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<H1><A name=top></A>[19] 继承 — 基础<BR><SMALL><SMALL>(Part of <A
href="http://www.sunistudio.com/cppfaq/index.html"><EM>C++ FAQ Lite</EM></A>, <A
href="http://www.sunistudio.com/cppfaq/copy-permissions.html#[1.2]">Copyright ©
1991-2001</A>, <A href="http://www.parashift.com/" target=OutsideTheFAQ>Marshall
Cline</A>, <A
href="mailto:cline@parashift.com">cline@parashift.com</A>)</SMALL></SMALL></H1>
<P>简体中文版翻译：<A href="http://www.sunistudio.com/nicrosoft">申旻</A>，<A
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<HR>
<H3>FAQs in section [19]:</H3>
<UL>
<LI><A
href="http://www.sunistudio.com/cppfaq/basics-of-inheritance.html#[19.1]">[19.1]
对于C++，继承是否重要？</A>
<LI><A
href="http://www.sunistudio.com/cppfaq/basics-of-inheritance.html#[19.2]">[19.2]
何时该使用继承？</A>
<LI><A
href="http://www.sunistudio.com/cppfaq/basics-of-inheritance.html#[19.3]">[19.3]
在C++中如何表达继承？</A> <IMG alt=UPDATED!
src="[19] Inheritance basics, C++ FAQ Lite.files/updated.gif">
<LI><A
href="http://www.sunistudio.com/cppfaq/basics-of-inheritance.html#[19.4]">[19.4]
将一个派生类型的指针转换成它的基类型可以吗？</A>
<LI><A
href="http://www.sunistudio.com/cppfaq/basics-of-inheritance.html#[19.5]">[19.5]
<TT>public:</TT>, <TT>private:</TT>和 <TT>protected:</TT>有什么不同？</A>
<LI><A
href="http://www.sunistudio.com/cppfaq/basics-of-inheritance.html#[19.6]">[19.6]
为什么派生类不能访问基类的<TT>private:</TT>成员？</A>
<LI><A
href="http://www.sunistudio.com/cppfaq/basics-of-inheritance.html#[19.7]">[19.7]
如何才能在改变类的内在部分时，保护其派生类不被破坏？</A> <IMG alt=UPDATED!
src="[19] Inheritance basics, C++ FAQ Lite.files/updated.gif"> </LI></UL>
<P>
<HR>
<P><A name=[19.1]></A>
<DIV class=FaqTitle>
<H3>[19.1] 对于C++，继承是否重要？</H3></DIV>
<P>是。
<P>继承是面向对象编程和抽象数据类型（ADT）编程的区分标志
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<HR>
<P><A name=[19.2]></A>
<DIV class=FaqTitle>
<H3>[19.2] 何时该使用继承？</H3></DIV>
<P>作为一种特化的机制。
<P>人们抽象事物有两种角度：“部分”和“种类”。Ford Taurus是一种（is-a-kind-of-a）汽车，并且 Ford Taurus 有（has-a）引擎，轮胎等。“部分”层次已经随着ADT风格而成为软件系统的一部分。继承则增加了另一种分解的角度。
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<HR>
<P><A name=[19.3]></A>
<DIV class=FaqTitle>
<H3>[19.3] 在C++中如何表达继承？ <IMG alt=UPDATED!
