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[C/C++] union初探

Posted on 2007-08-06 17:31 jacky_zz 閱讀(1225) 評(píng)論(4) 編輯收藏引用所屬分類(lèi): C/C++

這幾天在看MFC深入淺出，看到一個(gè)union的定義，代碼如下：

union MessageMapFunctions
2

{
3

AFX_PMSG pfn; // generic member function pointer
4

BOOL (AFX_MSG_CALL CCmdTarget::*pfn_b_D)(CDC*);
6

BOOL (AFX_MSG_CALL CCmdTarget::*pfn_b_b)(BOOL);
7

BOOL (AFX_MSG_CALL CCmdTarget::*pfn_b_u)(UINT);
8

BOOL (AFX_MSG_CALL CCmdTarget::*pfn_b_h)(HANDLE);
9

BOOL (AFX_MSG_CALL CCmdTarget::*pfn_b_W_u_u)(CWnd*, UINT, UINT);
10

BOOL (AFX_MSG_CALL CCmdTarget::*pfn_b_W_COPYDATASTRUCT)(CWnd*, COPYDATASTRUCT*);
11

BOOL (AFX_MSG_CALL CCmdTarget::*pfn_b_HELPINFO)(LPHELPINFO);
12

HBRUSH (AFX_MSG_CALL CCmdTarget::*pfn_B_D_W_u)(CDC*, CWnd*, UINT);
13

HBRUSH (AFX_MSG_CALL CCmdTarget::*pfn_B_D_u)(CDC*, UINT);
14

int (AFX_MSG_CALL CCmdTarget::*pfn_i_u_W_u)(UINT, CWnd*, UINT);
15

int (AFX_MSG_CALL CCmdTarget::*pfn_i_u_u)(UINT, UINT);
16

int (AFX_MSG_CALL CCmdTarget::*pfn_i_W_u_u)(CWnd*, UINT, UINT);
17

int (AFX_MSG_CALL CWnd::*pfn_i_s)(LPTSTR);
18

LRESULT (AFX_MSG_CALL CWnd::*pfn_l_w_l)(WPARAM, LPARAM);
19

LRESULT (AFX_MSG_CALL CWnd::*pfn_l_u_u_M)(UINT, UINT, CMenu*);
20

void (AFX_MSG_CALL CWnd::*pfn_v_v)();
21

int (AFX_MSG_CALL CWnd::*pfn_i_u)(UINT);
22

HCURSOR (AFX_MSG_CALL CWnd::*pfn_C_v)();
23

UINT (AFX_MSG_CALL CWnd::*pfn_u_u)(UINT);
24

BOOL (AFX_MSG_CALL CWnd::*pfn_b_v)();
25

void (AFX_MSG_CALL CWnd::*pfn_v_u)(UINT);
26

void (AFX_MSG_CALL CWnd::*pfn_v_u_u)(UINT, UINT);
27

void (AFX_MSG_CALL CWnd::*pfn_v_i_i)(int, int);
28

void (AFX_MSG_CALL CWnd::*pfn_v_u_u_u)(UINT, UINT, UINT);
29

void (AFX_MSG_CALL CWnd::*pfn_v_u_i_i)(UINT, int, int);
30

void (AFX_MSG_CALL CWnd::*pfn_v_w_l)(WPARAM, LPARAM);
31

void (AFX_MSG_CALL CWnd::*pfn_v_b_W_W)(BOOL, CWnd*, CWnd*);
32

void (AFX_MSG_CALL CWnd::*pfn_v_D)(CDC*);
33

void (AFX_MSG_CALL CWnd::*pfn_v_M)(CMenu*);
34

