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<h3><span lang=EN-US>31. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>不要编写对函数参数的求值顺序有依赖性的代码。</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"'>保持（求值）井然有序：</span><span lang=EN-US>C++</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>标准没有指定函数参数的求值顺序，因此不要依赖于某个特定的顺序。</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><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"'>在</span><span lang=EN-US>C</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>语言的早期，处理器的寄存器是一种稀有资源，为了能在高级语言的复杂表达式中有效地分配它们，编译器承受了很大的压力。为了允许产生更快的代码，</span><span
lang=EN-US>C</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的创造者给予了寄存器分配器（</span><span lang=EN-US>register
<span class=SpellE>allocator</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）额外的自由度：在调用一个函数时，它的参数的求值顺序故意没有指定。这样的动机对今天的处理器来说显然不够充分，但事实已经无法改变，在</span><span
lang=EN-US>C++</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>中参数的求值顺序不仅没有指定，而且正如我们所看到的，在各种编译器之间有很大的不同。（另外请参见第</span><span
lang=EN-US>30</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"'>这可能在不注意的时候导致大麻烦。考虑下面的代码：</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span><span
class=GramE>void</span> Transmogrify( <span class=SpellE>int</span>, <span
class=SpellE>int</span> );</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span></span></p>
<p class=MsoNormal style='text-indent:21.0pt'><span class=SpellE><span
class=GramE><span lang=EN-US>int</span></span></span><span lang=EN-US> count =
5;</span></p>
<p class=MsoNormal style='text-indent:21.0pt'><span lang=EN-US>Transmogrify(
++count, ++count );<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"'>求值的顺序不确定</span><span lang=EN-US><o:p></o:p></span></i></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"'>我们唯一能肯定的是一旦进入</span><span lang=EN-US>Transmogrify</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的函数体</span><span lang=EN-US>count</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>就会是</span><span lang=EN-US>7</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>——但我们无法知道哪个参数是</span><span
lang=EN-US>6</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>，哪个参数是</span><span lang=EN-US>7</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。这种不确定性同样适用于其它不怎么明显的情况，比如函数有副作用，会改变它的参数（或一些全局状态）：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span><span
class=SpellE><span class=GramE>int</span></span> Bump( <span class=SpellE>int</span>&
x ) {return ++x; }</span></p>
<p class=MsoNormal style='text-indent:21.0pt'><span lang=EN-US>Transmogrify(
Bump(count), Bump(count) );<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"'>仍然不确定</span><span lang=EN-US><o:p></o:p></span></i></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"'>根据第</span><span lang=EN-US>10</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条，应该首先避免使用全局及共享的变量。但是即使你不用，其他人的代码可能还是会用。例如，一些标准的函数确实有副作用（例如：</span><span
class=SpellE><span lang=EN-US>strtok</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，以及以</span><span
class=SpellE><span lang=EN-US>ostream</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>为其中一个参数的</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"'>的各种重载函数）。</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"'>解决办法很简单——使用具名对象来强制求值的顺序。（参见第</span><span
lang=EN-US>13</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 style='text-indent:21.0pt'><span class=SpellE><span
class=GramE><span lang=EN-US>int</span></span></span><span lang=EN-US> bumped =
++count;</span></p>
<p class=MsoNormal style='text-indent:21.0pt'><span lang=EN-US>Transmogrify(
bumped, ++count );<span style='mso-tab-count:1'> </span><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"'>没问题</span></i></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
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