📄 item_027.htm
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style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，如下所示：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>T& <span
class=SpellE>T::operator</span>@<span class=GramE>=(</span> const T& ) {</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </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"'>实现</span><span
lang=EN-US> …<o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span class=GramE>return</span> *this;</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>}</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>T <span
class=GramE>operator@(</span> const T& lhs, const T& <span
class=SpellE>rhs</span> ) {</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span>T <span class=GramE>temp(</span> lhs );</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span class=GramE>return</span> temp @= <span
class=SpellE>rhs</span>;</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>}</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"'>这两个函数协同工作。赋值版本执行实际的工作，并返回位于它左边的操作数。非赋值版本从</span><span
lang=EN-US>lhs</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>创建一个临时对象，调用赋值版本对它进行修改，然后返回该临时对象。</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"'>注意这里的</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"'>是一个非成员函数，因此它允许对左边和右边的操作数进行隐式转换而不会有任何区别，这正是我们想要的性质。（参见第</span><span
lang=EN-US>44</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条。）例如，如果定义了一个类</span><span lang=EN-US>String</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，它有一个以</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"'>为参数的隐式构造函数，那么把</span><span lang=EN-US>operator+( const
String&, const String& )</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>定义为非成员函数可以使得</span><span
lang=EN-US>char + String</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>String + char</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>都能用；而成员函数版本的</span><span
class=SpellE><span lang=EN-US>String::operator</span></span><span lang=EN-US>+(
const String& )</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>则只接受后一种形式。一个注重效率的实现可能会选择定义</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"'>的多个非成员重载函数，这样就可以避免在转换过程中产生的临时对象。</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"'>如果可能的话，应该把</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"'>也定义为非成员函数（参见第</span><span lang=EN-US>44</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条）。无论在哪种情况下，都应该把所有的非成员操作符放在同一个名字空间中，这样既便于调用方使用，又可以避免在名字查找时得到出乎意料的结果（参见第</span><span
lang=EN-US>57</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 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"'>一个变体是把</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"'>的第一个参数按值传递。采用这种方法，我们让编译器隐式地替我们执行复制操作，这同时也给了编译器更大的余地来进行优化：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>T& operator@<span
class=GramE>=(</span> T& lhs, const T& <span class=SpellE>rhs</span> )
{</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </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"'>实现</span><span
lang=EN-US> …<o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span class=GramE>return</span> lhs;</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>}</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt;tab-stops:252.0pt'><span
lang=EN-US>T <span class=GramE>operator@(</span> T lhs, const T& <span
class=SpellE>rhs</span> ) {<span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// lhs</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"'>通过值传递</span><span lang=EN-US><o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span class=GramE>return</span> lhs @= <span
class=SpellE>rhs</span>;</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>}</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"'>另一个变体是让</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"'>返回一个</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"'>值。这项技术的优点是可以禁止一些无意义的代码，如</span><span lang=EN-US>a + b = c</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，但是它同时也禁止了一些可能会有用的代码结构，如</span><span lang=EN-US>a = (b +
c).replace(pos, n, d)</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>——这个表达式一下子做了许多事情，把字符串</span><span
lang=EN-US>b</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"'>串联起来，替换掉其中一些字符，并把最终结果赋给</span><span lang=EN-US>a</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><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"'>示例</span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><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"'>示例：字符串的</span><span lang=EN-US>+=</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"'>操作符的一个实现。</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>在串联字符串时，事先知道字符串的长度是有用的，这样只需分配一次内存。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>String& <span
class=SpellE>String::operator</span>+<span class=GramE>=(</span> const
String& <span class=SpellE>rhs</span> ) {</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </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"'>实现</span><span
lang=EN-US> …<o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span class=GramE>return</span> *this;</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>}</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>String operator<span
class=GramE>+(</span> const String& lhs, const String& <span
class=SpellE>rhs</span> ) {</span></p>
<p class=MsoNormal style='margin-left:21.0pt;tab-stops:252.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span>String temp;<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"'>一开始为空</span><span lang=EN-US><o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:21.0pt;tab-stops:252.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span><span class=SpellE><span
class=GramE>temp.Reserve</span></span><span class=GramE>(</span> <span
class=SpellE>lhs.size</span>() + <span class=SpellE>rhs.size</span>() );<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"'>分配足够的内存</span><span lang=EN-US><o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:21.0pt;tab-stops:252.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span><span class=GramE>return</span>
(temp += lhs) += <span class=SpellE>rhs</span>;<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"'>附加字符串并返回</span></i></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>}</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"'>在某些情况下（例如：复数的</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"'>），一个操作符可能会大量改变它左边的操作数，在这种情况下，用</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"'>来实现</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"'>可能会比反过来的实现方式更有利。</span></p>
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