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<h3><span lang=EN-US>70. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>区分错误与非错误</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"'>违约即是错误：由于一个函数是一个工作单元，因此，操作失败应该被视为错误，或根据失败对函数造成的影响来决定。在函数</span><span
lang=EN-US>f</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>内，当且仅当违反了</span><span lang=EN-US>f</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的前置条件时，或阻碍了</span><span lang=EN-US>f</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>达成调用方的前置条件时，或阻碍了</span><span lang=EN-US>f</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>达成自己的任一后置条件时，或阻碍了</span><span lang=EN-US>f</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>重新确立它负责参与维护的不变性，</span> <span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>失败即是错误。</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"'>特别是，我们在这里把内部编程错误排除在外（即：调用方和被调用方由同一个人或小组负责，比如在一个模块内部），这是一个单独的类别，通常用断言来处理（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> </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"'>根据错误与非错误对函数产生的作用，清楚地区分它们是至关重要的，尤其是对定义安全性保证（参见第</span><span
lang=EN-US>71</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）这一目的来说。本条的关键字是前置条件，后置条件，以及不变性。</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"'>无论你用</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"'>以结构化的风格编程，以面向对象的风格编程，还是以泛型风格编程，函数都是基本的工作单元。函数就它的起始状态做了一定的假设（这被归档为它的前置条件，由调用方负责达成，由被调用方负责验证）并执行一个或多个操作（这被归档为结果或后置条件，由被调用方负责达成）。一个函数可能会负责参与维护一个或多个不变性。特别是，就定义而言，一个非私有的</span><span
lang=EN-US>mutation</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>成员函数是它的对象的一个工作单元，它必须使对象从一个保持了不变性的有效状态转变到另一个有效状态；在该成员函数的函数体中，对象的不变性可能（几乎可以肯定）被打破，这没有什么关系，只要对象的不变性在成员函数结束前被重新确立。高层的函数用低层的函数组成更大的工作单元。</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"'>错误就是任何一种失败，它们使函数无法继续执行。共有三种：</span></p>
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