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<H1><A name=top></A>[12] 赋值算符<BR><SMALL><SMALL>(Part of <A
href="http://www.sunistudio.com/cppfaq/index.html"><EM>C++ FAQ Lite</EM></A>, <A
href="http://www.sunistudio.com/cppfaq/copy-permissions.html#[1.2]">Copyright ©
1991-2001</A>, <A href="http://www.parashift.com/" target=OutsideTheFAQ>Marshall
Cline</A>, <A
href="mailto:cline@parashift.com">cline@parashift.com</A>)</SMALL></SMALL></H1>
<P>简体中文版翻译：<A href="http://www.sunistudio.com/nicrosoft">申旻</A>，<A
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<HR>
<H3>FAQs in section [12]:</H3>
<UL>
<LI><A
href="http://www.sunistudio.com/cppfaq/assignment-operators.html#[12.1]">[12.1]
什么是“自赋值”？</A>
<LI><A
href="http://www.sunistudio.com/cppfaq/assignment-operators.html#[12.2]">[12.2]
为什么应该当心“自赋值”？</A>
<LI><A
href="http://www.sunistudio.com/cppfaq/assignment-operators.html#[12.3]">[12.3]
好，好；我会处理自赋值的。但如何做呢？</A> <IMG alt=UPDATED!
src="[12] Assignment operators, C++ FAQ Lite.files/updated.gif"> </LI></UL>
<P>
<HR>
<P><A name=[12.1]></A>
<DIV class=FaqTitle>
<H3>[12.1] 什么是“自赋值”？</H3></DIV>
<P>自赋值就是将对象赋值给本身。例如，
<P>
<DIV
class=CodeBlock><TT> #include "Fred.hpp" </TT><EM>// 声明
<TT>Fred</TT>
类 </EM><TT><BR> <BR> void userCode(Fred& x)<BR> {<BR> x = x; </TT><EM>// 自赋值</EM><TT><BR> }
</TT></DIV>
<P>很明显，以上代码进行了显式的自赋值。但既然多个指针或引用可以指向相同对象（别名），那么进行了自赋值而不知道的情况也是可能的：
<P>
<DIV
class=CodeBlock><TT> #include "Fred.hpp" </TT><EM>// </EM><EM>声明
</EM><EM><TT>Fred</TT>
类</EM><TT><BR> <BR> void userCode(Fred& x, Fred& y)<BR> {<BR> x = y; </TT><EM>// 如果<TT>&x == &y</TT>就可能是自赋值</EM><TT><BR> }<BR> <BR> int main()<BR> {<BR> Fred z;<BR> userCode(z, z);<BR> }
</TT></DIV>
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<P><A name=[12.2]></A>
<DIV class=FaqTitle>
<H3>[12.2] 为什么应该当心“自赋值”？</H3></DIV>
<P>如果不注意<A
href="http://www.sunistudio.com/cppfaq/assignment-operators.html#[12.1]">自赋值</A>，将会使你的用户遭受非常微妙的并且一般来说非常严重的bug。例如，如下的类在自赋值的情况下将导致灾难：
<P>
<DIV
class=CodeBlock><TT> class Wilma { };<BR> <BR> class Fred {<BR> public:<BR> Fred() : p_(new Wilma()) { }<BR> Fred(const Fred& f) : p_(new Wilma(*f.p_)) { }<BR> ~Fred() { delete p_; }<BR> Fred& operator= (const Fred& f)<BR> {<BR> </TT><EM>// 差劲的代码：没有处理自赋值！</EM><TT><BR> delete p_; </TT><EM>// Line #1</EM><TT><BR> p_ = new Wilma(*f.p_); </TT><EM>// Line #2</EM><TT><BR> return *this;<BR> }<BR> private:<BR> Wilma* p_;<BR> };
</TT></DIV>
<P>如果有人将 <TT>Fred </TT>对象赋给其本身，由于<TT>*this</TT>和 <TT>f</TT> 是同一个对象，line
#1同时删除了<TT>this->p_</TT>和<TT>f.p_</TT>。而 line
#2使用了已经不存在的对象<TT>*f.p_</TT>，这样很可能导致严重的灾难。
<P>作为 <TT>Fred</TT><TT> </TT>类的作者，你最起码有责任<A
href="http://www.sunistudio.com/cppfaq/assignment-operators.html#[12.3]">确信在<TT>Fred</TT>对象上自赋值是无害的</A>。不要假设用户不会在对象上这样做。如果对象由于自赋值而崩溃，那是<I>你的</I>过失。
<BLOCKQUOTE>
<P><SMALL>另外：上述的</SMALL><SMALL><TT>Fred::operator= (const Fred&)</TT></SMALL><SMALL>还有第二个问题：如果在执行</SMALL><SMALL><TT>new Wilma(*f.p_)</TT></SMALL><SMALL>时，<A
href="http://www.sunistudio.com/cppfaq/exceptions.html">抛出了异常</A>（例如，<A
href="http://www.sunistudio.com/cppfaq/freestore-mgmt.html#[16.5]">内存不够的异常</A>或者</SMALL><A
href="http://www.sunistudio.com/cppfaq/exceptions.html#[17.2]"><TT><SMALL>Wilma</SMALL></TT><SMALL>的拷贝构造函数中的异常</SMALL><SMALL><!--rawtext:[17.2]:rawtext--></SMALL></A><SMALL>），</SMALL><SMALL>
<TT>this->p_</TT></SMALL><SMALL>将成为悬空指针——它所指向的内存不再是可用的。这可以通过在删除就对象前创建对象来解决。</SMALL></P></BLOCKQUOTE>
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<P><A name=[12.3]></A>
<DIV class=FaqTitle>
<H3>[12.3] 好，好；我会处理自赋值的。但如何做呢？ <IMG alt=UPDATED!
