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<div class=Section1>
<h3><span lang=EN-US>93. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>避免用</span><span lang=EN-US>static_cast</span><span
style='font-family:宋体;mso-ascii-font-family:Arial;mso-hansi-font-family:Arial'>来强制转换指针类型。</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</span><span lang=EN-US>static_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>动态对象的指针：有很多安全的做法，从</span><span lang=EN-US>dynamic_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>到重构到重新设计都可以。</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"'>请考虑用功能更强的</span><span lang=EN-US>dynamic_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>代替</span><span lang=EN-US>static_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，这样就不必惦记着</span><span lang=EN-US>static_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>在什么时候是安全的，而在什么时候又是危险的。虽然</span><span lang=EN-US>dynamic_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>可能效率稍低，但是它同时检测非法的类型转换（不要忘了第</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）。用</span><span lang=EN-US>static_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>而不用</span><span lang=EN-US>dynamic_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>就像是为了一年省九角钱，宁可冒摔断腿的危险晚上也不开楼道灯一样。</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"'>在设计时尽量消除向下转型：重构或重新设计代码，这样就不需要转型了。如果你发现自己正在传一个</span><span
lang=EN-US>Base</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>参数给某函数，而该函数事实上需要</span><span
lang=EN-US>Derived</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>才能工作，那么你应该检查函数的调用链，找出所需的类型信息是在哪里丢失的；修改函数的原型通常是很好的解决方案，它同时理清了类型信息的流向。</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"'>过多的向下转型可能说明了基类提供的接口不够充分。这会导致这样的设计：把更多的功能放在派生类中，然后在每次需要派生类接口时进行向下转型。一个比较好的解决方案是重新设计基类，让基类提供更多的功能。</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"'>当且仅当</span><span lang=EN-US>dynamic_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的开销确实影响很大时（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），可以考虑定义自己的转型，自定义的转型可以在调试时使用</span><span lang=EN-US>dynamic_cast</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，而在“全速无保证（</span><span lang=EN-US>all speed no guarantees</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）”模式下（参见</span><span lang=EN-US>[Stroustrup00]</span><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>static_cast</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>template<</span></span><span lang=EN-US>class To, class From>
To checked_cast( From* from ) {</span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span
style='mso-spacerun:yes'> </span><span class=GramE>assert(</span>
dynamic_cast<To>(from) == static_cast<To>(from) &&
"checked_cast failed" );</span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span
style='mso-spacerun:yes'> </span><span class=GramE>return</span>
static_cast<To>( from );</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> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span class=GramE><span
lang=EN-US>template<</span></span><span lang=EN-US>class To, class From>
To checked_cast( From& from ) {</span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span
style='mso-spacerun:yes'> </span><span class=GramE>assert(</span>
dynamic_cast<To>(from) == static_cast<To>(from) &&
"checked_cast failed" );</span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span
style='mso-spacerun:yes'> </span><span class=GramE>return</span>
static_cast<To>( from );</span></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 style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>这两个函数（一个针对指针，一个针对引用）只是测试一下两个转型是否一致。你可以根据自己的需要定制</span><span
lang=EN-US>checked_cast</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，或者用某个库提供的这类函数。</span></p>
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