item_016.htm

C++程序编写规范,适合C++中级读者

normal'><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>我不喜欢各种形式的预处理和宏。</span><span
lang=EN-US>C++</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"'>的目标之一就是使</span><span lang=EN-US>C</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"'>的预处理成为多余，因为我认为它的行为方式天生容易出错。——</span><span
lang=EN-US>[Stroustup94] §3.3.1<o:p></o:p></span></i></p>

<p class=MsoNormal style='margin-left:36.0pt'><i style='mso-bidi-font-style:
normal'><span lang=EN-US><o:p>&nbsp;</o:p></span></i></p>

<p class=MsoNormal style='margin-left: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"'>在</span><span lang=EN-US>C++</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"'>中几乎不需要宏。如果要定义易于理解的常量</span><span
lang=EN-US>[</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"'>参见第</span><span lang=EN-US>15</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"'>条</span><span lang=EN-US>]</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"'>，那么可以使用</span><span
lang=EN-US>const</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"'>（</span><span lang=EN-US>§5.4</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"'>）或</span><span class=SpellE><span
lang=EN-US>enum</span></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"'>（</span><span lang=EN-US>§4.8</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"'>）；如果要避免函数调用的开销</span><span lang=EN-US>[</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"'>但请参见第</span><span
lang=EN-US>8</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"'>条</span><span lang=EN-US>]</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"'>，那么可以使用</span><span lang=EN-US>inline</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"'>（</span><span
lang=EN-US>§7.1.1</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"'>）；如果要指定一个函数族和类型族</span><span lang=EN-US>[</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"'>参见第</span><span
lang=EN-US>64</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"'>到</span><span lang=EN-US>67</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"'>条</span><span lang=EN-US>]</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"'>，那么可以使用模板；如果要避免名字冲突</span><span
lang=EN-US>[</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"'>参见第</span><span lang=EN-US>57</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"'>到</span><span lang=EN-US>59</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"'>条</span><span
lang=EN-US>]</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"'>，那么可以使用</span><span lang=EN-US>namespace</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"'>（</span><span
lang=EN-US>8.2</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"'>）。——</span><span lang=EN-US>[Stroustup00] §1.6.1<o:p></o:p></span></i></p>

<p class=MsoNormal style='margin-left:36.0pt'><i style='mso-bidi-font-style:
normal'><span lang=EN-US><o:p>&nbsp;</o:p></span></i></p>

<p class=MsoNormal style='margin-left: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"'>关于宏的第一准则就是：除非没有其它办法，否则不要用它。几乎每个宏都揭示了编程语言中，程序中或程序员自身的问题。——</span><span
lang=EN-US>[Stroustup00] §7.8</span></i></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal><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"'>的宏的最主要问题在于它们看起来更适合做它们事实上并不适合做的事情。宏无视作用域，无视类型系统，无视所有其它的语言特性和规则，并从</span><span
lang=EN-US>#define</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>处开始将该符号劫持。宏调用看起来像是符号或函数调用，但实际上哪个都不是。宏不“卫生”，这意味着根据被使用时的上下文，它们会被扩展为完全不同并令人惊讶的东西。宏所进行的文本替换使得要写一个还堪适用的宏就像变魔法，而掌握这种魔法并不值得，因为它太乏味了。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>一些人认为与模板相关的错误像密码一样难以解读，他们可能还没有见过那些由宏的书写不当或使用不当而造成的错误。模板是</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"'>类型系统的一部分，因此编译器可以更好地处理它们（也的确是这样），而宏则是完全与语言分离的部分，因此难以控制。更糟糕的是，不像模板，宏可能会被放大成为一些杂音，在不需要的时候被编译，而这纯粹是意外。最后，宏定义中的错误只有在宏被扩展后才能被编译器指出，而不是在宏定义的时候。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>即使在极少数的情况下有正当的理由要写一个宏（参见例外），也绝对不要考虑用通用的单词或缩写。尽快地</span><span
lang=EN-US>#<span class=SpellE>undefine</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>宏，总是用大写字母给它们起一些醒目而难看的（</span><span
lang=EN-US>SCREAMING_UPPERCASE_AND_UGLY</span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）名字，并且避免把它们放在头文件里。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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><span lang=EN-US><o:p>&nbsp;</o:p></span></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：把一个模板实例化体传给宏</span></i><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>。宏仅能理解</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"'>的圆括号和方括号，并使括号匹配。而</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"'>定义了新的括号结构，也就是模板中使用的</span><span
lang=EN-US>&lt;</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>和</span><span lang=EN-US>&gt;</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。宏不能正确地使它们匹配，这意味着在下面这个宏调用中</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal style='text-indent:36.0pt'><span class=GramE><span
lang=EN-US>MACRO(</span></span><span lang=EN-US> <span class=SpellE>Foo</span>&lt;<span
class=SpellE>int</span>, double&gt; )</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>宏认为传入的参数有两个，分别是</span><span
class=SpellE><span lang=EN-US>Foo</span></span><span lang=EN-US>&lt;<span
class=SpellE>int</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>和</span><span
lang=EN-US>double&gt;</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，而实际上传入的是一个</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"'>实体。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>例外</span><span lang=EN-US><o:p></o:p></span></b></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>只有对少数的重要任务，宏仍是仅有的解决方案，如</span><span
lang=EN-US>#include</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>防护哨（参见第</span><span
lang=EN-US>24</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条），用于条件编译的</span><span lang=EN-US>#<span
class=SpellE>ifdef</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>和</span><span
lang=EN-US>#if defined</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，以及用来实现</span><span
lang=EN-US>assert</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>68</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>&nbsp;</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"'>对条件编译来说（例如：与系统相关的部分），要避免在代码中乱用</span><span
lang=EN-US>#<span class=SpellE>ifdef</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>。最好对代码进行组织，根据宏的使用与否来选择一个通用接口的某个实现，然后在所有的代码中使用该接口。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>在极端的情况下，如果只是需要复制和粘贴一些代码的话，你可能想要（谨慎地）使用宏。</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</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"'>我们注意到</span><span lang=EN-US>[C99]</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>和</span><span lang=EN-US>[Boost]</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>都分别对预处理器做了适度的和激进的扩展。</span></p>

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