📄 item_091.htm
字号:
{mso-level-tab-stop:18.0pt;
mso-level-number-position:left;
margin-left:18.0pt;
text-indent:-18.0pt;}
@list l9
{mso-list-id:-119;
mso-list-type:simple;
mso-list-template-ids:-457160462;}
@list l9:level1
{mso-level-number-format:bullet;
mso-level-text:\F0B7;
mso-level-tab-stop:18.0pt;
mso-level-number-position:left;
margin-left:18.0pt;
text-indent:-18.0pt;
font-family:Symbol;}
@list l10
{mso-list-id:1377124183;
mso-list-type:hybrid;
mso-list-template-ids:1846591246 67698689 67698691 67698693 67698689 67698691 67698693 67698689 67698691 67698693;}
@list l10:level1
{mso-level-number-format:bullet;
mso-level-text:\F0B7;
mso-level-tab-stop:36.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Symbol;}
@list l11
{mso-list-id:2145459888;
mso-list-type:hybrid;
mso-list-template-ids:28373944 67698689 67698691 67698693 67698689 67698691 67698693 67698689 67698691 67698693;}
@list l11:level1
{mso-level-number-format:bullet;
mso-level-text:\F0B7;
mso-level-tab-stop:36.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Symbol;}
ol
{margin-bottom:0cm;}
ul
{margin-bottom:0cm;}
-->
</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>91. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>依赖于对象类型，而不要依赖于对象的表示方法。</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>96</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条）：不要对对象在内存中的具体表示方法做任何假设。取而代之的是，让类型来决定如何在内存中读写该类型的对象。</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><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 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"'>标准对类型在内存中的表示方法做了有限的保证：</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:l11 level1 lfo11;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"'>保证整数以二进制表示。</span></li>
<li class=MsoNormal style='mso-list:l11 level1 lfo11;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"'>保证负整数以二进制补码表示。</span></li>
<li class=MsoNormal style='mso-list:l11 level1 lfo11;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"'>简单数据类型（</span><span lang=EN-US>Plain Old Data, POD</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）在内存中的布局与</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"'>兼容：成员变量根据它们声明的顺序来存储。</span></li>
<li class=MsoNormal style='mso-list:l11 level1 lfo11;tab-stops:list 36.0pt'><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"'>至少为</span><span
lang=EN-US>16</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>位。</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"'>尤其是，下面这些假设可能很普遍，但却无法保证在当前所有的计算机体系结构中都成立，而且很可能在新的体系结构出现时被打破：</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:l10 level1 lfo12;tab-stops:list 36.0pt'><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"'>不一定为</span><span
lang=EN-US>32</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>位，或其它任何一个固定的大小。</span></li>
<li class=MsoNormal style='mso-list:l10 level1 lfo12;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"'>指针和</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"'>的大小并不总是相同，并不总是能在它们之间随意地相互转换。</span></li>
<li class=MsoNormal style='mso-list:l10 level1 lfo12;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"'>类的布局并不总是根据基类和成员声明的顺序来存储的。</span></li>
<li class=MsoNormal style='mso-list:l10 level1 lfo12;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"'>类（甚至是</span><span lang=EN-US>POD</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）的成员之间可能存在用于地址对齐的间隙。</span></li>
<li class=MsoNormal style='mso-list:l10 level1 lfo12;tab-stops:list 36.0pt'><span
class=SpellE><span lang=EN-US>offsetof</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>仅能用于</span><span lang=EN-US>POD</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，而不是所有的类（但是编译器可能并不会报告错误）。</span></li>
<li class=MsoNormal style='mso-list:l10 level1 lfo12;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"'>类可能会包含隐藏的成员。</span></li>
<li class=MsoNormal style='mso-list:l10 level1 lfo12;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"'>指针可能看起来和整数完全不同。如果两个指针是有序的（</span><span lang=EN-US>ordered</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）并把它们强制转换为整数值，那么得到的值的顺序和原来不一定相同。</span></li>
<li class=MsoNormal style='mso-list:l10 level1 lfo12;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"'>在保证可移植性的前提下，无法对自动变量在内存中的位置做任何假设，也无法对栈增长的方向（译注：高地址</span><span
lang=EN-US>vs.</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>低地址）做任何假设。</span></li>
<li class=MsoNormal style='mso-list:l10 level1 lfo12;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"'>指向函数的指针可能和</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"'>的大小不同，即使有些</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"'>迫使你假设它们的大小是相同的。</span></li>
<li class=MsoNormal style='mso-list:l10 level1 lfo12;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"'>并不总是能在任意地址写入任意对象，即使有足够的内存空间也是这样，这是由于地址对齐的缘故。</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"'>只要适当地定义类型，然后用这些类型来读写数据，就不用考虑位元，字和地址。</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"'>的内存模型可以确保高效地执行，同时也不强迫程序员对数据的表示方法有任何依赖。所以不要依赖于对象的表示方法。</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 + -