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📁 C++程序编写规范,适合C++中级读者
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<div class=Section1>

<h3><span lang=EN-US>89. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>正确地编写函数对象（</span><span lang=EN-US>Function
Object</span><span style='font-family:宋体;mso-ascii-font-family:Arial;
mso-hansi-font-family:Arial'>）。</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></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"'>要成本低，要可配接：要把函数对象设计为复制成本低的值类型。如果可能和话，让它们继承自</span><span
class=SpellE><span lang=EN-US>unary_function</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>或</span><span class=SpellE><span lang=EN-US>binary_function</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，使之可配接。</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><a name=ch75index09></a><a name=ch75index08></a><a
name=ch75lev1sec2></a><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"'>函数对象是对函数指针的模仿。与函数指针类似，一般约定通过值来传递函数对象。标准库的所有算法都通过值来传递对象，你的算法也应该这样。例如：</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 lang=EN-US>template&lt;class
<span class=SpellE>InputIter</span>, class <span class=SpellE>Func</span>&gt;</span></p>

<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>Function <span
class=SpellE>for_each</span>( <span class=SpellE>InputIter</span> first, <span
class=SpellE>InputIter</span> last, Function f );</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"'>因此，函数对象必须复制成本低并且是单态的（不存在对象分割的问题，所以要避免虚函数；参见第</span><span
lang=EN-US>54</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）。但是大型的和</span><span lang=EN-US>/</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>或多态的对象也是有用的，使用它们也没有关系，只要用</span><span class=SpellE><span
lang=EN-US>Pimpl</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>惯用法（参见第</span><span
lang=EN-US>43</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）把对象的大小和丰富的状态隐藏起来，这样就使得外覆类一方面能达到低成本复制和单态的要求，另一方面仍能访问丰富的状态。外覆类应该：</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 lfo5;tab-stops:list 36.0pt'><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"'>可配接：</span></i><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>继承自</span><span class=SpellE><span lang=EN-US>unary_function</span></span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>或</span><span class=SpellE><span lang=EN-US>binary_function</span></span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>。（参见下面）</span></li>
</ul>

<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 lfo5;tab-stops:list 36.0pt'><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"'>使用</span><span
     lang=EN-US>Pimple</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"'>：</span></i><span style='font-family:宋体;mso-ascii-font-family:
     "Times New Roman";mso-hansi-font-family:"Times New Roman"'>存储一个指针（例如：</span><span
     class=SpellE><span lang=EN-US>shared_ptr</span></span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>），该指针指向一个包含大型</span><span lang=EN-US>/</span><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>丰富的状态的实现。</span></li>
</ul>

<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 lfo5;tab-stops:list 36.0pt'><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"'>有函数调用操作符：</span></i><span
     style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman"'>把这些调用转发给实现对象。</span></li>
</ul>

<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"'>除了可能需要提供的非默认构造函数，赋值操作符，以及析构函数之外，外覆类中所有需要的应该就是这些了。</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"'>函数对象应该可配接。标准的</span><span lang=EN-US>binder</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>adapter</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>要依赖于某些</span><span class=SpellE><span lang=EN-US>typedef</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，如果函数对象是继承自</span><span class=SpellE><span lang=EN-US>unary_function</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>或</span><span class=SpellE><span lang=EN-US>binary_function</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，那么这些</span><span class=SpellE><span lang=EN-US>typedef</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>就已经提供了，非常方便。在实例化</span><span class=SpellE><span lang=EN-US>unary_function</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>或</span><span class=SpellE><span lang=EN-US>binary_function</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>时，要用与</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"'>相同的参数类型和返回类型，另外对每个非指针类型来说，还要把项层的</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"'>和类型中的</span><span lang=EN-US>&amp;</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>去掉。</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"'>要避免提供多个</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"'>函数，因为那样会使配接变得困难。通常不可能提供用于配接的正确</span><span class=SpellE><span
lang=EN-US>typedef</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，因为对不同的</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"'>函数而言，同一个</span><span
class=SpellE><span lang=EN-US>typedef</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>会有不同的值。</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"'>并非所有的函数对象都是</span><span lang=EN-US>predicate</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>Predicate</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>是函数对象的一个子集。</span></p>

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