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<h3><span lang=EN-US>73. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>抛出值，捕获引用</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></b></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>学会正确地捕获异常：以值（不是指针）方式抛出异常并以（通常为常量）引用方式捕获异常。这种组合是异常语义的最佳搭配。在重新抛出同一个异常时，优先使用</span><span
lang=EN-US>throw; </span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>避免使用</span><span
lang=EN-US>throw e; </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"'>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"'>在抛出异常时，以值方式抛出对象。避免抛出指针，因为如果抛出指针的话，那么你就需要处理内存管理的问题：你不能抛出一个指向在栈上分配的对象的指针，因为当指针到达异常处理的代码时，栈将被展开，所以对象将不复存在。你可以抛出一个指向动态分配的内存的指针（如果你一开始要报告的错误不是“内存耗尽”的话），但是这样的话你就给异常处理代码增加了负担，它要负责释放内存。如果你觉得实在有必要抛出一个指针，那么可以考虑抛出诸如</span><span
class=SpellE><span lang=EN-US>shared_ptr</span></span><span lang=EN-US><T></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>之类的具有值语义的智能指针，而不要直接抛出</span><span lang=EN-US>T*</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><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>抛出值是最好的方式，因为编译器会负责处理异常对象所涉及的复杂的内存管理。你所需要关心的就是为你的异常类实现一个不抛出异常的复制构造函数（参见第</span><span
lang=EN-US>32</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"'>除非你要抛出一个智能指针，这种情况已经引入了一个间接层，可以保持多态性，否则捕获引用是唯一比较好的方法。以捕获值的方式来捕获异常会导致在异常处理代码处对象被截断（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），这极其过分地去除了对异常对象来说至关重要的多态性。以引用方式捕获异常可以保留异常对象的多态性。</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"'>在重新抛出一个异常</span><span lang=EN-US>e</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>时，最好只写</span><span lang=EN-US>throw; </span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>而不是</span><span lang=EN-US>throw e; </span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>因为前一种方法始终保持了被再次抛出的对象的多态性。</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><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：重新抛出一个经修改后的异常：</span></i><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>优先使用</span><span
lang=EN-US>throw;</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><i style='mso-bidi-font-style:normal'><span lang=EN-US><o:p> </o:p></span></i></p>
<p class=MsoNormal style='text-indent:36.0pt;tab-stops:270.0pt'><span
lang=EN-US>catch( <span class=SpellE>MyException</span>& e ) {<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"'>以非常量引用的方式捕获</span><span lang=EN-US><o:p></o:p></span></i></p>
<p class=MsoNormal style='text-indent:36.0pt;tab-stops:270.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span><span
style='mso-spacerun:yes'> </span><span class=SpellE>e.AppendContext</span>("Passed
through here");<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"'>修改</span><span lang=EN-US><o:p></o:p></span></i></p>
<p class=MsoNormal style='text-indent:36.0pt;tab-stops:270.0pt'><span
lang=EN-US><span style='mso-spacerun:yes'> </span><span
style='mso-spacerun:yes'> </span>throw;<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"'>重新抛出经修改后的对象</span></i></p>
<p class=MsoNormal style='text-indent:36.0pt'><span lang=EN-US>}</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
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