虚拟函数:允许派生类重写基类中的函数。当基类指针指向派生类对象时，调用派生类的虚拟函数。虚继承:解决多重继承中的菱形继承问题。确保每个基类在派生类中只存在一个实例。

C++ 虚拟函数与虚继承：揭示多重继承中的复杂性

虚拟函数

虚拟函数是 C++ 中一种特殊类型的成员函数，允许派生类重写基类中的函数。当基类指针或引用指向派生类对象时，将会调用派生类的虚拟函数。

class Shape {
public:
  virtual double area() const = 0; // 纯虚函数
};

class Rectangle : public Shape {
public:
  Rectangle(double width, double height) : m_width(width), m_height(height) {}
  double area() const override { return m_width * m_height; } // 重写虚函数

private:
  double m_width;
  double m_height;
};

虚继承

虚继承是一种技术，用于解决多重继承中发生的菱形继承问题（也称为多重继承的歧义性）。

class Animal {
public:
  virtual void speak() const { cout << "Animal speaks" << endl; }
};

class Dog : virtual public Animal { // 虚继承
public:
  void speak() const override { cout << "Dog barks" << endl; }
};

class Cat : virtual public Animal { // 虚继承
public:
  void speak() const override { cout << "Cat meows" << endl; }
};

class SiberianHusky : public Dog, public Cat { // 多重继承
public:
  void speak() const override { cout << "Siberian Husky howls" << endl; }
};

虚继承确保每个基类（Animal）在派生类（SiberianHusky）中只存在一个实例，从而避免菱形继承问题。

实战案例

#include <iostream>

using namespace std;

class Shape {
public:
  virtual double area() const = 0;
};

class Rectangle : public Shape {
public:
  Rectangle(double width, double height) : m_width(width), m_height(height) {}
  double area() const override { return m_width * m_height; }

private:
  double m_width;
  double m_height;
};

class Circle : public Shape {
public:
  Circle(double radius) : m_radius(radius) {}
  double area() const override { return M_PI * m_radius * m_radius; }

private:
  double m_radius;
};

int main() {
  // 创建形状的父类指针
  Shape* shape;

  // 创建矩形对象并将其分配给父类指针
  shape = new Rectangle(5, 10);
  cout << "矩形面积：" << shape->area() << endl;

  // 创建圆形对象并将其分配给父类指针
  shape = new Circle(3);
  cout << "圆形面积：" << shape->area() << endl;

  return 0;
}