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your C++/CLI programs (as long you re not using safe mode) if your design requires it. You just can t have it be a managed type. Another difference between inheritance in C++/CLI and classic C++ is the absence of private or protected inheritance. Inheritance is always public, but the public keyword is optional. Private inheritance was sometimes used in classic C++ to inherit an implementation: it is intended to model the has-a relationship, which can also be modeled by containing the type as a field. It is sometimes said that value types cannot participate in inheritance. This is not strictly true; value types can implement interfaces, but not inherit from other value classes or reference types. Value types do inherit from System::ValueType, although they have to be boxed in order to realize this inheritance relationship. Unboxed value types have no virtual function table with which to participate in polymorphism.

C++/CLI provides some additional ways to control whether functions in a derived class override a base class function, or whether they simply provide a new function that hides the base class function. The question is, Why The answer has to do with real-world problems associated with updating the base class to a new version. Library vendors often add new functions, including virtual functions, to their base classes to support new features. Consumers of these libraries generally have their own classes that use the library s base classes. It s possible that method name conflicts arise between the library consumer s derived classes and the library vendor s base classes. To make this a little more concrete, let s say we re creating an adventure game in which we have a player up against numerous monsters. Our game is extensible, so we ship our game library, and developers create their own monster types by inheriting from the Monster type in our library. We also have other objects in the game other than monsters, such as weapons, armor, and the like (all represented by an Item class) and tiles on the game map. So, our hierarchy looks like Listing 8-1. Listing 8-1. The Monster Hierarchy // game_library.cpp public ref class GameObject { }; public ref class Monster : GameObject { }; public ref class MapTile : GameObject { };

public ref class Item : GameObject { }; To explore versioning issues, suppose a client of this game library has a Scroll class (see Listing 8-2) representing magic spells written out on a scroll. This class supports a Read method that invokes a spell. Listing 8-2. A Class for Magic Spells // client_game.cpp #using "game_library.dll" ref class Scroll : Item { public: void Read() { /* read the scroll and invoke the spell */ } }; Let s say that we are shipping a new version of our library, and we add functionality to read and write GameObject instances from a file. So we add Read and Write methods, as in Listing 8-3. Listing 8-3. New Functionality for Game Objects public ref class GameObject { public: void Initialize(); virtual void Read() { /* read data from file */ } virtual void Write() { /* write data to file */ } }; They re virtual so that derived classes can read and write their own information, if needed. The question is, What happens when the user upgrades to the new version In C++, what might happen is that the virtual Read method for reading the scroll would silently override the Read method to read from a file. Then, at runtime, when we try to read data (as in Listing 8-4), the Read function in the Scroll class gets called and the game character casts a spell not what was intended at all. Listing 8-4. Reading with an Unintended Method void GameObject::Initialize() { if (reading_from_file) { Read(); // Oops! Scroll method called. } // ... }