📄 [c++对象模型][6]sizeof与对象内存布局 - itech's blog - 博客园.htm

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style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st2,level)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st2,level)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st2,number)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st2,number)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st2,math_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st2,math_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st2,Chinese_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st2,Chinese_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st2,sum_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st2,sum_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR><BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;sizeof(st3)=&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">sizeof</SPAN><SPAN 
style="COLOR: #000000">&nbsp;(st3)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st3,level)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st3,level)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st3,number)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st3,number)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st3,math_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st3,math_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st3,Chinese_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st3,Chinese_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp;&nbsp;offsetof(st3,sum_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;offsetof(st3,sum_grade)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>}</SPAN></SPAN></DIV>
<P>&nbsp;</P>
<P>运行结果如下;</P>
<P><IMG 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/2009022419503035.png"></P>
<P>基于上面的对struct的测试，我们是不是有些惊呆哦，对于C++的初学者更是情不自禁的说：“我靠！原来顺序不同所占空间都不同啊，还有那个pack是啥东东啊？”，其实这里蕴含了一个内存对齐的问题，在计算机的底层进行内存的读写的时候，如果内存对齐的话可以提高读写效率，下面是VC的默认规则：</P>
<P><SPAN style="COLOR: #ff0000">1) 结构体变量的首地址能够被其最宽基本类型成员的大小所整除； <BR>2) 
结构体每个成员相对于结构体首地址的偏移量（offset）都是成员大小的整数倍， 如有需要编译器会在成员之间加上填充字节（internal adding）； 
<BR>3) 结构体的总大小为结构体最宽基本类型成员大小的整数倍，如有需要编译器会在最末一个成员之后加上填充字节（trailing 
padding）。</SPAN></P>
<P><SPAN style="COLOR: #ff0000">当然VC提供了工程选项<SPAN 
id=ArticleContent1_ArticleContent1_lblContent>/Zp[1|2|4|8|16]</SPAN>可以修改对齐方式，当然我们也可以在代码中对部分类型实行特殊的内存对齐方式，修改方式为#pragma 
pack( n )，n为字节对齐 <BR>数，其取值为1、2、4、8、16，默认是8，取消修改用#pragma 
pack(),如果结构体某成员的sizeof大于你设置的，则按你的设置来对齐</SPAN><SPAN 
style="COLOR: #ff0000">。</SPAN></P>
<P>三 struct的嵌套</P>
<P>1）实例：</P>
<DIV class=cnblogs_code><IMG id=Code_Closed_Image_214953 style="DISPLAY: none" 
onclick="this.style.display='none'; document.getElementById('Code_Closed_Text_214953').style.display='none'; document.getElementById('Code_Open_Image_214953').style.display='inline'; document.getElementById('Code_Open_Text_214953').style.display='inline';" 
height=16 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/ContractedBlock.gif" 
width=11 align=top><IMG id=Code_Open_Image_214953 
onclick="this.style.display='none'; document.getElementById('Code_Open_Text_214953').style.display='none'; getElementById('Code_Closed_Image_214953').style.display='inline'; getElementById('Code_Closed_Text_214953').style.display='inline';" 
height=16 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/ExpandedBlockStart.gif" 
width=11 align=top><SPAN class=cnblogs_code_Collapse 
id=Code_Closed_Text_214953>Code</SPAN><SPAN id=Code_Open_Text_214953><BR><!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--><SPAN 
style="COLOR: #0000ff">struct</SPAN><SPAN 
style="COLOR: #000000">&nbsp;A<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">int</SPAN><SPAN 
style="COLOR: #000000">&nbsp;i;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">char</SPAN><SPAN 
style="COLOR: #000000">&nbsp;c;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">double</SPAN><SPAN 
style="COLOR: #000000">&nbsp;d;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">short</SPAN><SPAN 
style="COLOR: #000000">&nbsp;s;<BR>};&nbsp;</SPAN><SPAN 
style="COLOR: #008000">//</SPAN><SPAN 
style="COLOR: #008000">&nbsp;24</SPAN><SPAN 
style="COLOR: #008000"><BR></SPAN><SPAN style="COLOR: #000000"><BR></SPAN><SPAN 
style="COLOR: #0000ff">struct</SPAN><SPAN 
style="COLOR: #000000">&nbsp;B<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">char</SPAN><SPAN 
style="COLOR: #000000">&nbsp;cc;<BR>&nbsp;&nbsp;&nbsp;&nbsp;A&nbsp;a;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">int</SPAN><SPAN 
style="COLOR: #000000">&nbsp;ii;<BR>};&nbsp;</SPAN><SPAN 
style="COLOR: #008000">//</SPAN><SPAN 
style="COLOR: #008000">&nbsp;40</SPAN></SPAN></DIV>
<P>&nbsp;</P>
<P>布局：（使用VS的未发布的编译选项/d1 reportAllClassLayout 或 /d1 reportSingleClassLayout）</P>
<P><IMG 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/2009022421504116.png"></P>
<P>&nbsp;</P>
<P>2）实例：</P>
<DIV class=cnblogs_code><IMG id=Code_Closed_Image_220158 style="DISPLAY: none" 
