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計算機程式第十四單元 Polymorphism

授課教師：廖婉君教授

【 本 著 作 除 另 有 註 明 外 ， 採 取 創用 CC

「姓名標示－非商業性－相同方式分享」台灣 3.0

版授權釋出】本課程指定教材為 C++ How to Program, 7/e, Harvey M. Deitel and Paul J. Deitel, both from Deitel & Associates, Inc. ©

2010 。 本講義僅引用部分內容，請讀者自行準備。本作品轉載自 Microsoft Office 2007 多媒體藝廊，依據Microsoft服務合約及著作權法第 46 、 52 、 65 條合理使用。

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An Overview

• Polymorphism o To associate many meanings to one function name by means of the late

binding (runtime) mechanism

o Program in general, execute in specific

• Must work with class hierarchy

• Must work off base-class pointer or reference handles, not off name

handles

o Polymorphism: virtual function + late binding (i.e., run-time)/ dynamic

binding

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Virtual function• To make a function virtual is to tell the complier that “I

don’t know how the function is implemented. Wait until it is used in a program, and then get the implementation from the object instance.”

• e.g., virtual double bill() const; • Pure virtual function

o virtual double bill() const =0;

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More on Class Hierarchy• Upcasting is ok,

o e.g., vpet = vdog;

• Downcasting is not permitted unless dynamic_cast (it only works for pointer type) is explicitly used.

• e.g., (downcasting)

Pet *ppet; ppet=new Dog;

Dog *pdog = dynamic_cast<Dog *>(ppet);

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Virtual Destructor

• Make destructor in the base class virtual (then, the

destructor for the derived class is called), otherwise,

only the destructor for the base class is called. o e.g., SomeCass *p= new SomeClass;

delete p;

o e.g., Base *pBase = new Derived;

delete pBase;

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1-6本作品轉載自 Microsoft Office 2007 多媒體藝廊，依據 Microsoft 服務合約及著作權法第 46 、 52 、 65 條合理使用。