void (AFX_MSG_CALL CWnd::*pfn_v_M_u_b)(CMenu*, UINT, BOOL);
35

void (AFX_MSG_CALL CWnd::*pfn_v_W)(CWnd*);
36

void (AFX_MSG_CALL CWnd::*pfn_v_W_u_u)(CWnd*, UINT, UINT);
37

void (AFX_MSG_CALL CWnd::*pfn_v_W_p)(CWnd*, CPoint);
38

void (AFX_MSG_CALL CWnd::*pfn_v_W_h)(CWnd*, HANDLE);
39

void (AFX_MSG_CALL CWnd::*pfn_v_u_W)(UINT, CWnd*);
40

void (AFX_MSG_CALL CWnd::*pfn_v_u_W_b)(UINT, CWnd*, BOOL);
41

void (AFX_MSG_CALL CWnd::*pfn_v_u_u_W)(UINT, UINT, CWnd*);
42

void (AFX_MSG_CALL CWnd::*pfn_v_s)(LPTSTR);
43

void (AFX_MSG_CALL CWnd::*pfn_v_u_cs)(UINT, LPCTSTR);
44

void (AFX_MSG_CALL CWnd::*pfn_v_i_s)(int, LPTSTR);
45

int (AFX_MSG_CALL CWnd::*pfn_i_i_s)(int, LPTSTR);
46

UINT (AFX_MSG_CALL CWnd::*pfn_u_p)(CPoint);
47

UINT (AFX_MSG_CALL CWnd::*pfn_u_v)();
48

void (AFX_MSG_CALL CWnd::*pfn_v_b_NCCALCSIZEPARAMS)(BOOL, NCCALCSIZE_PARAMS*);
49

void (AFX_MSG_CALL CWnd::*pfn_v_v_WINDOWPOS)(WINDOWPOS*);
50

void (AFX_MSG_CALL CWnd::*pfn_v_u_u_M)(UINT, UINT, HMENU);
51

void (AFX_MSG_CALL CWnd::*pfn_v_u_p)(UINT, CPoint);
52

void (AFX_MSG_CALL CWnd::*pfn_v_u_pr)(UINT, LPRECT);
53

BOOL (AFX_MSG_CALL CWnd::*pfn_b_u_s_p)(UINT, short, CPoint);
54

LRESULT (AFX_MSG_CALL CWnd::*pfn_l_v)();
55

// type safe variant for thread messages
57

void (AFX_MSG_CALL CWinThread::*pfn_THREAD)(WPARAM, LPARAM);
58

// specific type safe variants for WM_COMMAND and WM_NOTIFY messages
60

void (AFX_MSG_CALL CCmdTarget::*pfnCmd_v_v)();
61

BOOL (AFX_MSG_CALL CCmdTarget::*pfnCmd_b_v)();
62

void (AFX_MSG_CALL CCmdTarget::*pfnCmd_v_u)(UINT);
63

BOOL (AFX_MSG_CALL CCmdTarget::*pfnCmd_b_u)(UINT);
64

void (AFX_MSG_CALL CCmdTarget::*pfnNotify_v_NMHDR_pl)(NMHDR*, LRESULT*);
66

BOOL (AFX_MSG_CALL CCmdTarget::*pfnNotify_b_NMHDR_pl)(NMHDR*, LRESULT*);
67

void (AFX_MSG_CALL CCmdTarget::*pfnNotify_v_u_NMHDR_pl)(UINT, NMHDR*, LRESULT*);
68

BOOL (AFX_MSG_CALL CCmdTarget::*pfnNotify_b_u_NMHDR_pl)(UINT, NMHDR*, LRESULT*);
69

