Why do we need virtual functions in C++? (轉貼)

Why do we need virtual functions in C++?

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I'm a C++ newbie myself, but here is how I understood not just what virtual functions are, but why they're required: Let's say you have these two classes: class Animal { public: void eat() { std::cout << "I'm eating generic food."; } }; class Cat : public Animal { public: void eat() { std::cout << "I'm eating a rat."; } }; In your main function: Animal *animal = new Animal; Cat *cat = new Cat; animal->eat(); // Outputs: "I'm eating generic food." cat->eat(); // Outputs: "I'm eating a rat." So far so good, right? Animals eat generic food, cats eat rats, all without virtual. Let's change it a little now so that eat() is called via an intermediate function (a trivial function just for this example): // This can go at the top of the main.cpp file void func(Animal *xyz) { xyz->eat(); } Now our main function is: Animal *animal = new Animal; Cat *cat = new Cat; func(animal); // Outputs: "I'm eating generic food." func(cat); // Outputs: "I'm eating generic food." Uh oh... we passed a Cat into func(), but it won't eat rats. Should you overload func() so it takes a Cat*? If you have to derive more animals from Animal they would all need their own func(). The solution is to make eat() from the Animal class a virtual function: class Animal { public: virtual void eat() { std::cout << "I'm eating generic food."; } }; class Cat : public Animal { public: void eat() { std::cout << "I'm eating a rat."; } }; Main: func(animal); // Outputs: "I'm eating generic food." func(cat); // Outputs: "I'm eating a rat." Done.