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virtual

This word is supplied for c++98 standard, decorate VIRTUAL with OVERRIDE and FINAL is C++11

Declare the function is a virtual function——allow derived class overwrite the virtual function

derived class own all members which is not private from base class

Runtime Polymorphism

#include <iostream>
#include <string>

// base class
class Animal {
public:
  Animal(){
          std::cout<<"Animal Constructor" <<std::endl;
  }
    // 虚函数：允许派生类重写
    virtual std::string speak() const {

        return "Animal Constructor";
    }

    // 虚析构函数（通常都应写）
    virtual ~Animal(){
      std::cout<<"Animal Destructor"<<std::endl;
    };
};

// 派生类
class Dog : public Animal {
public:
  Dog(){
          std::cout<<"Dog Constructor" <<std::endl;
  }
    std::string speak() const override {

        return "Woof!!!";
    }
    ~Dog(){
      std::cout<< "Dog Destructor"<<std::endl;
    }
};

class Cat : public Dog {
public:
  Cat(){
      std::cout<<"Cat Constructor" <<std::endl;
  }
    std::string speak() const override {

      return "Meow!!!";
    }
    ~Cat(){
      std::cout<<"Cat Destructor" <<std::endl;
    }
};


/// @brief global function accept Animal class or pointer
/// @param a derived class from Animal class
void make_sound(const Animal& a) {
    std::cout << a.speak() << std::endl;
}

int main() {

    Animal* cat = new Cat();
    delete cat;
    std::cout << "Demo con/destructor" << std::endl;
    Dog dog;
    make_sound(dog);   // 输出 "Woof!"（实际调用 Dog::speak）
    return 0;

//    make_sound(cat);   // 输出 "Meow!"（实际调用 Cat::speak）
}

const and override

const decorate the function which declare can’t modify self-object members
override declare the function inherit the same function signature from base class, whether thrrow except

construction and destruction

if base class add virtual keyword for destructor function, therefore all derived class for destructor implicitly own virtual attribute

pure virtual function

it indicate the derived class must overwrite the pure virtual function from base class

#include<iostream>
using namespace std;
class Shape {
public:
    virtual void draw() = 0;  // 纯虚函数
};

class Circle : public Shape {
public:
    void draw() override {
        cout << "Draw Circle" << endl;
    }
};

int main() {
    // Shape s; // 错误，抽象类不能实例化
    Shape* s = new Circle();
    s->draw(); // 输出 "Draw Circle"
    delete s;
}

Draw Circle