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<body lang=ZH-CN style='tab-interval:21.0pt;text-justify-trim:punctuation'>
<div class=Section1 style='layout-grid:15.6pt'>
<h3><span lang=EN-US>17</span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>．避免使用魔数（</span><span lang=EN-US>magic
numbers</span><span style='font-family:宋体;mso-ascii-font-family:Arial;
mso-hansi-font-family:Arial'>）。</span></h3>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>摘要</span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>编程序不是变戏法，所以不要念什么咒语：避免直接在代码中使用字面常量，如</span><span
lang=EN-US>42</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>或</span><span lang=EN-US>3.14159</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。它们自身不具备说明性，同时以一种难以觉察的形式引入了代码的重复，而这使维护变得复杂。最好用符号化的名字和表达式来代替字面常量，如</span><span
lang=EN-US>width * <span class=SpellE>aspectRatio</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>讨论</span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>符号化的名字加入了更多的信息，并引入了唯一需要维护的地方；而在代码各处重复的数值常量是匿名的，在维护时也很麻烦。常量应该是枚举类型或</span><span
lang=EN-US>const</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>值，并具有适当的作用域和命名。</span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>一个</span><span lang=EN-US>42</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的意义可能和另一个</span><span lang=EN-US>42</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>并不一样。更糟的是，如果后来要将</span><span lang=EN-US>42</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>改为另一个常量，程序员预先做的计算（如“这个</span><span lang=EN-US>84</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>是五行代码前用的</span><span lang=EN-US>42</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的两倍”）会使这个过程变得乏味并且很容易出错。</span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>最好用符号化的常量代替硬编码的字面值。将字面值与代码分开，这样就可以让非程序开发人员审查并更新它们，这样也减少了重复，并有助于国际化。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>示例</span><span lang=EN-US><o:p></o:p></span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><i style='mso-bidi-font-style:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>示例</span><span lang=EN-US>1</span></i><i style='mso-bidi-font-style:
normal'><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>：在名字空间层级的重要的领域相关的常量。</span></i></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:99.0pt 234.0pt'><span
class=GramE><span lang=EN-US>const</span></span><span lang=EN-US> <span
class=SpellE>size_t</span><span style='mso-tab-count:1'> </span>PAGE_SIZE<span
style='mso-tab-count:1'> </span>=
8192,</span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:99.0pt 234.0pt'><span
lang=EN-US><span style='mso-tab-count:1'> </span>WORDS_PER_PAGE<span
style='mso-tab-count:1'> </span>=
PAGE_SIZE / <span class=SpellE><span class=GramE>sizeof</span></span><span
class=GramE>(</span><span class=SpellE>int</span>),</span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:99.0pt 234.0pt'><span
lang=EN-US><span style='mso-tab-count:1'> </span>INFO_BITS_PER_PAGE<span
style='mso-tab-count:1'> </span>= 32 * CHAR_BIT;</span></p>
<p class=MsoNormal style='tab-stops:99.0pt 234.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><i style='mso-bidi-font-style:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>示例</span><span lang=EN-US>2</span></i><i style='mso-bidi-font-style:
normal'><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>：与类相关的常量。</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>可以在类定义中定义静态整型常量，其它类型的常量需要单独的定义或者简短的函数。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>// File <span
class=SpellE>widget.h</span></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>Class Widget {</span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:288.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span>static const <span
class=SpellE>int</span> <span class=SpellE>defaultWidth</span> = 400;<span
style='mso-tab-count:1'> </span>//
</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>在类声明中提供的值</span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:288.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span>static const double <span
class=SpellE>defaultPercent</span>;<span style='mso-tab-count:1'> </span>//
</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>在类定义中提供的值</span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span class=GramE>static</span> const
char* Name() { return “Widget”; } </span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>};</span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>// File <span
class=SpellE>widget.cpp</span></span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:288.0pt'><span
lang=EN-US>const double <span class=SpellE>Widget::defaultPercent</span> =
66.67;<span style='mso-tab-count:1'> </span>//
</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>在类定义中提供的值</span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:288.0pt'><span
lang=EN-US>const <span class=SpellE>int</span> <span class=SpellE>Widget::defaultWidth</span>;<span
style='mso-tab-count:1'> </span>//
</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>仍然需要定义</span></p>
<p class=MsoNormal style='tab-stops:252.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
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