📄 example3-19.htm
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<title>例3-19</title>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">#include
<iostream.h></font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">#include
<math.h></font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">class
Point</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">{</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">public:</font></p>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">Point(double
xx,double yy) { x=xx;y=yy;}</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">void
Getxy( );</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2"><font color="#FF6600">friend</font>
double Distance(Point &a,Point & b);</font></p>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">private:</font></p>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">double
x,y;</font></p>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">};</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">void
Point::Getxy( )</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">{
</font></p>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">cout<<"("<<x<<","<<y<<")"<<endl;
</font></p>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">}</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">double
Distance(Point &a,Point &b)</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">{</font></p>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">double
dx=a.x-b.x;</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2" color="#008000">//<font FACE="??ì?,SimSun" LANG="ZH-CN">对象的私有成员一般只有成员函数才能访问，这里友元函数中允许访问对象的私有成员</font></font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">double
dy=a.y-b.y;</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">return
sqrt(dx*dx+dy*dy);</font></p>
</blockquote>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">}</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">void
main( )</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">{</font></p>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">Point
p1(3.0,4.0),p2(6.0,8.0);</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">p1.Getxy(
);</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">p2.Getxy(
);</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">double
d=Distance(p1,p2);</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">cout<<"Distance
is"<<d<<endl;</font></p>
</blockquote>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">}</font></p>
</blockquote>
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<font SIZE="3">
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"></font><font FACE="??ì?,SimSun" LANG="ZH-CN" size="2">程序输出结果是：</font></p>
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<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">(3,4)</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">(6,8)</font></p>
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"><font size="2">Distance
is 5</font></p>
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</blockquote>
<font SIZE="3">
<p ALIGN="justify" style="line-height: 100%; margin-top: 0; margin-bottom: 0"></font><font size="2"><font FACE="??ì?,SimSun" LANG="ZH-CN">说明：该程序中的</font>Point<font FACE="??ì?,SimSun" LANG="ZH-CN">类中说明了一个友元函数</font>Distance(
)<font FACE="??ì?,SimSun" LANG="ZH-CN">，它在说明时前面加上了</font><font color="#008000">friend</font><font FACE="??ì?,SimSun" LANG="ZH-CN">关键字，表示它不是成员函数，而是友元函数。但是，它可以引用类中的私有成员，函数体中</font>a.x<font FACE="??ì?,SimSun" LANG="ZH-CN">，</font>b.x<font FACE="??ì?,SimSun" LANG="ZH-CN">，</font>a.y<font FACE="??ì?,SimSun" LANG="ZH-CN">，</font>b.y<font FACE="??ì?,SimSun" LANG="ZH-CN">都是类的私有成员，它们是通过对象引用的。在调用友元函数时，与普通函数的调用一样，不需要像成员函数那样调用。本例中，</font>p1.Getxy(
)<font FACE="??ì?,SimSun" LANG="ZH-CN">和</font>p2.Getxy( )<font FACE="??ì?,SimSun" LANG="ZH-CN">是成员函数的调用，因此要用对象来表示。而</font>Distance(p1,p2)<font FACE="??ì?,SimSun" LANG="ZH-CN">是友元函数的调用，它可直接调用，不需要用对象表示。但如果在友元函数中处理数据成员，都必须使用对象参数。</font></font></p>
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