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<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>你的库散布得越广，你对库的客户使用的构建（</span><span
lang=EN-US>build</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>）环境的控制就越小，你可以在库的外部接口中可靠地使用的类型就越少。模块间的交互涉及到二进制数据的交换。唉，</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"'>没有指定一个标准的二进制交互方法。为了和外界进行交互，广为散布的库可能要特别依赖于内建类型，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"'>和</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"'>。即便是编译相同的类型，使用相同的编译器，不同的编译选项，得到的二进制表示仍有可能不兼容。</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"'>一般来说，要么你能够控制用来构建模块的编译器和编译选项，这样你就可以使用任何类型——要么你无法控制这些，这样你就只能使用平台提供的类型和</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"'>的内建类型（即便如此，在后一种情况下你应该在文档中说明预期的类型大小和表示方法）。尤其是，不要在模块的接口中使用标准库的类型，除非使用它的所有其它模块会和它同时编译，并使用完全相同的标准库实现。</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"'>对由于使用非所有客户都能正确理解的类型而引起的问题，以及由于使用低层抽象而引起的问题，这里有一个权衡。抽象是重要的，如果某些客户只能理解低层的类型，那么你必须使用这些类型，但可以考虑另外提供可选的操作，这些操作使用高层类型。考虑函数</span><span
class=SpellE><span lang=EN-US>SummarizeFile</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，它的参数是将要被处理的文件。对该参数，有三种常见的选择：可以是一个</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"'>指针，指向以</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"'>风格字符串表示的文件名；可以是一个</span><span lang=EN-US>string</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>类型的文件名；也可以是一个</span><span class=SpellE><span lang=EN-US>istream</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>或自定义的</span><span lang=EN-US>File</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>对象。每个选择都是一个权衡：</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'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>选择</span><span lang=EN-US>1</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>char*</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。显然能够访问</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"'>类型的客户群最广。不幸的是，这也是最底层的选择。特别是，它不够健壮（例如：调用方和被调用方必须明确由谁分配和释放内存），比较容易遇到错误（例如：文件可能不存在），也不够安全（例如：对典型的缓冲区溢出攻击来说）。</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'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>选择</span><span lang=EN-US>2</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>string</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。能够访问</span><span lang=EN-US>string</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>类型的客户有更多的限制，它们必须用</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"'>编写，必须使用相同的标准库实现，相同的编译器，以及兼容的编译器设置。更多的限制换来的是更健壮（例如：调用方和被调用方可以不必关心内存管理的问题，但请参见第</span><span
lang=EN-US>60</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>条），更安全（例如：</span><span
lang=EN-US>string</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>会在需要时扩大内部的缓冲区，不易遭受缓冲区溢出攻击）。但这种选择仍然相对低层，因此还是会遇到错误，这些错误必须显式地检测（例如：文件可能不存在）。</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'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>选择</span><span lang=EN-US>3</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>istream</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>或</span><span lang=EN-US>File</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。如果你无论如何要使用类类型（</span><span lang=EN-US>class type</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>），这样就要求客户程序用</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"'>编写，并用相同的编译器和编译开关编译，那么请使用强抽象：</span><span class=SpellE><span
lang=EN-US>istream</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>（或自定义的</span><span
lang=EN-US>File</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>对象，它封装了</span><span
class=SpellE><span lang=EN-US>istream</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，这样就可以避免直接依赖于某个标准库实现）提高了抽象的层次，并使得</span><span
lang=EN-US>API</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>健壮得多。函数知道自己得到的是一个</span><span
lang=EN-US>File</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>或是相应的输入流，它不需要为文件名管理内存，也不受其它两种选择可能会遇到的许多有意无意的错误的影响。现在只剩很少的东西需要检查：文件必须已经被打开，文件内容的格式必须正确，但这些错误是无论如何都可能遇到的。</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"'>即使选择在模块的外部接口中使用较低层的抽象，也还是应该始终在内部使用最高层的抽象，并在模块的边界处将之转换为低层抽象。例如，如果你的一些客户不使用</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"'>，那么可以给客户代码使用不透明的</span><span
lang=EN-US>void*</span><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>int</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>句柄，但在内部仍然使用对象，并且只在模块的接口中进行低层抽象和高层抽象的相互转换。</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"'>示例</span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </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：在模块的接口中使用</span><span class=SpellE><span lang=EN-US>std::string</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></i><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>假设一个模块希望提供下面这个</span><span lang=EN-US>API</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 style='text-indent:36.0pt'><span class=SpellE><span
class=GramE><span lang=EN-US>std::string</span></span></span><span lang=EN-US>
Translate( const <span class=SpellE>std::string</span>& );</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"'>对于一个团队或公司内部使用的库，通常这样就行了。但是如果需要把该模块与使用了不同的</span><span
class=SpellE><span lang=EN-US>std::string</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>实现（具有不同的内存布局）的调用方代码动态链接到一起，就会发生一些奇怪的事情，因为客户和模块彼此不能理解对方的</span><span
lang=EN-US>string</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"'>我们见过一些开发人员，他们把</span><span
class=SpellE><span lang=EN-US>std::string</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>封装在自定义的</span><span
class=SpellE><span lang=EN-US>CustomString</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>中，试图以此来绕过这个问题，但是令他们吃惊的是他们还是会遇到一模一样的问题，这是因为他们无法控制调用方代码的构建过程。</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"'>一个解决方案是用可移植的（可能是内建的）类型，来代替函数中的</span><span
lang=EN-US>string</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>参数类型，或者也可以另外添加一个这样的函数。例如：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span class=GramE><span
lang=EN-US>void</span></span><span lang=EN-US> Translate( const char* <span
class=SpellE>src</span>, char* <span class=SpellE>dest</span>, <span
class=SpellE>size_t</span> <span class=SpellE>destSize</span> );</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"'>使用低层抽象的可移植性更好，但是增加了复杂性。比如在这个例子中，如果缓冲区不够长，那么调用方和被调用方必须明确处理可能发生的字符串截断。（注意这个版本使用的缓冲区由调用方分配，以避免在不同的模块中分配和释放内存所带来的隐患；参见第</span><span
lang=EN-US>60</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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