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<div class=Section1>

<h3><span lang=EN-US>97. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>不要用</span><span lang=EN-US>union</span><span
style='font-family:宋体;mso-ascii-font-family:Arial;mso-hansi-font-family:Arial'>来重新解释数据。</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><span lang=EN-US><o:p></o:p></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"'>不诚实也是一种欺骗：通过在</span><span lang=EN-US>union</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>中写入某个成员而读取另一个成员，</span><span lang=EN-US>union</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>可以被滥用来达到“不用强制的强制类型转换”。这比起</span><span class=SpellE><span
lang=EN-US>reinterpret_cast</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>来（参见第</span><span
lang=EN-US>92</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）更具欺骗性，也更无法预测。</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><span lang=EN-US><o:p></o:p></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"'>除非</span><span lang=EN-US>union</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>中的某个域是最后被写入的，否则不要读取该域。如果</span><span lang=EN-US>union</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>中的某个域不是最后被写入的，那么读取该域会导致未定义的行为，这甚至比</span><span class=SpellE><span
lang=EN-US>reinterpret_cast</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>（参见第</span><span
lang=EN-US>92</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）更糟糕，因为对后者来说，编译器仍有一线希望发出警告，或者发出“</span><span
lang=EN-US>impossible reinterpretation</span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>”对诸如把指针转换为字符的操作加以阻止。而在</span><span
lang=EN-US>union</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>被滥用时，任何对数据的重新解释都绝不会产生编译期错误，也不会得到可靠的结果。</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"'>考虑以下代码，它试图写入某一类型（</span><span
lang=EN-US>char*</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>）的值并以另一种不同的类型（</span><span lang=EN-US>long</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）来读取该值：</span></p>

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

<p class=MsoNormal style='margin-left:36.0pt'><span class=GramE><span
lang=EN-US>union</span></span><span lang=EN-US> {</span></p>

<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'>&nbsp;</span><span
style='mso-spacerun:yes'>&nbsp;</span><span class=GramE>long</span> <span
class=SpellE>intValue</span>_;</span></p>

<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'>&nbsp;</span><span
style='mso-spacerun:yes'>&nbsp;</span><span class=GramE>char</span>* <span
class=SpellE>pointerValue</span>_;</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>&nbsp;</o:p></span></p>

<p class=MsoNormal style='margin-left:36.0pt'><span class=SpellE><span
lang=EN-US>pointerValue</span></span><span lang=EN-US>_ = <span class=SpellE>somePointer</span>;</span></p>

<p class=MsoNormal style='margin-left:36.0pt'><span class=GramE><span
lang=EN-US>long</span></span><span lang=EN-US> <span class=SpellE>int</span>
gotcha = <span class=SpellE>intValue</span>_;</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"'>这段代码有两个问题：</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'><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>做了太多的假设：它假设</span><span class=SpellE><span lang=EN-US>sizeof</span></span><span
     lang=EN-US>(long)</span><span style='font-family:宋体;mso-ascii-font-family:
     "Times New Roman";mso-hansi-font-family:"Times New Roman"'>和</span><span
     class=SpellE><span lang=EN-US>sizeof</span></span><span lang=EN-US>(char*)</span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>是相等的，并且位元的表示（</span><span lang=EN-US>bit representation</span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>）也完全一样。这样的假设并不总是成立（参见第</span><span lang=EN-US>91</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'><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>使你的意图对阅读代码的人和编译器变得更晦涩：玩</span><span lang=EN-US>union</span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>的把戏使得编译器更难发现真正的类型错误，使得代码阅读者更难发现逻辑错误，这比起声名狼籍的</span><span
     class=SpellE><span lang=EN-US>reinterpret_cast</span></span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>（参见第</span><span lang=EN-US>92</span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>条）有过之而无不及。</span></li>
</ul>

<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><span lang=EN-US><o:p></o:p></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"'>如果两个</span><span lang=EN-US>POD <span
class=SpellE>struct</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>是某个</span><span
lang=EN-US>union</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的成员，并且结构中以相同类型的成员开始，那么对这些相互匹配的成员来说，写入其中一个再读取另一个是可以的。</span></p>

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