📄 item_020.htm
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<h3><span lang=EN-US>20. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>避免太长的函数。避免太深的嵌套。</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"'>简短比冗长要好，没有嵌套比有嵌套要好：过长的函数和嵌套的代码块常常是因为没有能给每个函数分配一份内聚的职责（参见第</span><span
lang=EN-US>5</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条），而这些通常都可以通过更好的重构来解决。</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"'>每个函数应该是一个内聚的工作任务单元，并具有提示性的名字（参见第</span><span
lang=EN-US>5</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条和第</span><span lang=EN-US>70</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条的讨论）。如果一个函数试图将许多此类小型概念元素合并到长函数体中，那么它最终做的就太多了。</span></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"'>过长的函数体和过深的代码块嵌套（例如</span><span
lang=EN-US>if</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>，</span><span lang=EN-US>for</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，</span><span lang=EN-US>while</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>try</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>代码块）是两个肇事者，它们使函数更难理解和维护，而且这经常是不必要的。</span></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"'>每一层的嵌套都会增加对理解力的要求，因为你必须在脑子里维护一个栈（例如，进入条件判断，进入循环，进入</span><span
lang=EN-US>try</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>，进入条件判断，</span><span lang=EN-US>…</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）。你有没有曾经在别人的代码中看到一个右括号，并想知道它和那么多的</span><span lang=EN-US>for</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，</span><span lang=EN-US>while</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>if</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>中的哪个相对应？最好根据功能进行解耦，这样有助于避免强迫代码阅读者同时记住太多的上下文。</span></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"'>运用常识和合理性：限制函数的长度和深度。以下所有的好建议都有助于减少函数的长度和嵌套：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </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"'>保持内聚性</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>：给每个函数一份职责（参见第</span><span lang=EN-US>5</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"'>不要重复</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>：最好创建一个具名函数而不要复制相似的代码段。</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"'>最好使用</span><span
lang=EN-US>&&</span></i><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>：在能用</span><span
lang=EN-US>&&</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>条件判断的地方避免使用嵌套的连续的</span><span
lang=EN-US>if</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>。</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"'>不要</span><span
lang=EN-US>try</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"'>得太利害了</span></i><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>；在跨越</span><span
lang=EN-US>try</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>代码块时，最好用析构函数自动进行清理（参见第</span><span
lang=EN-US>13</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"'>最好使用（</span><span
lang=EN-US>STL</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"'>）算法</span></i><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>：它们没有嵌套，通常也更好（参见第</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"'>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"'>不要对类型名称用</span><span
lang=EN-US>switch</span></i><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>：最好使用多态函数（参见第</span><span
lang=EN-US>90</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> </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><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><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>当一个函数的功能无法被合理地分割为单独的子任务时，它变得太长或嵌套太深可能是合理的，因为每个潜在的重构都需要传递许多局部变量和上下文（这样的结果是可读性更差而不是更好）。但是如果这些潜在的函数中的多个具有相似的参数，那么可能可以使这些参数成为一个新类的成员。</span></p>
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