onclick="this.style.display='none'; document.getElementById('Code_Closed_Text_220158').style.display='none'; document.getElementById('Code_Open_Image_220158').style.display='inline'; document.getElementById('Code_Open_Text_220158').style.display='inline';" 
height=16 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/ContractedBlock.gif" 
width=11 align=top><IMG id=Code_Open_Image_220158 
onclick="this.style.display='none'; document.getElementById('Code_Open_Text_220158').style.display='none'; getElementById('Code_Closed_Image_220158').style.display='inline'; getElementById('Code_Closed_Text_220158').style.display='inline';" 
height=16 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/ExpandedBlockStart.gif" 
width=11 align=top><SPAN class=cnblogs_code_Collapse 
id=Code_Closed_Text_220158>Code</SPAN><SPAN id=Code_Open_Text_220158><BR><!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--><SPAN 
style="COLOR: #0000ff">#pragma</SPAN><SPAN 
style="COLOR: #000000">&nbsp;pack(4)</SPAN><SPAN 
style="COLOR: #000000"><BR></SPAN><SPAN 
style="COLOR: #0000ff">struct</SPAN><SPAN 
style="COLOR: #000000">&nbsp;A2<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">int</SPAN><SPAN 
style="COLOR: #000000">&nbsp;i;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">char</SPAN><SPAN 
style="COLOR: #000000">&nbsp;c;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">double</SPAN><SPAN 
style="COLOR: #000000">&nbsp;d;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">short</SPAN><SPAN 
style="COLOR: #000000">&nbsp;s;<BR>};&nbsp;</SPAN><SPAN 
style="COLOR: #008000">//</SPAN><SPAN 
style="COLOR: #008000">&nbsp;20</SPAN><SPAN 
style="COLOR: #008000"><BR></SPAN><SPAN 
style="COLOR: #0000ff">#pragma</SPAN><SPAN 
style="COLOR: #000000">&nbsp;pack()</SPAN><SPAN 
style="COLOR: #000000"><BR><BR></SPAN><SPAN 
style="COLOR: #0000ff">struct</SPAN><SPAN 
style="COLOR: #000000">&nbsp;B2<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">char</SPAN><SPAN 
style="COLOR: #000000">&nbsp;cc;<BR>&nbsp;&nbsp;&nbsp;&nbsp;A2&nbsp;a;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">int</SPAN><SPAN 
style="COLOR: #000000">&nbsp;ii;<BR>};&nbsp;</SPAN><SPAN 
style="COLOR: #008000">//</SPAN><SPAN 
style="COLOR: #008000">&nbsp;28</SPAN></SPAN></DIV>
<P>&nbsp;</P>
<P>布局：（使用VS的未发布的编译选项/d1 reportAllClassLayout 或 /d1 reportSingleClassLayout）</P>
<P><IMG 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/2009022422022940.png"></P>
<P>总结：</P>
<P>　<SPAN 
style="COLOR: #ff0000">　由于结构体的成员可以是复合类型，比如另外一个结构体，所以在寻找最宽基本类型成员时，应当包括复合类型成员的子成员，而不是把复合成员看成是一个整体。但在确定复合类型成员的偏移位置时则是将复合类型作为整体看待。 
</SPAN></P>
<P>四 空struct/class和const，static成员</P>
<P>实例：</P>
<DIV class=cnblogs_code><IMG id=Code_Closed_Image_202416 style="DISPLAY: none" 
onclick="this.style.display='none'; document.getElementById('Code_Closed_Text_202416').style.display='none'; document.getElementById('Code_Open_Image_202416').style.display='inline'; document.getElementById('Code_Open_Text_202416').style.display='inline';" 
height=16 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/ContractedBlock.gif" 
width=11 align=top><IMG id=Code_Open_Image_202416 
onclick="this.style.display='none'; document.getElementById('Code_Open_Text_202416').style.display='none'; getElementById('Code_Closed_Image_202416').style.display='inline'; getElementById('Code_Closed_Text_202416').style.display='inline';" 
height=16 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/ExpandedBlockStart.gif" 
width=11 align=top><SPAN class=cnblogs_code_Collapse 
id=Code_Closed_Text_202416>Code</SPAN><SPAN id=Code_Open_Text_202416><BR><!--<br /><br />Code highlighting produced by Actipro CodeHighlighter (freeware)<br />http://www.CodeHighlighter.com/<br /><br />--><SPAN 
style="COLOR: #0000ff">struct</SPAN><SPAN 
style="COLOR: #000000">&nbsp;empty{};&nbsp;</SPAN><SPAN 
style="COLOR: #008000">//</SPAN><SPAN style="COLOR: #008000">&nbsp;1</SPAN><SPAN 
style="COLOR: #008000"><BR></SPAN><SPAN 
style="COLOR: #0000ff">struct</SPAN><SPAN 
style="COLOR: #000000">&nbsp;constAndStatic<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">const</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #0000ff">int</SPAN><SPAN 
style="COLOR: #000000">&nbsp;i;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">static</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">char</SPAN><SPAN 
style="COLOR: #000000">&nbsp;c;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">const</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">double</SPAN><SPAN 
style="COLOR: #000000">&nbsp;d;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">static</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">void</SPAN><SPAN 
style="COLOR: #000000">&nbsp;TestStatic(){}<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">void</SPAN><SPAN 
style="COLOR: #000000">&nbsp;TestNoStatic(){}<BR>};&nbsp;</SPAN><SPAN 
style="COLOR: #008000">//</SPAN><SPAN
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