item_057.htm

C++程序编写规范,适合C++中级读者

mso-hansi-font-family:"Times New Roman"'>，这里不需要特殊的查找，因为显然会在</span><span
lang=EN-US>X</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的作用域内查找</span><span lang=EN-US>f</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）和非成员函数接口（例如：</span><span class=SpellE><span lang=EN-US>cout</span></span><span
lang=EN-US> &lt;&lt; x</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，这里调用了</span><span
lang=EN-US>X</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的非成员函数</span><span lang=EN-US>operator&lt;&lt;</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）。</span><span lang=EN-US>ADL</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>确保了用到了</span><span
lang=EN-US>X</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>并且与</span><span lang=EN-US>X</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的定义一起提供的非成员函数也是</span><span lang=EN-US>X</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>接口的一等公民，就像</span><span lang=EN-US>X</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的直接成员函数一样。</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"'>特别是，当</span><span lang=EN-US>X</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>为</span><span class=SpellE><span lang=EN-US>std::string</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>（参见</span><span lang=EN-US>[Sutter00]</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）时，这就是</span><span lang=EN-US>ADL</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>产生的主要动机。</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"'>考虑定义于名字空间</span><span lang=EN-US>N</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>中的类</span><span lang=EN-US>X</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>&nbsp;</o:p></span></p>

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

<p class=MsoNormal style='margin-left:36.0pt'><span class=GramE><span
lang=EN-US>public</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;&nbsp;</span><span class=GramE>void</span> f();</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 lang=EN-US>X operator<span
class=GramE>+(</span> const X&amp;, const X&amp; );</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"'>一般来说，调用方会希望写这样的代码，这里</span><span
lang=EN-US>x1</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>，</span><span lang=EN-US>x2</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>x3</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>都是</span><span
lang=EN-US>X</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>类型的对象：</span></p>

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

<p class=MsoNormal style='text-indent:36.0pt'><span class=GramE><span
lang=EN-US>x3</span></span><span lang=EN-US> = x1 + x2;</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"'>如果</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"'>是在与</span><span lang=EN-US>X</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>相同的名字空间中声明的，那么没有任何问题，这样的代码会正常工作，因为提供的</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"'>会根据</span><span
lang=EN-US>ADL</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>来查找。</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"'>如果</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"'>不是在与</span><span lang=EN-US>X</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>相同的名字空间中声明的，那么调用方的代码就无法正常工作。调用方有两种方法来绕过这个障碍。第一种方法是使用显式的限定符：</span></p>

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

<p class=MsoNormal style='text-indent:36.0pt'><span class=GramE><span
lang=EN-US>x3</span></span><span lang=EN-US> = <span class=SpellE>N::operator</span>+(
x1, x2 );</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"'>这真是既可悲又可耻，因为它需要用户放弃直观的操作符语法，而这正是操作符重载最主要的用途。另一种方法是使用</span><span
lang=EN-US>using</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>语句：</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>using</span></span><span lang=EN-US> <span class=SpellE>N::operator</span>+;</span></p>

<p class=MsoNormal style='margin-left:36.0pt'><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"'>或者：</span><span lang=EN-US>using namespace N;<o:p></o:p></span></i></p>

<p class=MsoNormal style='margin-left:36.0pt'><span class=GramE><span
lang=EN-US>x3</span></span><span lang=EN-US> = x1 + x2;</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"'>随便写哪个</span><span lang=EN-US>using</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>语句都是完全可以接受的（参见第</span><span lang=EN-US>59</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</span><span lang=EN-US>X</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的作者做了正确的事情，把</span><span lang=EN-US>X</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>operator+</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>放在与</span><span lang=EN-US>X</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>相同的名字空间中的话，那么调用方就不用这么麻烦了。</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"'>关于此问题的另一面，请参见第</span><span lang=EN-US>58</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>&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"'>示例</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><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</span><span lang=EN-US>1</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。</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>用来对</span><span lang=EN-US>X</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>类型的对象进行流操作的</span><span
lang=EN-US>operator&lt;&lt;</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>operator&gt;&gt;</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>可能是最有说服力的例子了，因为它们显然是类</span><span
lang=EN-US>X</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的接口的一部分，但却始终是非成员函数（这是必要的，因为左边的参数是流，而不是</span><span
lang=EN-US>X</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>）。对</span><span lang=EN-US>X</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>对象的其它非成员操作符来说，也是同样的道理。要确保操作符与它们操作的类出现在同一个名字空间中。如果可以选择，那么应该优先把操作符和其它函数作为非成员非友元函数（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）。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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</span><span lang=EN-US>2</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"'>：其它函数。</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>如果</span><span lang=EN-US>X</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的作者提供了一些辅助函数，它们用到了</span><span
lang=EN-US>X</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>对象，那么这些函数应该被放在同一个名字空间中，否则使用了</span><span
lang=EN-US>X</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>对象的调用方代码将无法使用这些函数，除非用显式的限定符或</span><span
lang=EN-US>using</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>语句。</span></p>

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