[C/C++] union初探

这几天在看MFC深入浅出，看到一个union的定义，代码如下：

union MessageMapFunctions
2

{
3

AFX_PMSG pfn; // generic member function pointer
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfn_b_D)(CDC * );
6

BOOL (AFX_MSG_CALL CCmdTarget:: * pfn_b_b)(BOOL);
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfn_b_u)(UINT);
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfn_b_h)(HANDLE);
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfn_b_W_u_u)(CWnd * , UINT, UINT);
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfn_b_W_COPYDATASTRUCT)(CWnd * , COPYDATASTRUCT * );
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfn_b_HELPINFO)(LPHELPINFO);
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HBRUSH (AFX_MSG_CALL CCmdTarget:: * pfn_B_D_W_u)(CDC * , CWnd * , UINT);
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HBRUSH (AFX_MSG_CALL CCmdTarget:: * pfn_B_D_u)(CDC * , UINT);
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int (AFX_MSG_CALL CCmdTarget:: * pfn_i_u_W_u)(UINT, CWnd * , UINT);
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int (AFX_MSG_CALL CCmdTarget:: * pfn_i_u_u)(UINT, UINT);
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int (AFX_MSG_CALL CCmdTarget:: * pfn_i_W_u_u)(CWnd * , UINT, UINT);
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int (AFX_MSG_CALL CWnd:: * pfn_i_s)(LPTSTR);
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LRESULT (AFX_MSG_CALL CWnd:: * pfn_l_w_l)(WPARAM, LPARAM);
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LRESULT (AFX_MSG_CALL CWnd:: * pfn_l_u_u_M)(UINT, UINT, CMenu * );
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void (AFX_MSG_CALL CWnd:: * pfn_v_v)();
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int (AFX_MSG_CALL CWnd:: * pfn_i_u)(UINT);
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HCURSOR (AFX_MSG_CALL CWnd:: * pfn_C_v)();
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UINT (AFX_MSG_CALL CWnd:: * pfn_u_u)(UINT);
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BOOL (AFX_MSG_CALL CWnd:: * pfn_b_v)();
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void (AFX_MSG_CALL CWnd:: * pfn_v_u)(UINT);
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_u)(UINT, UINT);
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void (AFX_MSG_CALL CWnd:: * pfn_v_i_i)( int , int );
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_u_u)(UINT, UINT, UINT);
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_i_i)(UINT, int , int );
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void (AFX_MSG_CALL CWnd:: * pfn_v_w_l)(WPARAM, LPARAM);
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void (AFX_MSG_CALL CWnd:: * pfn_v_b_W_W)(BOOL, CWnd * , CWnd * );
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void (AFX_MSG_CALL CWnd:: * pfn_v_D)(CDC * );
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void (AFX_MSG_CALL CWnd:: * pfn_v_M)(CMenu * );
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void (AFX_MSG_CALL CWnd:: * pfn_v_M_u_b)(CMenu * , UINT, BOOL);
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void (AFX_MSG_CALL CWnd:: * pfn_v_W)(CWnd * );
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void (AFX_MSG_CALL CWnd:: * pfn_v_W_u_u)(CWnd * , UINT, UINT);
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void (AFX_MSG_CALL CWnd:: * pfn_v_W_p)(CWnd * , CPoint);
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void (AFX_MSG_CALL CWnd:: * pfn_v_W_h)(CWnd * , HANDLE);
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_W)(UINT, CWnd * );
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_W_b)(UINT, CWnd * , BOOL);
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_u_W)(UINT, UINT, CWnd * );
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void (AFX_MSG_CALL CWnd:: * pfn_v_s)(LPTSTR);
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_cs)(UINT, LPCTSTR);
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void (AFX_MSG_CALL CWnd:: * pfn_v_i_s)( int , LPTSTR);
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int (AFX_MSG_CALL CWnd:: * pfn_i_i_s)( int , LPTSTR);
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UINT (AFX_MSG_CALL CWnd:: * pfn_u_p)(CPoint);
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UINT (AFX_MSG_CALL CWnd:: * pfn_u_v)();
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void (AFX_MSG_CALL CWnd:: * pfn_v_b_NCCALCSIZEPARAMS)(BOOL, NCCALCSIZE_PARAMS * );
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void (AFX_MSG_CALL CWnd:: * pfn_v_v_WINDOWPOS)(WINDOWPOS * );
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_u_M)(UINT, UINT, HMENU);
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_p)(UINT, CPoint);
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void (AFX_MSG_CALL CWnd:: * pfn_v_u_pr)(UINT, LPRECT);
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BOOL (AFX_MSG_CALL CWnd:: * pfn_b_u_s_p)(UINT, short, CPoint);
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LRESULT (AFX_MSG_CALL CWnd:: * pfn_l_v)();
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// type safe variant for thread messages
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void (AFX_MSG_CALL CWinThread:: * pfn_THREAD)(WPARAM, LPARAM);
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// specific type safe variants for WM_COMMAND and WM_NOTIFY messages
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void (AFX_MSG_CALL CCmdTarget:: * pfnCmd_v_v)();
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfnCmd_b_v)();
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void (AFX_MSG_CALL CCmdTarget:: * pfnCmd_v_u)(UINT);
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfnCmd_b_u)(UINT);
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void (AFX_MSG_CALL CCmdTarget:: * pfnNotify_v_NMHDR_pl)(NMHDR * , LRESULT * );
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfnNotify_b_NMHDR_pl)(NMHDR * , LRESULT * );
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void (AFX_MSG_CALL CCmdTarget:: * pfnNotify_v_u_NMHDR_pl)(UINT, NMHDR * , LRESULT * );
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfnNotify_b_u_NMHDR_pl)(UINT, NMHDR * , LRESULT * );
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void (AFX_MSG_CALL CCmdTarget:: * pfnCmdUI_v_C)(CCmdUI * );
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void (AFX_MSG_CALL CCmdTarget:: * pfnCmdUI_v_C_u)(CCmdUI * , UINT);
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void (AFX_MSG_CALL CCmdTarget:: * pfnCmd_v_pv)(void * );
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BOOL (AFX_MSG_CALL CCmdTarget:: * pfnCmd_b_pv)(void * );
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// OLD
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// specific type safe variants for WM - style messages
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// BOOL (AFX_MSG_CALL CWnd:: * pfn_bD)(CDC * );
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// BOOL (AFX_MSG_CALL CWnd:: * pfn_bb)(BOOL);
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// BOOL (AFX_MSG_CALL CWnd:: * pfn_bWww)(CWnd * , UINT, UINT);
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// BOOL (AFX_MSG_CALL CWnd:: * pfn_bHELPINFO)(HELPINFO * );
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// BOOL (AFX_MSG_CALL CWnd:: * pfn_bWCDS)(CWnd * , COPYDATASTRUCT * );
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// HBRUSH (AFX_MSG_CALL CWnd:: * pfn_hDWw)(CDC * , CWnd * , UINT);
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// HBRUSH (AFX_MSG_CALL CWnd:: * pfn_hDw)(CDC * , UINT);
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// int (AFX_MSG_CALL CWnd:: * pfn_iwWw)(UINT, CWnd * , UINT);
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// int (AFX_MSG_CALL CWnd:: * pfn_iww)(UINT, UINT);
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// int (AFX_MSG_CALL CWnd:: * pfn_iWww)(CWnd * , UINT, UINT);
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// int (AFX_MSG_CALL CWnd:: * pfn_is)(LPTSTR);
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// LRESULT (AFX_MSG_CALL CWnd:: * pfn_lwl)(WPARAM, LPARAM);
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// LRESULT (AFX_MSG_CALL CWnd:: * pfn_lwwM)(UINT, UINT, CMenu * );
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// void (AFX_MSG_CALL CWnd:: * pfn_vv)(void);
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// void (AFX_MSG_CALL CWnd:: * pfn_vw)(UINT);
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// void (AFX_MSG_CALL CWnd:: * pfn_vww)(UINT, UINT);
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// void (AFX_MSG_CALL CWnd:: * pfn_vvii)( int , int );
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// void (AFX_MSG_CALL CWnd:: * pfn_vwww)(UINT, UINT, UINT);
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// void (AFX_MSG_CALL CWnd:: * pfn_vwii)(UINT, int , int );
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// void (AFX_MSG_CALL CWnd:: * pfn_vwl)(WPARAM, LPARAM);
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// void (AFX_MSG_CALL CWnd:: * pfn_vbWW)(BOOL, CWnd * , CWnd * );
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// void (AFX_MSG_CALL CWnd:: * pfn_vD)(CDC * );
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// void (AFX_MSG_CALL CWnd:: * pfn_vM)(CMenu * );
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// void (AFX_MSG_CALL CWnd:: * pfn_vMwb)(CMenu * , UINT, BOOL);
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103

