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<h3><span lang=EN-US>25. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>通过值，（智能）指针，或引用适当地取得参数。</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"'>精心定义参数：要区分输入，输出，及输入</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"'>输出参数，还要区分值和引用参数。要适当地取得它们。</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"'>为了定义参数，在值，引用，及指针间精心选择是一个良好的习惯，这可以同时最大化安全性和效率。</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"'>虽然效率不应该是我们在一开始最关心的问题（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），但我们也不应该在其它情况，包括清晰性，都相同的情况下（参见第</span><span
lang=EN-US>9</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）编写一些不必要的低效率的代码。</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"'>在选择如何取得参数时，最好是遵循下面的指导方针。对仅用于输入的参数来说：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<ul style='margin-top:0cm' type=disc>
<li class=MsoNormal style='mso-list:l1 level1 lfo3;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>始终给仅用于输入的指针或引用参数加上</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"'>限定符。</span></li>
</ul>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<ul style='margin-top:0cm' type=disc>
<li class=MsoNormal style='mso-list:l1 level1 lfo3;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>最好是通过原始类型（例如：</span><span lang=EN-US>char</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>float</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）和可以通过值来复制并且复制成本低的值对象（例如：</span><span lang=EN-US>Point</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>complex<float></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）来取得参数。</span></li>
</ul>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<ul style='margin-top:0cm' type=disc>
<li class=MsoNormal style='mso-list:l1 level1 lfo3;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>对其它自定义类型的输入，最好是通过</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"'>引用来取得。</span></li>
</ul>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<ul style='margin-top:0cm' type=disc>
<li class=MsoNormal style='mso-list:l1 level1 lfo3;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>如果函数需要参数的复本，那么可以考虑用传递值来代替传递引用。从概念上说，这等价于取得一个</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"'>引用再做一次复制，它可以帮助编译器更好地优化掉临时对象。</span></li>
</ul>
<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"'>对输出或输入</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"'>输出参数来说：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<ul style='margin-top:0cm' type=disc>
<li class=MsoNormal style='mso-list:l2 level1 lfo4;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>如果参数是可选的（因此调用方可以传递空指针来表示“不可用”或“不关心”的值），或者函数要保存指针的一个复本或操控参数的所有权，那么最好是通过（智能）指针传递。</span></li>
</ul>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<ul style='margin-top:0cm' type=disc>
<li class=MsoNormal style='mso-list:l2 level1 lfo4;tab-stops:list 36.0pt'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>如果参数是必需的，而且函数无需保存指向该参数的指针或无需操控参数的所有权，那么最好是通过引用传递。这表明该参数是必需的，并让调用方来负责提供一个有效的对象。</span></li>
</ul>
<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"'>不要使用</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"'>风格的</span><span lang=EN-US>varargs</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>（参见第</span><span lang=EN-US>98</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>条）。</span></p>
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