src="[12] Assignment operators, C++ FAQ Lite.files/updated.gif"></H3></DIV><SMALL><EM>[Recently
reworded the last paragraph (on 7/00). <A
href="http://www.sunistudio.com/cppfaq/operator-overloading.html#[13.3]">Click
here to go to the next FAQ in the "chain" of recent changes<!--rawtext:[13.3]:rawtext--></A>.]</EM></SMALL>
<P><A
href="http://www.sunistudio.com/cppfaq/assignment-operators.html#[12.2]">在你创建类的每时每刻，都应该当心自赋值</A>。这并不意味着需要为你所有的类都增加额外的代码：只要对象优美地处理自赋值，而不管是否必须增加额外的代码。
<P>如果不需要为赋值算符增加额外代码，这里有一个简单而有效的技巧：
<P>
<DIV
class=CodeBlock><TT> Fred& Fred::operator= (const Fred& f)<BR> {<BR> if (this == &f) return *this; </TT><EM>// 优美地处理<A
href="http://www.sunistudio.com/cppfaq/assignment-operators.html#[12.1]">自赋值</A></EM><TT><BR> <BR> </TT><EM>// 此处写正常赋值的代码...</EM><TT><BR> <BR> return *this;<BR> }
</TT></DIV>
<P>显式的测试并不总是必要的。例如，如果修正<A
href="http://www.sunistudio.com/cppfaq/assignment-operators.html#[12.2]">前一个 FAQ<!--rawtext:[12.2]:rawtext-->中的赋值算符</A>使之<A
href="http://www.sunistudio.com/cppfaq/freestore-mgmt.html#[16.5]">处理<TT>new</TT><!--rawtext:[16.5]:rawtext-->抛出的异常</A>和／或<TT>Wilma</TT>类的<A
href="http://www.sunistudio.com/cppfaq/exceptions.html#[17.2]">拷贝构造函数抛出的异常</A>，可能会写出如下的代码。注意这段代码有（令人高兴的）自动处理自赋值的附带效果：
<P>
<DIV
class=CodeBlock><TT> Fred& Fred::operator= (const Fred& f)<BR> {<BR> </TT><EM>// 这段代码优美地（但隐含的）处理<A
href="http://www.sunistudio.com/cppfaq/assignment-operators.html#[12.1]">自赋值</A></EM><TT><BR> Wilma* tmp = new Wilma(*f.p_); </TT><EM>// 如果<A
href="http://www.sunistudio.com/cppfaq/exceptions.html">异常</A>在此处被抛出也没有问题</EM><TT><BR> delete p_;<BR> p_ = tmp;<BR> return *this;<BR> }
</TT></DIV>
<P>在象这个例子的情况下（自赋值是无害的但是低效），一些程序员想通过增加另外的不必要的测试，如“<TT>if (this == &f) return *this;</TT>”来改善自赋值时的效率。通常来说，使自赋值情况更高效而使得非自赋值情况更低效的折衷是错误的。例如，为<TT>Fred</TT>类的赋值算符增加如上的<TT>if</TT>测试会使得非自赋值情况更低效（一个额外的(而且不必要的)条件分支）。如果自赋值实际上一千次才发生一次，那么
<TT>if</TT><TT> </TT>将浪费99.9%的时间周期。
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