📄 item_094.htm
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"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"'>即使是撒小谎，有些也会让你受到惩罚：强制去除</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"'>有时会导致未定义的结果，即使是合法的，这也是拙劣的编程风格的一种主要成份。</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"'>一旦用了</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"'>，就绝不（应该）再回头。如果一个对象原先确实定义为</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"'>，那么强制去除</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"'>将使所有的假设失效并导致未定义的行为。例如，编译器可以（也的确是这样做的）将常量数据放在只读存储器（</span><span
lang=EN-US>ROM</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>）中或受到写保护的内存页（</span><span lang=EN-US>RAM
pages</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>）中，强制去除此类真正的</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"'>对象的</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"'>限定符是一种欺骗行为，应该受到惩罚，它经常表现为内存故障。</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"'>即使强制去除</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"'>不会导致程序崩溃，它仍然毁坏了承诺，并且不会表现得如很多人预期地那样。例如，下面的代码不会分配一个大小可变的数组：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span class=GramE><span
lang=EN-US>void</span></span><span lang=EN-US> Foolish( unsigned <span
class=SpellE>int</span> n ) {</span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span class=GramE>const</span> unsigned <span
class=SpellE>int</span> size = 1;</span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:252.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span><span class=SpellE>const_cast</span><unsigned
<span class=SpellE>int</span>&>(size) = n;<span style='mso-tab-count:
1'> </span><i
style='mso-bidi-font-style:normal'>// </i></span><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"'>糟糕：别这样做</span></i></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:252.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span>char buffer[size];<span
style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// </i></span><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"'>分配的大小总是为</span><span lang=EN-US>1</span></i></p>
<p class=MsoNormal style='margin-left:36.0pt'><i style='mso-bidi-font-style:
normal'><span lang=EN-US><span style='mso-spacerun:yes'> </span>// ...<o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>}</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><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"'>有一个隐式的</span><span
class=SpellE><span lang=EN-US>const_cast</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，用来把一个字符串“起死回生”为一个</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"'>：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span class=GramE><span
lang=EN-US>char</span></span><span lang=EN-US>* weird = “Trick or treat?”;</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"'>编译器不声不响地用</span><span class=SpellE><span
lang=EN-US>const_cast</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>把</span><span
lang=EN-US>const char[16]</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>强制转换为</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"'>。允许这样做是为了保持与</span><span lang=EN-US>C
API</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的兼容性，但这是</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"'>类型系统中的一个漏洞。字符串是可以放在</span><span lang=EN-US>ROM</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>中的，试图修改这样的字符串很有可能导致一个内存故障。</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"'>例外</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"'>为了调用</span><span lang=EN-US>const-incorrect
API</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>15</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条），强制去除</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"'>可能是必要的。另外如果一个函数必须接收并返回一个相同类型的引用，该函数有重载的</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"'>和非</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"'>版本，且其中一个通过调用另一个来实现，那么强制去除</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"'>也是有用的：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span class=GramE><span
lang=EN-US>const</span></span><span lang=EN-US> Object& f( const
Object& );</span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>Object& <span
class=GramE>f(</span> Object& <span class=SpellE>obj</span> ) {</span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span>Const Object& ref = <span
class=SpellE>obj</span>;</span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:270.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span>return </span><span
class=SpellE><span lang=EN-US style='font-family:"Arial Black"'>const_cast</span></span><span
lang=EN-US style='font-family:"Arial Black"'><Object&>(</span><span
lang=EN-US> f(ref) </span><span lang=EN-US style='font-family:"Arial Black"'>)</span><span
lang=EN-US>;<span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// </i></span><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"'>必须强制去除返回</span><span lang=EN-US><o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:270.0pt'><span
lang=EN-US><span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// </i></span><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"'>类型的</span><span lang=EN-US>const</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"'>限定符</span></i></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>}</span></p>
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