C++ 中的Virtual Function (虚函数)

1.C++ Virtual 用法

这里只讲语法，因为讲原理比较难。还没有涉及到构造函数。那么就直接上代码了：

// VitualFunction.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <iostream>
#include <vector>

using namespace std;
//base class
class Animal{
public:
  virtual void eat(){
    cout << "animal eat" << endl;
  }
  virtual void die(){
    cout << "animal die" << endl;
  }
};

class Dog : public Animal{
  public:
  void eat(){
    cout << "dog eat" << endl;
               Animal::die();//use base class's function
  }
};

class Cat : public Animal{
  public:
  void eat(){
    cout << "cat eat" << endl;
  }
};

class Lion : public Animal{

};

int _tmain(int argc, _TCHAR* argv[])
{
  vector<Animal*> someAnimals;
  someAnimals.push_back(new Animal());
  someAnimals.push_back(new Dog());
  someAnimals.push_back(new Cat());
  someAnimals.push_back(new Lion());

  for(int i = 0; i < someAnimals.size(); ++i){
    someAnimals[i]->eat();
  }

  system("pause");
  return 0;
}

我总是觉得C++的这块语法有点不对，因为我是先搞过C#和Java的。当子类重写父类方法时，连个关键词都没有，就重写了。还有调用父类的方法，连个关键词都没有，直接名字加::就调用了，也太不尊重父类了。这可能是C++支持多重继承的语法决定。

2.C#中的virtual用法

using System;
using System.Collections.Generic;

using System.Text;

namespace VirutalFunctionCShape
{
 //base class  
 public class Animal{
        public virtual void eat(){
            Console.WriteLine("Animal eat");
        }
        public virtual void die(){
            Console.WriteLine("Animal die");
        }
    }

    public class Dog : Animal{
        public override void eat()
        {
            Console.WriteLine("Dog eat");
            base.die();
        }
    }

    public class Cat : Animal{
        public override void eat()
        {
            Console.WriteLine("Cat eat");
        }
    }

    public class Lion : Animal{
     
    }

    class Program
    {
        static void Main(string[] args)
        {
            IList<Animal> someAnimals = new List<Animal>();
            someAnimals.Add(new Animal());
            someAnimals.Add(new Dog());
            someAnimals.Add(new Cat());
            someAnimals.Add(new Lion());

            foreach (Animal animal in someAnimals)
            {
                animal.eat();
            }

            Console.ReadLine();
        }
    }
}

C#中比C++好多了，重写的时候加上了override关键词，调用父类的方法时候加上了base关键词。

3.java 中的 Virtual

import java.util.LinkedList;
import java.util.List;

public class Animal {
  public void eat(){
    System.out.println("Animal eat");
  }

  public void die() {
    System.out.println("Animal die");
  }

  /**
   * @param args
   */
  public static void main(String[] args) {
    List<Animal> someAnimals = new LinkedList<Animal>();
    someAnimals.add(new Animal());
    someAnimals.add(new Dog());
    someAnimals.add(new Cat());
    someAnimals.add(new Lion());

    for(Animal animal : someAnimals){
      animal.eat();
    }
  }

}

public class Cat extends Animal{
  @Override
  public void eat() {
    System.out.println("Cat eat");
  }
}
public class Dog extends Animal{
  @Override
  public void eat(){
    System.out.println("Dog eat");
    super.die();
  }
}
public class Lion extends Animal{

}

java这里好方便，尼玛所有函数默认就是virtual函数，连个virtual关键词都没有，好了，你们要随便重写你们自己搞。重写的时候加上个@override关键词。好像有时候不用也行，看jdk版本。调用父类方法的关键词是super。还行。

参考：

​​C++ Virtual Function​​