📄 item_072.htm
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<div class=Section1>
<h3><span lang=EN-US>72. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>尽量用异常来报告错误</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"'>受到伤害时，要（用异常来）抵制：与错误码相比，要尽量用异常来报告错误。对一些无法使用异常的错误（参见第</span><span
lang=EN-US>62</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条），或者一些不属于错误的情况，可以用状态码（</span><span
lang=EN-US>status code</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，例如：返回码，</span><span
class=SpellE><span lang=EN-US>errno</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）来报告。如果不可能或不需要从错误中恢复，那么可以使用其它方法，比如正常或非正常地终止程序。</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"'>在过去</span><span lang=EN-US>20</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>年产生的现代编程语言中，大多数都使用异常作为主要的错误报告机制，这并不是一个巧合。正如它的定义，异常是用来报告正常处理过程的例外情况，也称为“错误”，它在第</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"'>条中被定义为对前置条例，后置条件和不变性的违反。正如所有的错误报告机制，异常不应该出自正常的，成功的操作。</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"'>我们用术语“状态码”来涵盖通过编码来报告状态（包括返回码，</span><span
class=SpellE><span lang=EN-US>errno</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，</span><span
class=SpellE><span lang=EN-US>GetLastError</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>函数，以及其它用来返回或取得编码的策略）的所有形式，用“错误码”特指用来报告错误的状态码。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，和用错误码来报告错误相比，用异常来报告错误具有许多明显的优势，所有这些都使得你的代码更健壮：</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:l1 level1 lfo1;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>程序员不能无视异常：错误码的最糟糕的缺点就是在默认情况下它们会被忽略；即使是给予错误码微不足道的关注，都必须显示地编写代码，以接受错误并做出反应。程序员因为偶然（或因为懒惰）而忘记关注错误码是很平常的事。这使得代码复查变得更困难。程序员不能无视异常；要忽略异常，必须显式地捕获它（即使只是用</span><span
lang=EN-US>catch(...)</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），然后不对之进行处理。</span></li>
</ul>
<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:l1 level1 lfo1;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>异常会自动传递：默认情况下错误码不会跨作用域传递；为了把一个低层的错误码通知高层的调用函数，程序员必须在在中间层的代码中显式地手工编写代码以传递该错误。异常会自动地跨作用域传递，直到被处理为止。（“试图使每个函数都成为防火墙并不是什么好主意。”</span><span
lang=EN-US>[Stroustrup94, §16.8] </span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）</span></li>
</ul>
<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:l1 level1 lfo1;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>异常处理从主控制流中去除了错误处理及恢复：错误码的检测及处理，一旦要写的话，就必须夹杂在主控制流中（并使之变得难以理解）。这使得主控制流以及错误处理的代码都更难以理解和维护。异常处理很自然地把错误检测及恢复移到醒目的</span><span
lang=EN-US>catch</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>代码块中，换句话说，它使错误处理既醒目，又易于使用，而不是像意大利面条那样纠缠在主控制流中。</span></li>
</ul>
<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:l1 level1 lfo1;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>对从构造函数和操作符中报告错误来说，异常处理比其它选择要好：复制构造函数和操作符都有预定义的签名（</span><span
lang=EN-US>signature</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），它们没有给返回值留下余地。特别是，构造函数根本就没有返回类型（连</span><span
lang=EN-US>void</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>都不是），又比如每个</span><span
lang=EN-US>operator+</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>必须有两个参数，并返回一个对象（以某个规定的类型；参见第</span><span
lang=EN-US>26</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>条）。对操作符来说，虽然使用错误码并不能令人满意，但至少还是可行的；这需要类似</span><span
class=SpellE><span lang=EN-US>errno</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的方法，或者更差的方法，比如将状态信息与对象包在一起。对构造函数来说，用错误码是不可行的，因为</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"'>语言把构造函数异常和构造函数失败紧紧地绑定在了一起，因此这两者无疑是同义词；如果我们想用类似</span><span
class=SpellE><span lang=EN-US>errno</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的方法取而代之，比如：</span></li>
</ul>
<p class=MsoNormal style='margin-left:18.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:54.0pt;text-indent:18.0pt;tab-stops:288.0pt'><span
class=SpellE><span lang=EN-US>SomeType</span></span><span lang=EN-US> <span
class=SpellE>anObject</span>;<span style='mso-tab-count:1'> </span><i
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