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    <p>The key to understanding the practical usage of virtual functions is to keep in mind that an object of a certain class can be assigned another object of a class derived from the first object's class.</p> <p>E.g.: </p> <pre><code>class Animal { public void eat() {...} } class FlyingAnimal : Animal { public void eat() {...} } Animal a = new FlyingAnimal(); </code></pre> <p>The <code>Animal</code> class has a function <code>eat()</code> that generally describes how an animal should eat (e.g. put the object in mouth and swallow). </p> <p>However, the <code>FlyingAnimal</code> class should define a new <code>eat()</code> method, because flying animals have a particular way of eating.</p> <p>So the question that comes to mind here is: after I declared the variable <code>a</code> of type <code>Animal</code> and asigned it a new object of type <code>FlyingAnimal</code>, what will <code>a.eat()</code> do? Which of the two methods is called?</p> <p>The answer here is: because <code>a</code> is of type <code>Animal</code>, it will call <code>Animal</code>'s method. The compiler is dumb and doesn't know that you are going to assign an object of another class to the <code>a</code> variable.</p> <p>Here is where the <code>virtual</code> keyword comes in action: if you declare the method as <code>virtual void eat() {...}</code>, you are basically telling the compiler "be careful that I am doing some clever stuff here that you cannot handle because you're not as smart". So the compiler will not attempt to link the call <code>a.eat()</code> to either of the two methods, but instead it tells the system to do it <strong>at runtime</strong>!</p> <p>So only when the code executes, the system will look at <code>a</code>'s <strong>content type</strong> not at its declared type and executes <code>FlyingAnimal</code>'s method.</p> <p>You may wonder: why the hell would I want to do that? Why not say right from the start <code>FlyingAnimal a = new FlyingAnimal()</code> ?</p> <p>The reason for that is that, for example, you may have many derived classes from <code>Animal</code>: <code>FlyingAnimal</code>, <code>SwimmingAnimal</code>, <code>BigAnimal</code>, <code>WhiteDog</code> etc. And at one point you want to define a world containing many <code>Animal</code>s, so you say:</p> <pre><code>Animal[] happy_friends = new Animal[100]; </code></pre> <p>We have a world with 100 happy animals. You initialize them at some point:</p> <pre><code>... happy_friends[2] = new AngryFish(); ... happy_friends[10] = new LoudSnake(); ... </code></pre> <p>And at the end of the day, you want everybody to eat before they go to sleep. So you want to say:</p> <pre><code>for (int i=0; i&lt;100; i++) { happy_friends[i].eat(); } </code></pre> <p>So as you can see, each animal has its own method of eating. Only by using <strong>virtual</strong> functions can you achieve this functionality. Otherwise, everyone would be forced to "eat" in the exact same way: as described in the most general <code>eat</code> function inside the <code>Animal</code> class.</p> <p>EDIT: This behavior is actually <strong>default</strong> in common high-level languages like Java.</p>
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