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📁 C++程序编写规范,适合C++中级读者
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<h3><span lang=EN-US>18. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>尽可能局部地声明变量</span></h3>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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>&nbsp;</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"'>避免范围膨胀，对需求来说是这样，对变量的作用域来说也是一样：变量引入了状态，而程序应该尽可能少地处理状态，而且状态的生命期也越短越好。这是第</span><span
lang=EN-US>10</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条的一个值得单独讨论特例。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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>&nbsp;</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"'>生命期长得过分的变量有以下缺点：</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<ul style='margin-top:0cm' type=disc>
 <li class=MsoNormal style='mso-list:l0 level1 lfo1;tab-stops:list 36.0pt'><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"'>它们使程序更难理解和维护</span></i><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>：例如，如果代码只更改了当前驱动器，那么它是否应该更新整个模块范围内的</span><span
     lang=EN-US>path</span><span style='font-family:宋体;mso-ascii-font-family:
     "Times New Roman";mso-hansi-font-family:"Times New Roman"'>字符串？</span></li>
 <li class=MsoNormal style='mso-list:l0 level1 lfo1;tab-stops:list 36.0pt'><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"'>它们的名字污染了它们所在的上下文</span></i><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>：这一点的直接推论就是，名字空间层级的变量是最糟糕的，因为它们对所有的代码都是可见的（参见第</span><span
     lang=EN-US>10</span><span style='font-family:宋体;mso-ascii-font-family:
     "Times New Roman";mso-hansi-font-family:"Times New Roman"'>条）。</span></li>
 <li class=MsoNormal style='mso-list:l0 level1 lfo1;tab-stops:list 36.0pt'><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"'>它们不能被合理地初始化</span></i><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>：在能够合理地初始化一个变量之前，绝对不要声明它。未初始化的变量是</span><span
     lang=EN-US>C</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
     mso-hansi-font-family:"Times New Roman"'>和</span><span lang=EN-US>C++</span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>程序中错虫的根源，并具有渗透性，它们需要程序员主动地关注，因为编译器无法在任何情况下都检测出未初始化的变量（参见第</span><span
     lang=EN-US>19</span><span style='font-family:宋体;mso-ascii-font-family:
     "Times New Roman";mso-hansi-font-family:"Times New Roman"'>条）。</span></li>
</ul>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>特别是，</span><span lang=EN-US>[C99]</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>之前的老版本的</span><span lang=EN-US>C</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>只允许变量在作用域的开始处定义，这种风格在</span><span lang=EN-US>C++</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>中已经过时了。这种限制的一个严重问题就是在作用域的开始处，程序常常还没有足够的相关信息来初始化变量。这样程序员只有两种选择——要么用一些缺省的空值（如零）来初始化，这通常都比较浪费，如果变量在被赋予一个有用的状态之前被使用，那么这还可能导致错误；要么就不初始化，而这是危险的。一个用户自定义类型的变量如果未经初始化，那么该变量会自己初始化为某个空值。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>对策很简单：尽可能局部地声明每个变量，这通常恰好是在程序具备了足够的数据来初始化变量之后，并紧接着首次使用该变量之前。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>例外</span></b></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>有时将变量从循环内提出到循环外是有益的（参见第</span><span
lang=EN-US>9</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）。</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"'>由于常量不增加状态，因此本条对常量不适用（参见第</span><span
lang=EN-US>17</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）。</span></p>

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