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<title>friend 函式、friend 类别</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: friend 函式、friend 类别</a></h1>
在定义类别成员时，私用成员只能被同一个类别定义的成员存取，不可以直接由外界进行存取，然而有些时候，您希望提供私用成员给某些外部函式来存取，这时您
可以设定类别的“好友”，只有好友才可以直接存取自家的私用成员。 <br>
<br>
下面这个程式中使用friend关键字来设定类别的好友函式，该好友可以直接存取该类别的私用成员： <br>
<ul>
<li>Ball.h</li>
</ul>
<pre>class Ball;<br><br>int compare(Ball&, Ball&);<br><br>class Ball { <br>public: <br> Ball(double, char*); <br> <br> double radius() {<br> return _radius;<br> }<br> <br> char* name() {<br> return _name; <br> }<br> <br> void radius(double radius) {<br> _radius = radius;<br> } <br> <br> void name(char *name) {<br> _name = name;<br> }<br> <br> // 宣告朋友函式 <br> friend int compare(Ball&, Ball&);<br> <br>private:<br> double _radius; // 半径 <br> char *_name; // 名称 <br>};</pre>
<br>
<ul>
<li>Ball.cpp</li>
</ul>
<pre>#include "Ball.h"<br><br>// compare 为 Ball 的 friend <br>int compare(Ball &b1, Ball &b2) {<br> // 可直接存取私用成员<br> if(b1._radius == b2._radius)<br> return 0;<br> else if(b1._radius > b2._radius)<br> return 1;<br> else<br> return -1;<br>}<br><br>Ball::Ball(double radius, char *name) { <br> _radius = radius; <br> _name = name;<br>}<br></pre>
<br>
<ul>
<li>main.cpp</li>
</ul>
<pre>#include <iostream><br>#include "Ball.h"<br>using namespace std;<br><br>int main() {<br> Ball b1(10, "RBall");<br> Ball b2(20, "GBall");<br> <br> switch(compare(b1, b2)) {<br> case 1:<br> cout << b1.name() << " 较大" << endl;<br> break;<br> case 0:<br> cout << b1.name() << " 等于 " << b2.name() << endl;<br> break;<br> case -1:<br> cout << b2.name() << " 较大" << endl;<br> break;<br> }<br> <br> return 0;<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">
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<td style="background-color: rgb(0, 0, 0);"><small><span style="color: rgb(255, 255, 255);">GBall 较大</span></small><span style="color: rgb(255, 255, 255);"><br>
</span></td>
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<br>
使用friend函式通常是基于效率的考量，以直接存取私用成员而不透过函式呼叫的方式，来省去函式呼叫的负担，另外您也可以使用friend来重载
（Overload）运算子，之后的主题中会介绍。<br>
<br>
您也可以将某个类别宣告为friend类别，被宣告为friend的类别可以直接存取私用成员，例如：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">class Ball;</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">int
compare(Ball&, Ball&);</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">class Ball { </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;"> ....</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;">
// 宣告朋友类别</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
friend class SomeClass;</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;"> ....</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>
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<br>
如上宣告的话，SomeClass的实例就可以存取Ball实例的私用成员。<br>
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