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<title>纯虚拟函式、抽象类别（Abstract class）</title>
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<h3><a href="http://caterpillar.onlyfun.net/GossipCN/index.html">From
Gossip@caterpillar</a></h3>
<h1><a href="CppGossip.html">C++
Gossip: 纯虚拟函式、抽象类别（Abstract class）</a></h1>
C++预设函式成员都不是虚拟函式，如果要将某个函式成员宣告为虚拟函式，则要加上"virtual"关键字，然而C++提供一种语法定义“纯虚拟函式”
（Pure virtual
function），指明某个函式只是提供一个介面，要求继承的子类别必须重新定义该函式，定义纯虚拟函式除了使用关键字"virtual"之外，要在函
式定义之后紧跟着'='并加上一个0，例如：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">class Some {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">public:<br>
// 纯虚拟函式<br style="font-weight: bold;">
virtual void someFunction() = 0;</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> ....</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">};</span><br>
</div>
<br>
一个类别中如果含有虚拟函式，则该类别为一“抽象类别”（Abstract class），该类别只能被继承，而不能用来直接生成实例，如果试图使用一个抽象类别来生成实例，则会发生编译错误。<br>
<br>
以下举个实际的例子，先假设您设计了两个类别：ConcreteCircle与HollowCircle：<br>
<div style="margin-left: 40px; font-weight: bold; font-family: Courier New,Courier,monospace;">class ConcreteCircle {<br>
public:<br>
void radius(double radius) {<br>
_radius = radius;<br>
}<br>
double radius() {<br>
return _radius;<br>
}<br>
void render() {<br>
cout << "画一个半径 " <br>
<< _radius <br>
<< " 的实心圆"<br>
<< endl;<br>
}<br>
private:<br>
double _radius;<br>
};<br>
</div>
<br style="font-family: Courier New,Courier,monospace;">
<br style="font-family: Courier New,Courier,monospace;">
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">class HollowCircle {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">public:</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> void radius(double radius) {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> _radius = radius;</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> double radius() {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> return _radius;</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> void render() {</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> cout << "画一个半径 " </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> << _radius </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> << " 的空心圆"</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> << endl;</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> }</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">private:</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> double _radius;</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">};</span><br>
</div>
<br>
显然的，这两个类别除了render()方法的实作内容不同之外，其它的定义是一样的，而且这两个类别所定义的显然都是“圆”的一种类型，您可以定义一个
抽象的AbstractCircle类别，将ConcreteCircle与HollowCircle中相同的行为与定义提取（Pull
up）至抽象类别中：<br>
<ul>
<li>AbstractCircle.h</li>
</ul>
<pre>#ifndef ABSTRACTCIRCLE<br>#define ABSTRACTCIRCLE<br><br>class AbstractCircle {<br>public:<br> void radius(double radius) {<br> _radius = radius;<br> }<br> double radius() {<br> return _radius;<br> }<br> // 宣告虚拟函式<br> virtual void render() = 0;<br> <br>protected:<br> double _radius;<br>};<br><br>#endif</pre>
<span class="postbody"><br>
注意到在类别宣告了虚拟函式render()，所以AbstractCircle是个抽象类别，它只能被继承，继承了AbstractCircle的类别
必须实作render()函式，接着您可以让ConcreteCircle与HollowCircle类别继承AbstractCircle方法并实作
render()函式：<br>
</span>
<ul>
<li>HollowCircle.h</li>
</ul>
<pre>#include <iostream> <br>#include "AbstractCircle.h"<br>using namespace std; <br><br>class HollowCircle : public AbstractCircle {<br>public:<br> void render() {<br> cout << "画一个半径 " <br> << _radius <br> << " 的空心圆"<br> << endl;<br> }<br><br>};</pre>
<ul>
<li>ConcreteCircle.h</li>
</ul>
<pre>#include <iostream> <br>#include "AbstractCircle.h"<br>using namespace std; <br><br>class ConcreteCircle : public AbstractCircle {<br>public:<br> void render() {<br> cout << "画一个半径 " <br> << _radius <br> << " 的实心圆"<br> << endl;<br> }<br><br>};</pre>
<span class="postbody"><br>
由于共同的定义被提取至AbstractCircle类别中，并于衍生类别中继承了下来，所以在ConcreteCircle与HollowCircle
中无需重覆定义，只要定义个别对render()的处理方式就行了，而由于ConcreteCircle与HollowCircle都是
AbstractCircle的子类别，因而可以使用AbstractCircle上所定义的虚拟操作介面，来操作子类别实例上的方法，如下所示：<br>
</span>
<ul>
<li>main.cpp</li>
</ul>
<pre>#include <iostream> <br>#include "AbstractCircle.h"<br>#include "ConcreteCircle.h"<br>#include "HollowCircle.h"<br>using namespace std; <br><br>void render(AbstractCircle &circle) {<br> circle.render();<br>}<br><br>int main() {<br> ConcreteCircle concrete;<br> concrete.radius(10.0);<br> render(concrete);<br> <br> HollowCircle hollow;<br> hollow.radius(20.0);<br> render(hollow);<br> <br> return 0;<br>}<br></pre>
<span class="postbody"><br>
执行结果：</span><br>
<table style="text-align: left; width: 100%;" border="0" cellpadding="2" cellspacing="2">
<tbody>
<tr>
<td style="background-color: rgb(0, 0, 0);"><small><span style="color: rgb(255, 255, 255);">画一个半径 10 的实心圆<br>
画一个半径 20 的空心圆</span></small><span style="color: rgb(255, 255, 255);"><br>
</span></td>
</tr>
</tbody>
</table>
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<br>
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