程序示例

例如，一个基类 Animal 有一个虚函数 eat。子类 Fish 要实做一个函数 eat()，这个子类 Fish 与子类 Wolf 是完全不同的，但是你可以引用类别 Animal 底下的函数 eat() 定义，而使用子类 Fish 底下函数 eat() 的进程。

C++

以下代码是 C++ 的程序示例。要注意的是，这个示例没有异常处理的代码。尤其是 new 或是 vector::push_back 丢出一个异常时，程序在运行时有可能会出现崩溃或是错误的现象。

类别 Animal 的区块图

# include

# include

using namespace std;

class Animal

{

public:

virtual void eat() const { cout << "I eat like a generic Animal." << endl; }

virtual ~Animal() {}

};

class Wolf : public Animal

{

public:

void eat() const { cout << "I eat like a wolf!" << endl; }

};

class Fish : public Animal

{

public:

void eat() const { cout << "I eat like a fish!" << endl; }

};

class GoldFish : public Fish

{

public:

void eat() const { cout << "I eat like a goldfish!" << endl; }

};

class OtherAnimal : public Animal

{

};

int main()

{

std::vector animals;

animals.push_back( new Animal() );

animals.push_back( new Wolf() );

animals.push_back( new Fish() );

animals.push_back( new GoldFish() );

animals.push_back( new OtherAnimal() );

for( std::vector::const_iterator it = animals.begin();

it != animals.end(); ++it)

{

(*it)->eat();

delete *it;

}

return 0;

}

以下是虚函数 Animal::eat() 的输出：

I eat like a generic Animal.

I eat like a wolf!

I eat like a fish!

I eat like a goldfish!

I eat like a generic Animal.

当 Animal::eat() 不是被宣告为虚函数时，输出如下所示：

I eat like a generic Animal.

I eat like a generic Animal.

I eat like a generic Animal.

I eat like a generic Animal.

I eat like a generic Animal.

Java

在Java语言中, 所有的方法默认都是"虚函数". 只有以关键字 final 标记的方法才是非虚函数. 以下是 Java 中虚方法的一个例子:

import java.util.*;

public class Animal {

public void eat() { System.out.println("I eat like a generic Animal."); }

public static void main(String[] args) {

List animals = new LinkedList();

animals.add(new Animal());

animals.add(new Wolf());

animals.add(new Fish());

animals.add(new OtherAnimal());

for (Animal currentAnimal : animals) {

currentAnimal.eat();

}

}

}

public class Wolf extends Animal {

public void eat() { System.out.println("I eat like a wolf!"); }

}

public class Fish extends Animal {

public void eat() { System.out.println("I eat like a fish!"); }

}

public class OtherAnimal extends Animal {}

输出:

I eat like a generic Animal.

I eat like a wolf!

I eat like a fish!

I eat like a generic Animal.

C#

在 C# 语言中, 对基类中的任何虚方法必须用 virtual 修饰, 而派生类中由基类继承而来的重载方法必须用 override 修饰. 以下是 C# 的一个程序实例:

using System;

using System.Collections.Generic;

namespace ConsoleApplication1

{

public class Animal

{

public virtual void Eat()

{

Console.WriteLine("I eat like a generic Animal.");

}

}

public class Wolf : Animal

{

public override void Eat()

{

Console.WriteLine("I eat like a wolf!");

}

}

public class Fish : Animal

{

public override void Eat()

{

Console.WriteLine("I eat like a fish!");

}

}

public class GoldFish : Fish

{

public override void Eat()

{

Console.WriteLine("I eat like a goldfish!");

}

}

public class OtherAnimal : Animal

{

// Eat() method is not overridden, so the base class method will be used.

}

public class Program

{

public static void Main(string[] args)

{

IList animals = new List();

animals.Add(new Animal());

animals.Add(new Wolf());

animals.Add(new Fish());

animals.Add(new GoldFish());

animals.Add(new OtherAnimal());

foreach (Animal currentAnimal in animals)

{

currentAnimal.Eat();

}

}

}

}

输出:

I eat like a generic Animal.

I eat like a wolf!

I eat like a fish!

I eat like a goldfish!

I eat like a generic Animal.