📄 item_096.htm
字号:
<html xmlns:v="urn:schemas-microsoft-com:vml"
xmlns:o="urn:schemas-microsoft-com:office:office"
xmlns:w="urn:schemas-microsoft-com:office:word"
xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv=Content-Type content="text/html; charset=gb2312">
<meta name=ProgId content=Word.Document>
<meta name=Generator content="Microsoft Word 11">
<meta name=Originator content="Microsoft Word 11">
<link rel=File-List href="filelist-66.xml.css" tppabs="http://www.gesoftfactory.com/developer/cppcodingstandards/Item_096_files/filelist.xml">
<title>96</title>
<!--[if gte mso 9]><xml>
<o:DocumentProperties>
<o:Author>SGuy</o:Author>
<o:LastAuthor>Dove</o:LastAuthor>
<o:Revision>2</o:Revision>
<o:TotalTime>80</o:TotalTime>
<o:Created>2005-08-07T04:53:00Z</o:Created>
<o:LastSaved>2005-08-07T04:53:00Z</o:LastSaved>
<o:Pages>1</o:Pages>
<o:Words>135</o:Words>
<o:Characters>770</o:Characters>
<o:Lines>6</o:Lines>
<o:Paragraphs>1</o:Paragraphs>
<o:CharactersWithSpaces>904</o:CharactersWithSpaces>
<o:Version>11.5606</o:Version>
</o:DocumentProperties>
</xml><![endif]--><!--[if gte mso 9]><xml>
<w:WordDocument>
<w:SpellingState>Clean</w:SpellingState>
<w:GrammarState>Clean</w:GrammarState>
<w:PunctuationKerning/>
<w:ValidateAgainstSchemas/>
<w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid>
<w:IgnoreMixedContent>false</w:IgnoreMixedContent>
<w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText>
<w:Compatibility>
<w:BreakWrappedTables/>
<w:SnapToGridInCell/>
<w:WrapTextWithPunct/>
<w:UseAsianBreakRules/>
<w:DontGrowAutofit/>
<w:UseFELayout/>
</w:Compatibility>
<w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel>
</w:WordDocument>
</xml><![endif]--><!--[if gte mso 9]><xml>
<w:LatentStyles DefLockedState="false" LatentStyleCount="156">
</w:LatentStyles>
</xml><![endif]-->
<style>
<!--
/* Font Definitions */
@font-face
{font-family:宋体;
panose-1:2 1 6 0 3 1 1 1 1 1;
mso-font-alt:SimSun;
mso-font-charset:134;
mso-generic-font-family:auto;
mso-font-pitch:variable;
mso-font-signature:3 135135232 16 0 262145 0;}
@font-face
{font-family:"\@宋体";
panose-1:2 1 6 0 3 1 1 1 1 1;
mso-font-charset:134;
mso-generic-font-family:auto;
mso-font-pitch:variable;
mso-font-signature:3 135135232 16 0 262145 0;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{mso-style-parent:"";
margin:0cm;
margin-bottom:.0001pt;
mso-pagination:widow-orphan;
font-size:12.0pt;
font-family:"Times New Roman";
mso-fareast-font-family:宋体;}
h3
{mso-style-next:Normal;
margin-top:12.0pt;
margin-right:0cm;
margin-bottom:3.0pt;
margin-left:0cm;
mso-pagination:widow-orphan;
page-break-after:avoid;
mso-outline-level:3;
font-size:13.0pt;
font-family:Arial;}
span.SpellE
{mso-style-name:"";
mso-spl-e:yes;}
@page Section1
{size:612.0pt 792.0pt;
margin:72.0pt 90.0pt 72.0pt 90.0pt;
mso-header-margin:35.4pt;
mso-footer-margin:35.4pt;
mso-paper-source:0;}
div.Section1
{page:Section1;}
-->
</style>
<!--[if gte mso 10]>
<style>
/* Style Definitions */
table.MsoNormalTable
{mso-style-name:"Table Normal";
mso-tstyle-rowband-size:0;
mso-tstyle-colband-size:0;
mso-style-noshow:yes;
mso-style-parent:"";
mso-padding-alt:0cm 5.4pt 0cm 5.4pt;
mso-para-margin:0cm;
mso-para-margin-bottom:.0001pt;
mso-pagination:widow-orphan;
font-size:10.0pt;
font-family:"Times New Roman";
mso-ansi-language:#0400;
mso-fareast-language:#0400;
mso-bidi-language:#0400;}
</style>
<![endif]--><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="2050"/>
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1"/>
</o:shapelayout></xml><![endif]-->
</head>
<body lang=ZH-CN style='tab-interval:36.0pt'>
<div class=Section1>
<h3><span lang=EN-US>96. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>不要对非</span><span lang=EN-US>POD</span><span
style='font-family:宋体;mso-ascii-font-family:Arial;mso-hansi-font-family:Arial'>类型使用</span><span
class=SpellE><span lang=EN-US>memcpy</span></span><span style='font-family:
宋体;mso-ascii-font-family:Arial;mso-hansi-font-family:Arial'>或</span><span
class=SpellE><span lang=EN-US>memcmp</span></span><span style='font-family:
宋体;mso-ascii-font-family:Arial;mso-hansi-font-family:Arial'>。</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><span lang=EN-US><o:p></o:p></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"'>不要给对象拍</span><span lang=EN-US>X</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>光片（参见第</span><span lang=EN-US>91</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条）：除非对象的表示就是一块原始的内存，否则不要用</span><span class=SpellE><span
lang=EN-US>memcpy</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