// void (AFX_MSG_CALL CWnd:: * pfn_vW)(CWnd * );
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// void (AFX_MSG_CALL CWnd:: * pfn_vWww)(CWnd * , UINT, UINT);
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// void (AFX_MSG_CALL CWnd:: * pfn_vWp)(CWnd * , CPoint);
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// void (AFX_MSG_CALL CWnd:: * pfn_vWh)(CWnd * , HANDLE);
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// void (AFX_MSG_CALL CWnd:: * pfn_vwW)(UINT, CWnd * );
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// void (AFX_MSG_CALL CWnd:: * pfn_vwWb)(UINT, CWnd * , BOOL);
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// void (AFX_MSG_CALL CWnd:: * pfn_vwwW)(UINT, UINT, CWnd * );
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// void (AFX_MSG_CALL CWnd:: * pfn_vwwx)(UINT, UINT);
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// void (AFX_MSG_CALL CWnd:: * pfn_vs)(LPTSTR);
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// void (AFX_MSG_CALL CWnd:: * pfn_vOWNER)( int , LPTSTR); // force return TRUE
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// int (AFX_MSG_CALL CWnd:: * pfn_iis)( int , LPTSTR);
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// UINT (AFX_MSG_CALL CWnd:: * pfn_wp)(CPoint);
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// UINT (AFX_MSG_CALL CWnd:: * pfn_wv)(void);
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void (AFX_MSG_CALL CWnd:: * pfn_vPOS)(WINDOWPOS * );
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void (AFX_MSG_CALL CWnd:: * pfn_vCALC)(BOOL, NCCALCSIZE_PARAMS * );
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void (AFX_MSG_CALL CWnd:: * pfn_vwp)(UINT, CPoint);
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void (AFX_MSG_CALL CWnd:: * pfn_vwwh)(UINT, UINT, HANDLE);
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BOOL (AFX_MSG_CALL CWnd:: * pfn_bwsp)(UINT, short, CPoint);
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// void (AFX_MSG_CALL CWnd:: * pfn_vws)(UINT, LPCTSTR);
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};

