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<h3><span lang=EN-US>10. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>将全局和共享的数据减至最少</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 style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>共享会导致竞争：避免使用共享的数据，尤其是全局数据。共享的数据加强了耦合，而这降低了可维护性，通常也降低了性能。</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 style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>本条较之第</span><span lang=EN-US>18</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条的特殊处理更为通用。</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"'>避免使用位于名字空间作用域中的具有外部链接的数据，避免把数据作为类的静态成员。它们使程序的逻辑变得更加复杂，并导致程序的不同部分（更糟的是，相距甚远的部分）之间更紧密的耦合。共享的数据削弱了单元测试，因为如果一段代码使用了共享的数据，那么它的正确性不仅取决于过去对数据的更改，更取决于大量的目前未知的代码是否正常工作，如果这些代码在后来进一步使用了该共享数据。</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"'>另外，全局名字空间中的对象的名字污染了全局名字空间。</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"'>如果必须使用全局的，名字空间作用域的，或静态的类对象，那么一定要小心地初始化这些对象。这些对象在不同的编译单元中的初始化顺序是未定义的，因此需要特殊的技术来正确地处理（参见参考资料）。初始化顺序的规则并不显而易见，所以要尽量避免，但如果确实非用不可，那么就得熟悉这些规则并小心使用。</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"'>位于名字空间作用域中的对象，作为静态成员的对象，或在线程或进程间共享的对象会在多线程及多进程的环境中降低并行性，并通常是产生性能和可伸缩性的瓶颈的根源（参见第</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"'>条）。力争“不共享任何数据”，最好用通信机制（例如，消息队列）代替数据共享。</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"'>最好降低类之间的耦合性，并将类之间的交互减至最少。（参见</span><span
lang=EN-US>[Cargill92]</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><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></b></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>在整个程序范围内的设备</span><span class=SpellE><span
lang=EN-US>cin</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，</span><span
class=SpellE><span lang=EN-US>cout</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，及</span><span
class=SpellE><span lang=EN-US>cerr</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>是特殊的，并经过特殊实现。一个（类）工厂必须维护一个注册表，以知道要调用哪个函数来创建某一给定类型，并且一般来说整个程序中只有一个注册表（但这最好只作为工厂的内部实现，而不是共享的全局对象；参见第</span><span
lang=EN-US>11</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 style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>代码如果确实在不同的线程中共享了对象，那么应该总是串行化对共享的对象的访问（参见第</span><span
lang=EN-US>12</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条和</span><span lang=EN-US>[Sutter04c]</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>
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