src="[19] Inheritance basics, C++ FAQ Lite.files/updated.gif"></H3></DIV><SMALL><EM>[Recently
added "derived class of" to the list of synonyms (on 7/00). <A
href="http://www.sunistudio.com/cppfaq/basics-of-inheritance.html#[19.7]">Click
here to go to the next FAQ in the "chain" of recent changes<!--rawtext:[19.7]:rawtext--></A>.]</EM></SMALL>
<P>通过 <TT>:public</TT> 语法：
<P>
<DIV
class=CodeBlock><TT> class Car : public Vehicle {<BR> public:<BR> </TT><EM>// ...</EM><TT><BR> };
</TT></DIV>
<P>我们有几种方式声明以上的关系：<BR>
<UL>
<LI><TT>Car</TT>
是 “一种”（"a kind of a"）<TT>Vehicle（交通工具）</TT>
<LI><TT>Car</TT> 起源于（"derived from"）<TT>Vehicle</TT>
<LI><TT>Car</TT> 是一种特殊化的（"a specialized"） <TT>Vehicle</TT>
<LI><TT>Car</TT> 是<TT>Vehicle</TT>的一个子类（"subclass"）
<LI><TT>Car</TT> 是<TT>Vehicle</TT>的一个派生类（“derived class”）
<LI><TT>Vehicle</TT> 是<TT>Car</TT>的基类（"base class"）
<LI><TT>Vehicle</TT> 是<TT>Car</TT> 的超类（"superclass"）(这在C++ 社群中不常用) </LI></UL>
<P>(注意: 本 FAQ 的论述仅与公有继承（<TT>public</TT> inheritance）有关; <A
href="http://www.sunistudio.com/cppfaq/private-inheritance.html">私有和保护继承</A>并不相同)
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<HR>
<P><A name=[19.4]></A>
<DIV class=FaqTitle>
<H3>[19.4] 将一个派生类型的指针转换成它的基类型可以吗？</H3></DIV>
<P>可以。<BR><BR>派生类对象是基类对象的一种。因此从派生类指针到基类指针的转换是非常安全的，并且始终会发生。例如，如果有一个 car 类型的指针，而实际上指向了 vehicle，这种从 <TT>Car*</TT> 到 <TT>Vehicle*
</TT>的转换是非常安全的和常规的：<BR>
<DIV
class=CodeBlock><TT> void f(Vehicle* v);<BR> void g(Car* c) { f(c); } </TT><EM>//
</EM><I>非常安全; 不用转换</I> </DIV>
<P>(注意: 本 FAQ 的论述仅与公有继承（<TT>public</TT> inheritance）有关; <A
href="http://www.sunistudio.com/cppfaq/private-inheritance.html">私有和保护继承</A>并不相同)
<P><SMALL>[ <A
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<HR>
<P><A name=[19.5]></A>
<DIV class=FaqTitle>
<H3>[19.5] <TT>public:</TT>, <TT>private:</TT>和
<TT>protected:</TT>有什么不同？</H3></DIV>
<UL>
<LI>在类的<TT>private:</TT>节中声明的成员（无论数据成员或是成员函数）仅仅能被类的成员函数和<A
href="http://www.sunistudio.com/cppfaq/friends.html">友元</A>访问。
<LI>在类的<TT>protected:</TT> 节中声明的成员（无论数据成员或是成员函数）仅仅能被类的成员函数，<A
href="http://www.sunistudio.com/cppfaq/friends.html">友元</A>以及子类的成员函数和<A
href="http://www.sunistudio.com/cppfaq/friends.html">友元</A>访问。
<LI>在类的<TT>public:</TT>节中声明的成员（无论数据成员或是成员函数）能被任何人访问。 </LI></UL>
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<HR>
<P><A name=[19.6]></A>
<DIV class=FaqTitle>
<H3>[19.6] 为什么派生类不能访问基类的<TT>private:</TT>成员？</H3></DIV>
<P>为了使派生类在将来基类改变时不受影响。<BR><BR>派生类无法访问基类的私有成员。这样在对基类私有成员作任何改变时，就有效地锁定了派生类。
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<HR>
<P><A name=[19.7]></A>
<DIV class=FaqTitle>
<H3>[19.7] 如何才能在改变类的内在部分时，保护其派生类不被破坏？<IMG alt=UPDATED!
src="[19] Inheritance basics, C++ FAQ Lite.files/updated.gif"></H3></DIV><SMALL><EM>[Recently
renamed "subclass" to "derived class" (on 7/00). <A
href="http://www.sunistudio.com/cppfaq/proper-inheritance.html#[21.4]">Click
here to go to the next FAQ in the "chain" of recent changes<!--rawtext:[21.4]:rawtext--></A>.]</EM></SMALL>
<P>类有两套截然不同的接口，它们分别面向两个截然不同的客户：</P>
<UL>
<LI>有为无关类服务的<TT>public:</TT> 接口
<LI>有为派生类服务的<TT>protected:</TT> 接口 </LI></UL>
<P>除非你期望你的所有子类全部由你自己的团队建立，否则你应该考虑让基类部分成为<TT>private:</TT>，并且用 <TT>protected:</TT>来内联供子类访问基类私有数据的访问函数。使用这种方法，私有部分可以被改变，但是派生类的代码不会被破坏（除非你改变了<TT>protected</TT>的访问函数）。
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