C++——多态（核心中的核心）

多态 (Polymorphism)

1. 静态多态 (Static Polymorphism)

• 改进前:  函数重载时，仅返回值类型不同, 编译器无法区分调用哪个函数

  #include <iostream>
  using namespace std;
  
  int add(int a, int b) {
      return a + b;
  }
  
  double add(int a, int b) { // 错误: 与之前的函数声明冲突
      return a + b;
  }
  
  int main() {
      int result1 = add(1, 2); 
      double result2 = add(1.5, 2.5); 
  
      cout << "result1 = " << result1 << endl;
      cout << "result2 = " << result2 << endl;
      return 0;
  }
  

• 改进后:  修改函数名或参数列表，避免函数重载冲突

  #include <iostream>
  using namespace std;
  
  int addInt(int a, int b) {
      return a + b;
  }
  
  double addDouble(double a, double b) { 
      return a + b;
  }
  
  int main() {
      int result1 = addInt(1, 2); 
      double result2 = addDouble(1.5, 2.5); 
  
      cout << "result1 = " << result1 << endl;
      cout << "result2 = " << result2 << endl;
      return 0;
  }
  

2. 动态多态 (Dynamic Polymorphism)——（虚函数）

• 改进前:  如果没有将基类函数声明为虚函数，则动态绑定不会发生，调用的是基类的函数

  #include <iostream>
  
  using namespace std;
  
  class Shape {
  public:
      void draw() const { cout << "Drawing a shape." << endl; } // 非虚函数
  };
  
  class Circle : public Shape {
  public:
      void draw() const { cout << "Drawing a circle." << endl; } 
  };
  
  class Square : public Shape {
  public:
      void draw() const { cout << "Drawing a square." << endl; } 
  };
  
  int main() {
      Shape* shape1 = new Circle();
      Shape* shape2 = new Square();
  
      shape1->draw(); // 调用 Shape::draw() 
      shape2->draw(); // 调用 Shape::draw() 
  
      delete shape1;
      delete shape2;
      return 0;
  }
  

• 改进后:  将基类函数声明为虚函数，实现动态绑定

  #include <iostream>
  
  using namespace std;
  
  class Shape {
  public:
      virtual void draw() const { cout << "Drawing a shape." << endl; } // 虚函数
  };
  
  class Circle : public Shape {
  public:
      void draw() const override { cout << "Drawing a circle." << endl; } 
  };
  
  class Square : public Shape {public:
      void draw() const override { cout << "Drawing a square." << endl; } 
  };
  
  int main() {
      Shape* shape1 = new Circle();
      Shape* shape2 = new Square();
  
      shape1->draw(); // 调用 Circle::draw() 
      shape2->draw(); // 调用 Square::draw() 
  
      delete shape1;
      delete shape2;
      return 0;
  }
  

虚函数的多态行为需要满足以下三个必要条件：

• 基类声明虚函数
• 派生类重写（override）虚函数、
• 通过指针或引用调用