求类的大小

#include "iostream.h"
<br>class CTestA
<br>
<br>{
<br>
<br>public:
<br>
<br>    CTestA(){}
<br>
<br>virtual ~ CTestA(){}
<br>
<br>protected:
<br>
<br>int     m_A;
<br>
<br>};
<br>void main()
<br>{
<br>cout&lt;&lt;sizeof(CTestA);
<br>}
<br>结果是8。为什么

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