class=SpellE><span lang=EN-US>memcmp</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>来复制或比较任何对象。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span class=SpellE><span lang=EN-US>memcpy</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 class=SpellE><span lang=EN-US>memcmp</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>破坏了类型系统。用</span><span class=SpellE><span lang=EN-US>memcpy</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>来复制对象就象是用复印机来印钱一样。用</span><span class=SpellE><span
lang=EN-US>memcmp</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>来比较对象就象是根据斑点的数量来比较豹子一样。这两种方式可能看起来也行得通，但是要达到真正可接受的程度，它们就显得太粗糙了。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </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"'>对象到处都有信息隐藏（可能是软件工程中最有用的原则之一，参见第</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）：对象隐藏了数据（参见第</span><span lang=EN-US>41</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条），并通过复制构造函数和赋值操作符（参见第</span><span lang=EN-US>52</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>至</span><span lang=EN-US>55</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>条）提供了复制该数据所需的精确抽象。用</span><span
class=SpellE><span lang=EN-US>memcpy</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>来处理这些数据严重违背了信息隐藏的原则，并经常导致内存和资源的泄漏（最好的情况），或程序崩溃（更糟糕），或是未定义的行为（最差的情况）。例如：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>{</span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:279.0pt'><a
name="OLE_LINK2"></a><a name="OLE_LINK1"><span style='mso-bookmark:OLE_LINK2'><span
lang=EN-US><span style='mso-spacerun:yes'> </span></span></span></a><span
class=SpellE><span style='mso-bookmark:OLE_LINK1'><span style='mso-bookmark:
OLE_LINK2'><span lang=EN-US>shared_ptr</span></span></span></span><span
style='mso-bookmark:OLE_LINK1'><span style='mso-bookmark:OLE_LINK2'><span
lang=EN-US><<span class=SpellE>int</span>> p1( new <span class=SpellE>int</span>
), p2( new <span class=SpellE>int</span> );<span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// </i></span></span></span><span
style='mso-bookmark:OLE_LINK1'><span style='mso-bookmark:OLE_LINK2'><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"'>在堆上创建两个</span><span
class=SpellE><span lang=EN-US>int</span></span></i></span></span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:279.0pt'><span
style='mso-bookmark:OLE_LINK1'><span style='mso-bookmark:OLE_LINK2'><span
lang=EN-US><span style='mso-spacerun:yes'> </span><span class=SpellE>memcpy</span>(
&p1, &p2, <span class=SpellE>sizeof</span>(p1) );<span
style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// </i></span></span></span><span
style='mso-bookmark:OLE_LINK1'><span style='mso-bookmark:OLE_LINK2'><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"'>糟糕：令人发指的错误</span><span
lang=EN-US><o:p></o:p></span></i></span></span></p>
<span style='mso-bookmark:OLE_LINK2'></span><span style='mso-bookmark:OLE_LINK1'></span>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>} <i
style='mso-bidi-font-style:normal'>// </i></span><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"'>内存泄漏：</span><span lang=EN-US>p2</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"'>未被释放</span></i></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span><span
style='mso-spacerun:yes'> </span><i style='mso-bidi-font-style:normal'>// </i></span><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"'>非法访问内存：</span><span
lang=EN-US>p1</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</span><span class=SpellE><span lang=EN-US>int</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"'>被释放了两次</span><span
lang=EN-US><o:p></o:p></span></i></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"'>滥用</span><span class=SpellE><span
lang=EN-US>memcpy</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>可能会影响到一些非常基本的方面，如对象的类型和标识。编译器通常会在多态对象中嵌入一些隐藏的数据（所谓的虚函数表，或</span><span
class=SpellE><span lang=EN-US>vptr</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），这些数据是对象在运行时候的标识。如果是多重继承，那么对象中会有多个这样的</span><span
class=SpellE><span lang=EN-US>vptr</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>共存，位于不同的偏移处，而在涉及虚继承的时候，大部分编译器实现都会加入更多的内部指针。一般情况下，编译器会负责管理所有隐藏的数据，而</span><span
class=SpellE><span lang=EN-US>memcpy</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>则只会带来灾难。</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"'>与此类似，除了位元数据，</span><span class=SpellE><span
lang=EN-US>memcmp</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>也不适合用来比较任何更为复杂的对象。有时候，它做得太少了（例如：比较</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"'>风格的字符串和比较</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"'>对象的指针是不同的）。荒谬的是，有时候它又做得太多了（例如：</span><span class=SpellE><span
lang=EN-US>memcmp</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>会比较一些不属于对象状态的无用信息，如由编译器插入的用于对齐的填充字节）。无论是哪种情况，比较的结果都是错的。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
</div>
</body>
</html>
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -