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

最为全面的c++内存模型探秘。不管是初学者还是老手

style="COLOR: #000000">&lt;&lt;</SPAN><SPAN style="COLOR: #000000"> endl; 
<BR>&nbsp;&nbsp;&nbsp; cout </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp; sizeof(1.5f)= </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"> ( </SPAN><SPAN 
style="COLOR: #800080">1.5f</SPAN><SPAN style="COLOR: #000000"> ) </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN style="COLOR: #000000"> endl; 
<BR>&nbsp;&nbsp;&nbsp; cout </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp; sizeof(1.5)= </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"> ( </SPAN><SPAN 
style="COLOR: #800080">1.5</SPAN><SPAN style="COLOR: #000000"> ) </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN style="COLOR: #000000"> endl; 
<BR><BR>&nbsp;&nbsp;&nbsp; cout </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp; sizeof(Good!)= </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"> ( </SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #800000">Good!</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000"> ) </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN style="COLOR: #000000"> endl ; 
<BR><BR>&nbsp;&nbsp;&nbsp; </SPAN><SPAN style="COLOR: #0000ff">char</SPAN><SPAN 
style="COLOR: #000000">&nbsp; str[] </SPAN><SPAN 
style="COLOR: #000000">=</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">CharArray!</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">; 
<BR>&nbsp;&nbsp;&nbsp; </SPAN><SPAN style="COLOR: #0000ff">int</SPAN><SPAN 
style="COLOR: #000000">&nbsp; a[</SPAN><SPAN 
style="COLOR: #800080">10</SPAN><SPAN style="COLOR: #000000">];&nbsp; 
<BR>&nbsp;&nbsp;&nbsp; </SPAN><SPAN style="COLOR: #0000ff">double</SPAN><SPAN 
style="COLOR: #000000">&nbsp; xy[</SPAN><SPAN 
style="COLOR: #800080">10</SPAN><SPAN style="COLOR: #000000">]; 
<BR>&nbsp;&nbsp;&nbsp; cout </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp; char str[] = \</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #000000">CharArray</SPAN><SPAN 
style="COLOR: #000000">!</SPAN><SPAN style="COLOR: #000000">\</SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #800000">,</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: #800000">"</SPAN><SPAN 
style="COLOR: #800000"> sizeof(str)= </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"> (str) 
</SPAN><SPAN style="COLOR: #000000">&lt;&lt;</SPAN><SPAN style="COLOR: #000000"> 
endl; <BR>&nbsp;&nbsp;&nbsp; cout </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp; int a[10],</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: #800000">"</SPAN><SPAN 
style="COLOR: #800000"> sizeof(a)= </SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
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style="COLOR: #0000ff">sizeof</SPAN><SPAN style="COLOR: #000000"> (a) 
</SPAN><SPAN style="COLOR: #000000">&lt;&lt;</SPAN><SPAN style="COLOR: #000000"> 
endl; <BR>&nbsp;&nbsp;&nbsp; cout </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp; double xy[10],</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: #800000">"</SPAN><SPAN 
style="COLOR: #800000"> sizeof(xy)= </SPAN><SPAN 
style="COLOR: #800000">"</SPAN><SPAN style="COLOR: #000000">&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN style="COLOR: #000000">&nbsp;&nbsp; 
</SPAN><SPAN style="COLOR: #0000ff">sizeof</SPAN><SPAN style="COLOR: #000000"> 
(xy) </SPAN><SPAN style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000"> endl; <BR><BR>&nbsp;&nbsp;&nbsp; cout </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;</SPAN><SPAN style="COLOR: #800000">"</SPAN><SPAN 
style="COLOR: #800000">&nbsp; sizeof(void*)= </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">(</SPAN><SPAN 
style="COLOR: #0000ff">void</SPAN><SPAN style="COLOR: #000000">*</SPAN><SPAN 
style="COLOR: #000000">) </SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN style="COLOR: #000000"> endl; 
<BR>}</SPAN></SPAN></DIV>
<P><SPAN style="COLOR: #000000"></SPAN></P>
<P><SPAN style="COLOR: #000000"><BR></SPAN></P>
<P><SPAN style="COLOR: #000000">运行结果如下：</SPAN></P>
<P><SPAN style="COLOR: #000000"><IMG 
src="[C++对象模型][6]sizeof与对象内存布局 - iTech's Blog - 博客园.files/2009022419400533.png"><BR></SPAN></P>
<P>二 struct/class的大小</P>
<P>在C++中我们知道struct和class的唯一区别就是默认的访问级别不同，struct默认为public，而class的默认为private。所以考虑对象的大小，我们均以struct为例。对于struct的大小对于初学者来说还确实是个难回答的问题，我们就通过下面的一个struct定义加逐步的变化来引出相关的知识。</P>
<P>代码如下：</P>
<DIV class=cnblogs_code><IMG id=Code_Closed_Image_195003 style="DISPLAY: none" 
onclick="this.style.display='none'; document.getElementById('Code_Closed_Text_195003').style.display='none'; document.getElementById('Code_Open_Image_195003').style.display='inline'; document.getElementById('Code_Open_Text_195003').style.display='inline';" 
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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_195003 
onclick="this.style.display='none'; document.getElementById('Code_Open_Text_195003').style.display='none'; getElementById('Code_Closed_Image_195003').style.display='inline'; getElementById('Code_Closed_Text_195003').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_195003>Code</SPAN><SPAN id=Code_Open_Text_195003><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;st1<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">short</SPAN><SPAN 
style="COLOR: #000000">&nbsp;number;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">float</SPAN><SPAN 
style="COLOR: #000000">&nbsp;math_grade;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">float</SPAN><SPAN 
style="COLOR: #000000">&nbsp;Chinese_grade;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">float</SPAN><SPAN 
style="COLOR: #000000">&nbsp;sum_grade;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">char</SPAN><SPAN 
style="COLOR: #000000">&nbsp;&nbsp;level;<BR>}; //20<BR><BR></SPAN><SPAN 
style="COLOR: #0000ff">struct</SPAN><SPAN 
style="COLOR: #000000">&nbsp;st2<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">char</SPAN><SPAN 
style="COLOR: #000000">&nbsp;&nbsp;level;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">short</SPAN><SPAN 
style="COLOR: #000000">&nbsp;number;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">float</SPAN><SPAN 
style="COLOR: #000000">&nbsp;math_grade;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">float</SPAN><SPAN 
style="COLOR: #000000">&nbsp;Chinese_grade;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">float</SPAN><SPAN 
style="COLOR: #000000">&nbsp;sum_grade;<BR>};//16<BR><BR></SPAN><SPAN 
style="COLOR: #0000ff">#pragma</SPAN><SPAN 
style="COLOR: #000000">&nbsp;pack(1)</SPAN><SPAN 
style="COLOR: #000000"><BR></SPAN><SPAN 
style="COLOR: #0000ff">struct</SPAN><SPAN 
style="COLOR: #000000">&nbsp;st3<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">char</SPAN><SPAN 
style="COLOR: #000000">&nbsp;&nbsp;level;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">short</SPAN><SPAN 
style="COLOR: #000000">&nbsp;number;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">float</SPAN><SPAN 
style="COLOR: #000000">&nbsp;math_grade;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">float</SPAN><SPAN 
style="COLOR: #000000">&nbsp;Chinese_grade;<BR>&nbsp;&nbsp;&nbsp;&nbsp;</SPAN><SPAN 
style="COLOR: #0000ff">float</SPAN><SPAN 
style="COLOR: #000000">&nbsp;sum_grade;<BR>}; //15<BR></SPAN><SPAN 
style="COLOR: #0000ff">#pragma</SPAN><SPAN 
style="COLOR: #000000">&nbsp;pack()&nbsp;</SPAN><SPAN 
style="COLOR: #000000"><BR><BR></SPAN><SPAN 
style="COLOR: #0000ff">void</SPAN><SPAN 
style="COLOR: #000000">&nbsp;TestStructSizeOf()<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;__FUNCTION__&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<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(st1)=&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;(st1)&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(st1,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(st1,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(st1,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(st1,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(st1,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(st1,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(st1,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(st1,sum_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(st1,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(st1,level)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<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(st2)=&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;(st2)&nbsp;</SPAN><SPAN 
style="COLOR: #000000">&lt;&lt;</SPAN><SPAN 
style="COLOR: #000000">&nbsp;endl;<BR>&nbsp;&nbsp;&nbsp;&nbsp;cout&nbsp;</SPAN><SPAN