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<h3><span lang=EN-US>29. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>考虑通过重载来避免隐式的类型转换。</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><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"'>不要过分地复制对象（</span><span lang=EN-US>Occam's
Razor</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>）：隐式的类型转换提供了语法上的方便性（但请参见第</span><span
lang=EN-US>40</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）。但是如果创建临时对象是不必要的而且优化是适当的（参见第</span><span
lang=EN-US>8</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条），那么可以提供重载函数，让重载函数的签名（</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"'>）完全匹配常见的参数类型，这样就不会引起类型转换。</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><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"'>如果你在办公室里，没纸了，那么你会怎么办？当然，你会走向你可靠的复印机并复印出几张白纸。</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"'>听起来很愚蠢，但通常这正是隐式转换所做的事情：不必要地创建一些临时对象，目的仅仅是执行一些无关紧要的操作，然后把临时对象扔掉（参见第</span><span
lang=EN-US>40</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）。一个常见的例子是字符串比较：</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>class String {<i
style='mso-bidi-font-style:normal'>// …</i></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span>String( const char* text );<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"'>允许隐式转换</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></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>bool operator==(
const String&, const String& );</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><i style='mso-bidi-font-style:
normal'><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"'>代码中的某处</span><span lang=EN-US> …<o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>if( someString
== "Hello" ) {... }</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"'>编译器在看到上面的定义后，会把字符串比较操作编译为</span><b
style='mso-bidi-font-weight:normal'><span lang=EN-US>someString ==
String("Hello")</span></b><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，就像是你编写了这样的代码。考虑到没有必要为了读取字符串而把它复制一遍，这就显得比较浪费了。解决这个问题的办法很简单：定义重载函数，避免转换。例如：</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>bool operator==(
const String& lhs, const String& rhs );<span
style='mso-spacerun:yes'> </span>// #1</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>bool operator==(
const String& lhs, const char* rhs );<span
style='mso-spacerun:yes'> </span>// #2</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>bool operator==(
const char* lhs, const String& rhs );<span
style='mso-spacerun:yes'> </span>// #3</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"'>这看起来好像是大量代码的复制，但实际上这只是“函数签名的复制”，因为一般来说所有这三个函数会用同一个函数来实现。你不太可能会因为这些简单的重载函数而被认为是进行了过早的优化（参见第</span><span
lang=EN-US>8</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条），而且提供这些重载函数会是有益的，尤其是在设计一个程序库的时候，因为很难预先知道哪些通用的类型会被用于有高性能要求的代码中。</span></p>
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