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This program does not compile because get_access( ) is attempting to access a nonstatic variable You may not see an immediate need for static members, but as you continue to write programs in C++, you will find them very useful in certain situations because they allow you to avoid the use of global variables

As you know, in C++, the virtual keyword is used to declare virtual functions that will be overridden by a derived class However, virtual also has another use that enables you to specify virtual base classes To understand what a virtual base class is and why the keyword virtual has a second meaning, let s begin with the short, incorrect program shown here:

Set the version number of the MyPhotos application to 16.4. HIDE THE TOOLBAR IN OUR CHILD FORM Action 1 2 Locate the OnLoad method in the MainForm.cs code window. Update this method to hide the toolbar when this form is an MDI child form. Result

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// This program contains an error and will not compile #include <iostream> using namespace std; class base { public: int i; };

protected override void OnLoad(EventArgs e) { if (IsMdiChild) { menuExit.Text = "&Close"; toolBarMain.Visible = false; } base.OnLoad(e); }

// d1 inherits base class d1 : public base { public: int j; }; // 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; di dj dk dm = = = = 10; 20; 30; 40; // this is ambiguous, which i

ambiguous, which i di << " "; dj << " " << dk << " "; dm;

Not very exciting, but it does the job. A similar argument could be made for the status bar. In this case, since the status bar is still accurate and provides some useful information related to the displayed album, we will simply leave this control alone. As for a Toolbar control in the parent form, we will create a simple control with five buttons to demonstrate how this is done. Our buttons will correspond to the New, Open, Save, Previous, and Next menu items in our merged menu. The one 544

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return 0; }

change from what we saw in chapter 13 when creating our original toolbar is that the Save, Previous, and Next buttons must operate on the active child form, rather than the parent form. The ActiveMdiChild property in the Form class is used to identify the active child for a form. We will use this to implement our toolbar buttons in the parent form. We will also use the C# internal keyword here to expose some of our MenuItem objects in the MainForm class to other classes in our assembly, and in particular to our ParentForm class. This keyword is an access modifier like public or protected, and permits any other class in the same assembly to have access to the class member. Let s see how these constructs are used by creating the Toolbar control for our parent form. Since toolbars were discussed in chapter 13, we will simply highlight the required changes without too much detailed discussion.

As the comments in the program indicate, both d1 and d2 inherit base However, d3 inherits both d1 and d2 This means there are two copies of base present in an object of type d3 Therefore, in an expression like

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CREATE A TOOLBAR IN THE PARENT FORM Action 3 Add an ImageList called imageListParent to the ParentForm class in the ParentForm.cs [Design] window. 4 Add the following images to the Images property of this list: bitmaps/OffCtlBr/Small/Color/ NEW.BMP bitmaps/OffCtlBr/Small/Color/ OPEN.BMP bitmaps/OffCtlBr/Small/Color/ SAVE.BMP icons/arrows/ARW08LT.ICO icons/arrows/ARW08RT.ICO 5 Add a ToolBar control to the ParentForm class.

di = 20;

26:

Now that the controls are on the form, we can load an image into the PictureBox control using the OpenFileDialog class. Up until this point we really haven t typed any C# code for our MyPhotos application. We simply set values via Visual Studio and let the environment do the work on our behalf. In this section we finally get our hands dirty. The result of our labors will allow a file to be selected as shown in figure 2.2.