📄 item_098.htm
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<h3><span lang=EN-US>98. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>不要使用</span><span class=SpellE><span
lang=EN-US>varargs</span></span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>（省略号）</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 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"'>省略号会导致各种问题：省略号是从</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"'>中继承过来的危险品。要避免使用省略号，以高级的</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"'>结构和库取而代之。</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"'>参数数量可变的函数是不错的常用工具，但是不应该用</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 class=SpellE><span
lang=EN-US>varargs</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>来实现。</span><span
class=SpellE><span lang=EN-US>varargs</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>有许多缺点：</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:l0 level1 lfo1;tab-stops:list 36.0pt'><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"'>缺少类型安全性：</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>从本质上说，省略号等于是告诉编译器：“从这里开始由我来接管，我要开始进行</span><span
class=SpellE><span lang=EN-US>reinterpret_cast</span></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>92</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条）。</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:l0 level1 lfo1;tab-stops:list 36.0pt'><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"'>调用方与被调用方之间紧密耦合，两者之间还需要手动地协作：</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>因为</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"'>语言的类型检查被关闭了，所以调用</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"'>被调用代码必须以另一种方式来交流参数的类型。因为无法进行完全的检查，或由调用方或被调用方的其中一方强制施行（参见第</span><span
lang=EN-US>99</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>条），所以这样的协议（例如：</span><span
class=SpellE><span lang=EN-US>printf</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的格式字符串）既容易出错又不安全，并因此声名狼籍。</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:l0 level1 lfo1;tab-stops:list 36.0pt'><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"'>对类类型（</span><span
lang=EN-US>class type</span></i><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"'>）的对象来说，行为未定义：</span></i><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"'>中，除非用</span><span
class=SpellE><span lang=EN-US>varargs</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>传递的是基本类型或简单数据结构（</span><span
lang=EN-US>plain old data, POD</span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），否则行为是未定义的。不幸的是，大多数编译器甚至不会就此发出警告。</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:l0 level1 lfo1;tab-stops:list 36.0pt'><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"'>参数的数量未知：</span></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>对参数数量可变的函数来说，即使是最简单的函数（例如：</span><span lang=EN-US>min</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>），已经知道它的参数类型（例如：</span><span class=SpellE><span
lang=EN-US>int</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），你仍然需要一个协议来给出参数的数量。（讽刺的是，这反而是件好事，因为这样就更不鼓励使用</span><span
class=SpellE><span lang=EN-US>varargs</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）</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"'>应该避免在函数的</span><span lang=EN-US>signature</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>中使用</span><span class=SpellE><span lang=EN-US>varargs</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。应该避免调用使用了</span><span class=SpellE><span lang=EN-US>varargs</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的函数，包括从前的函数和标准</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"'>库函数如</span><span class=SpellE><span lang=EN-US>sprintf</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。诚然，和使用</span><span class=SpellE><span lang=EN-US>stringstream</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>来进行格式化并用</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"'>来输出相比，调用</span><span class=SpellE><span lang=EN-US>sprintf</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>通常看起来更紧凑并易于阅读——就像跳进一辆没有安全带和保险杠的车更容易一样。这样冒险不值得。</span><span
class=SpellE><span lang=EN-US>printf</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>相关的函数易于遭受攻击，在写本书的时候（参见</span><span
lang=EN-US>[Cowan01]</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），这依然是一个严重的安全隐患，也正因为如此，所以开发工具还存在一个完整的分支来帮助查找这种与类型有关的错误（参见</span><span
lang=EN-US>[Tsai01]</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"'>有些库用其它方法（而不是</span><span class=SpellE><span
lang=EN-US>varargs</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）来支持数量可变的参数，它们是类型安全的，尽量使用这些库。例如，</span><span
lang=EN-US>[Boost] format</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>库使用了</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"'>的高级特性，把类型安全性，速度和便于使用融为一体。</span></p>
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