When the function performs the math, it’s actually using the original value as pointed to by the reference, which means that this is a kind of pointer, just not passed from the caller. The changes made in MessMeUpC() are reflected in the original value because you’re using a pointer to that value and not a copy of the value in local memory.

I highly recommend that anyone trying to understand how code works use the debugger to trace through that code. It’s instructive to look at each of the functions to see how they look. Place a breakpoint in each of the functions and then start the debugger. Open the Watches window, if necessary, by choosing Debug | Debugging Windows | Watches. Here is the view of the Watches window when calling MessMeUpA().

PassingData02

What you see here is a function argument—an integer that contains the value of 30. Now, let’s look at MessMeUpB().

PassingData03

In this case, you see a local variable, currentValue, that contains the updated value and a pointer to the variable used by the caller in myparam. The pointer gives you access to the value held by the caller and allows you to change it. Finally, let’s look at MessMeUpC().

PassingData04

Look at the function argument, myparam. The debugger is giving you a valuable insight here. See the address provided with the variable? Match that address to the pointer provided to MessMeUpB() and you’ll see that they’re the same. The reference is simply another sort of pointer, but one that gives you direct access to the memory without having to go through the extra steps required by pointers. The debugger even shows you that the current value of that memory is 30 (the math hasn’t been performed as of yet).

A function argument is simply a box that holds either a value or a pointer. Whether that pointer is an actual pointer or a reference depends on how you write you code. That’s the point of the example. Pointers don’t have to be mysterious or hardthey’re simply an address for an area of memory (a box if you will) that holds a specific value. Please let me know if you have any questions about this issue at [email protected].