程序示例

例如，一個基類 Animal 有一個虛函數 eat。子類 Fish 要實做一個函數 eat()，這個子類 Fish 與子類 Wolf 是完全不同的，但是你可以引用類別 Animal 底下的函數 eat() 定義，而使用子類 Fish 底下函數 eat() 的進程。

C++

以下代碼是 C++ 的程序示例。要注意的是，這個示例沒有異常處理的代碼。尤其是 new 或是 vector::push_back 丟出一個異常時，程序在運行時有可能會出現崩潰或是錯誤的現象。

類別 Animal 的區塊圖

# 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;

}

以下是虛函數 Animal::eat() 的輸出：

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.

當 Animal::eat() 不是被宣告為虛函數時，輸出如下所示：

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語言中, 所有的方法默認都是"虛函數". 只有以關鍵字 final 標記的方法才是非虛函數. 以下是 Java 中虛方法的一個例子:

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 {}

輸出:

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# 語言中, 對基類中的任何虛方法必須用 virtual 修飾, 而派生類中由基類繼承而來的重載方法必須用 override 修飾. 以下是 C# 的一個程序實例:

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();

}

}

}

}

輸出:

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.

總結

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