而调用的方法只是指定了一个函数指针，而函数指针的定义为：
typedef void (AFX_MSG_CALL CCmdTarget::*AFX_PMSG)(void);
起初一点也不明白这样的定义怎么指向实际执行的函数？怀着这样的疑问，我自己做了个测试，才明白其中的真实含义。union与struct的区别是：union为每个成员共享一个地址空间，而struct为每个成员都分配一个地址空间。这样就好理解了，不管union里定义了多少个成员，每个成员都使用同一个内存地址，而struct的每个成员的内存地址却都不相同。
测试代码如下：

#include " stdafx.h "
2

typedef void ( * FUNC)(void);
4

typedef void ( * Func01)( int , int );
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typedef double ( * Func02)( double , double );
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typedef int ( * Func03)( int , int );
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void Test01( int , int );
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double Test02( double , double );
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int Test03( int , int );
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union UnionTest {
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FUNC pFunc;
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Func01 func01;
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Func02 func02;
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Func03 func03;
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};
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int _tmain( int argc, char * argv[])
22

{
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system( " cls " );
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printf( " 此程序模仿了MFC中的消息函数分配机制\n " );
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union UnionTest test;
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printf( " (FUNC)(static_cast< void (*)(int, int) > (Test01))\n " );
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test.pFunc = (FUNC)(static_cast < void ( * )( int , int ) > (Test01));
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printf( " %p, %p, %p\n " , test.func01, test.func02, test.func03);
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test.func01( 10 , 10 );
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test.func02( 10.001 , 20.002 );
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test.func03( 10 , 10 );
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printf( " \n " );
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printf( " (FUNC)(static_cast< double (*)(double, double) > (Test02))\n " );
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test.pFunc = (FUNC)(static_cast < double ( * )( double , double ) > (Test02));
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printf( " %p, %p, %p\n " , test.func01, test.func02, test.func03);
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test.func01( 10 , 10 );
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double xx = test.func02( 10.001 , 20.002 );
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test.func03( 10 , 10 );
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printf( " \n " );
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printf( " (FUNC)(static_cast< int (*)(int, int) > (Test03))\n " );
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test.pFunc = (FUNC)(static_cast < int ( * )( int , int ) > (Test03));
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printf( " %p, %p, %p\n " , test.func01, test.func02, test.func03);
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test.func01( 10 , 10 );
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double y01 = test.func02( 10.001 , 20.002 );
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int yy = test.func03( 10 , 10 );
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return 0 ;
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}
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void Test01( int x, int y) {
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printf( " hehe, test it. x=%d, y=%d\n " , x, y);
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}
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double Test02( double x, double y) {
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double sum = x + y;
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printf( " sum double: %10.10f+%10.10f=%10.10f\nx address:%p\ny address:%p\n " ,x,y,sum, & x, & y);
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return sum;
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}
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int Test03( int x, int y) {
67

int sum = x + y;
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printf( " sum int: %d+%d=%d\n " ,x,y,sum);
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return sum;
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}

输出结果如下：

此程序模仿了MFC中的消息函数分配机制
2

(FUNC)(static_cast < void ( * )( int , int ) > (Test01))
3

00401000 , 00401000 , 00401000
4

hehe, test it. x = 10 , y = 10
5

hehe, test it. x = 309237645 , y = 1076101251
6

hehe, test it. x = 10 , y = 10
7

(FUNC)(static_cast < double ( * )( double , double ) > (Test02))
9

00401020 , 00401020 , 00401020
10

sum double : 0.0000000000 + 0.0000000000 = 0.0000000000
11

x address:0012FEB0
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y address:0012FEB8
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sum double : 10.0010000000 + 20.0020000000 = 30.0030000000
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x address:0012FED8
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y address:0012FEE0
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sum double : 0.0000000000 + 10.0010000000 = 10.0010000000
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x address:0012FED0
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y address:0012FED8
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(FUNC)(static_cast < int ( * )( int , int ) > (Test03))
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00401070 , 00401070 , 00401070
22

sum int : 10 + 10 = 20
23

sum int : 309237645 + 1076101251 = 1385338896
24

sum int : 10 + 10 = 20

从输出结果可以看出，union的每个成员的内存地址都是一样的。这样就好理解union MessageMapFunctions的真实作用了。

[C/C++] union初探

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