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which i is being referred to The one in d1 or the one in d2 Since there are two copies of base present in object d, there are two di s As you can see, the statement is inherently ambiguous There are two ways to remedy the preceding program The first is to apply the scope resolution operator to i and manually select one i For example, this version of the program does compile and run as expected:

Intra Video In Integer Transform Motion Estimation Motion Comp/ De-Blocking Inverse Integer Transform Inverse Quantization Integer Transform Quantization Entropy Coding Blu-ray Disc Demystified Inter Quantization

Figure 2.2 The dialog used to select a file in our application. This dialog is created using the OpenFileDialog class.

#include <iostream> using namespace std; class base { public: int i; }; // d1 inherits base class d1 : public base { public: int j; }; C++ // d2 inherits base class d2 : public base { public: int k; }; /* d3 inherits both d1 and d2 This means that there are two copies of base in d3! */ class d3 : public d1, public d2 { public: int m; }; int main() { d3 d; dd2::i = 10; // scope resolved, using d2's i dj = 20; dk = 30; dm = 40;

There are a couple of topics worth discussing here. First we will discuss how to support the dialog shown in figure 2.2. Then we will discuss how to handle the case where the user selects an invalid file. 2.3.1 Event handlers in Visual Studio .NET As discussed in chapter 1, an event is a predefined action that a program can respond to, such as a user clicking a button or resizing a window. In chapter 1 we handled the event that occurs when the user clicks on the Load button. Here we will do the same using Visual Studio rather than a text editor. As before, the Load button handler will allow the user to select a file and then load a Bitmap image of the file into our PictureBox control. If you recall, this involves GETTING STARTED WITH VISUAL STUDIO .NET

// scope resolved, using d2's i cout << dd2::i << " "; cout << dj << " " << dk << " "; cout << dm; return 0; }

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By applying the ::, the program has manually selected d2 s version of base However, this solution raises a deeper issue: What if only one copy of base is actually required Is there some way to prevent two copies from being included in d3 The answer, as you probably have guessed, is yes And this solution is achieved by using virtual base classes When two or more classes are derived from a common base class, you can prevent multiple copies of the base class from being present in a class derived from those classes by declaring the base class as virtual when it is inherited For example, here is another version of the example program in which d3 contains only one copy of base:

setting a Click event handler for the button and using the OpenFileDialog class to prompt the user for an image to load. Let s duplicate our code from chapter 1 in Visual Studio. Our code for the event handler will be identical to that already shown and discussed, so if you skipped ahead and missed this discussion, go back to chapter 1

#include <iostream> using namespace std; class base { public: int i; }; // d1 inherits base as virtual class d1 : virtual public base { public: int j; }; // d2 inherits base as virtual class d2 : virtual public base { public: int k; };

Set the version number of the application to 2.3. IMPLEMENT A CLICK HANDLER FOR THE BTNLOAD BUTTON Action 1 Display the MainForm.cs [Design] window (the Windows Forms Designer window). Add a Click event handler for the Load button.

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/* d3 inherits both d1 and d2 However, now there is only one copy of base in d3 */ class d3 : public d1, public d2 { public: int m; }; int main() { d3 d; di dj dk dm = = = = 10; // no longer ambiguous 20; 30; 40;

protected void btnLoad_Click(object sender, System.EventArgs e) { }

cout << di << " "; // no longer ambiguous cout << dj << " " << dk << " "; cout << dm; C++ return 0; }