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#include <iostream> using namespace std; class Base { public: virtual void who() { cout << "Base\n"; } }; class first_d : public Base { public: void who() { cout << "First derivation\n"; } }; class second_d : public Base { // who() not defined }; int main() { Base base_obj; Base *p; first_d first_obj; second_d second_obj; p = &base_obj; p->who(); // access Base's who() p = &first_obj; p->who(); // access first_d's who() p = &second_obj; p->who(); /* access Base's who() because second_d does not redefine it */

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_album[index].Caption = caption;

23:

try { _album.Save(_album.FileName); } catch (Exception) { MessageBox.Show("Unable to save new " + "caption to album file."); } return true; }

return 0; }

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One further change we can make here is to alter the text displayed in the corresponding menu item. This will provide visual feedback to the user on which property they are actually changing, especially when the Details view is not displayed.

This program now outputs the following:

Keep in mind that inherited characteristics are hierarchical To illustrate this point, imagine that in the preceding example second_d is derived from first_d instead of Base When who( ) is referenced relative to an object of type second_d (in which who( ) is not defined) it is the version of who( ) declared inside first_d that is called, since it is the class closest to second_d In general, when a class does not override a virtual function, C++ uses the first definition that it finds in reverse order of derivation

MODIFY THE TEXT DISPLAYED IN THE EDIT LABEL MENU Action 7 In the MainForm.cs [Design] window, add a Popup event handler for the menuEditLabel menu. Enable the contained menus only if a single item is selected in the view. Set the menu s text to Name or Caption depending on which type of object is displayed in the list. Result

As stated at the start of this chapter, virtual functions in combination with derived types allow C++ to support run-time polymorphism Polymorphism is essential to object-oriented programming because it allows a generalized class to specify those functions that will be common to any derivative of that class, while allowing a derived class to specify the exact implementation of those functions In other words, the base class dictates the general interface that any object derived from that class will have, but it lets the derived class define the actual method This is why the phrase one interface, multiple methods is often used to describe polymorphism Part of the key to successfully applying polymorphism is understanding that base and derived classes form a hierarchy that moves from greater to lesser generalization (base to derived) Hence, when used correctly, the base class provides all elements that a derived class can use directly plus the basis for those functions that the derived class must implement on its own Having a consistent interface with multiple implementations is important because it helps the programmer handle increasingly complex programs For example, when you develop a program, all objects you derive from a particular base class are accessed in the same general way, even if the specific actions vary from one derived class to the next This means that you need to remember only one interface rather than several Further, the separation of interface and implementation allows the creation of class libraries, which can be provided by a third party If these libraries are implemented correctly, they provide a common interface that you can use to derive your own specific classes

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private void menuEdit_Popup (object sender, System.EventArgs e) {

To get an idea of the power of the one interface, multiple methods concept, examine this short program It creates a base class called figure This class is used to store the dimensions of various two-dimensional objects and to compute their areas The function set_dim( ) is a standard member function because its operation is common to all derived classes However, show_area( ) is declared as virtual because the way the area of each object is computed varies The program uses figure to derive two specific classes, called square and triangle