void (AFX_MSG_CALL CCmdTarget::*pfnCmdUI_v_C)(CCmdUI*);
70

void (AFX_MSG_CALL CCmdTarget::*pfnCmdUI_v_C_u)(CCmdUI*, UINT);
71

void (AFX_MSG_CALL CCmdTarget::*pfnCmd_v_pv)(void*);
73

BOOL (AFX_MSG_CALL CCmdTarget::*pfnCmd_b_pv)(void*);
74

//OLD
76

// specific type safe variants for WM-style messages
77

// BOOL (AFX_MSG_CALL CWnd::*pfn_bD)(CDC*);
78

// BOOL (AFX_MSG_CALL CWnd::*pfn_bb)(BOOL);
79

// BOOL (AFX_MSG_CALL CWnd::*pfn_bWww)(CWnd*, UINT, UINT);
80

// BOOL (AFX_MSG_CALL CWnd::*pfn_bHELPINFO)(HELPINFO*);
81

// BOOL (AFX_MSG_CALL CWnd::*pfn_bWCDS)(CWnd*, COPYDATASTRUCT*);
82

// HBRUSH (AFX_MSG_CALL CWnd::*pfn_hDWw)(CDC*, CWnd*, UINT);
83

// HBRUSH (AFX_MSG_CALL CWnd::*pfn_hDw)(CDC*, UINT);
84

// int (AFX_MSG_CALL CWnd::*pfn_iwWw)(UINT, CWnd*, UINT);
85

// int (AFX_MSG_CALL CWnd::*pfn_iww)(UINT, UINT);
86

// int (AFX_MSG_CALL CWnd::*pfn_iWww)(CWnd*, UINT, UINT);
87

// int (AFX_MSG_CALL CWnd::*pfn_is)(LPTSTR);
88

// LRESULT (AFX_MSG_CALL CWnd::*pfn_lwl)(WPARAM, LPARAM);
89

// LRESULT (AFX_MSG_CALL CWnd::*pfn_lwwM)(UINT, UINT, CMenu*);
90

// void (AFX_MSG_CALL CWnd::*pfn_vv)(void);
91

// void (AFX_MSG_CALL CWnd::*pfn_vw)(UINT);
93

// void (AFX_MSG_CALL CWnd::*pfn_vww)(UINT, UINT);
94

// void (AFX_MSG_CALL CWnd::*pfn_vvii)(int, int);
95

// void (AFX_MSG_CALL CWnd::*pfn_vwww)(UINT, UINT, UINT);
96

// void (AFX_MSG_CALL CWnd::*pfn_vwii)(UINT, int, int);
97

// void (AFX_MSG_CALL CWnd::*pfn_vwl)(WPARAM, LPARAM);
98

// void (AFX_MSG_CALL CWnd::*pfn_vbWW)(BOOL, CWnd*, CWnd*);
99

// void (AFX_MSG_CALL CWnd::*pfn_vD)(CDC*);
100

// void (AFX_MSG_CALL CWnd::*pfn_vM)(CMenu*);
101

// void (AFX_MSG_CALL CWnd::*pfn_vMwb)(CMenu*, UINT, BOOL);
102

103

// void (AFX_MSG_CALL CWnd::*pfn_vW)(CWnd*);
104

// void (AFX_MSG_CALL CWnd::*pfn_vWww)(CWnd*, UINT, UINT);
105

// void (AFX_MSG_CALL CWnd::*pfn_vWp)(CWnd*, CPoint);
106

// void (AFX_MSG_CALL CWnd::*pfn_vWh)(CWnd*, HANDLE);
107

// void (AFX_MSG_CALL CWnd::*pfn_vwW)(UINT, CWnd*);
108

// void (AFX_MSG_CALL CWnd::*pfn_vwWb)(UINT, CWnd*, BOOL);
109

// void (AFX_MSG_CALL CWnd::*pfn_vwwW)(UINT, UINT, CWnd*);
110

// void (AFX_MSG_CALL CWnd::*pfn_vwwx)(UINT, UINT);
111

// void (AFX_MSG_CALL CWnd::*pfn_vs)(LPTSTR);
112

// void (AFX_MSG_CALL CWnd::*pfn_vOWNER)(int, LPTSTR); // force return TRUE
113

// int (AFX_MSG_CALL CWnd::*pfn_iis)(int, LPTSTR);
114

// UINT (AFX_MSG_CALL CWnd::*pfn_wp)(CPoint);
115

// UINT (AFX_MSG_CALL CWnd::*pfn_wv)(void);
116

void (AFX_MSG_CALL CWnd::*pfn_vPOS)(WINDOWPOS*);
117

void (AFX_MSG_CALL CWnd::*pfn_vCALC)(BOOL, NCCALCSIZE_PARAMS*);
118

void (AFX_MSG_CALL CWnd::*pfn_vwp)(UINT, CPoint);
119

void (AFX_MSG_CALL CWnd::*pfn_vwwh)(UINT, UINT, HANDLE);
120

BOOL (AFX_MSG_CALL CWnd::*pfn_bwsp)(UINT, short, CPoint);
121

// void (AFX_MSG_CALL CWnd::*pfn_vws)(UINT, LPCTSTR);
122

};

而調(diào)用的方法只是指定了一個(gè)函數(shù)指針，而函數(shù)指針的定義為：
typedef void (AFX_MSG_CALL CCmdTarget::*AFX_PMSG)(void);
起初一點(diǎn)也不明白這樣的定義怎么指向?qū)嶋H執(zhí)行的函數(shù)？懷著這樣的疑問(wèn)，我自己做了個(gè)測(cè)試，才明白其中的真實(shí)含義。union與struct的區(qū)別是：union為每個(gè)成員共享一個(gè)地址空間，而struct為每個(gè)成員都分配一個(gè)地址空間。這樣就好理解了，不管union里定義了多少個(gè)成員，每個(gè)成員都使用同一個(gè)內(nèi)存地址，而struct的每個(gè)成員的內(nèi)存地址卻都不相同。
測(cè)試代碼如下：

#include "stdafx.h"
2

typedef void (*FUNC)(void);
4

typedef void (*Func01)(int, int);
6

typedef double (*Func02)(double, double);
7

typedef int (*Func03)(int, int);
8

void Test01(int, int);
10

double Test02(double, double);
11

int Test03(int, int);
12

union UnionTest {
14

FUNC pFunc;
15

Func01 func01;
17

Func02 func02;
18

Func03 func03;
19

};
20

int _tmain(int argc, char* argv[])
22

{
23

system("cls");
24

printf("此程序模仿了MFC中的消息函數(shù)分配機(jī)制\n");
25

union UnionTest test;
26

printf("(FUNC)(static_cast< void (*)(int, int) > (Test01))\n");
28

test.pFunc = (FUNC)(static_cast< void (*)(int, int) > (Test01));
29

printf("%p, %p, %p\n", test.func01, test.func02, test.func03);
30

test.func01(10,10);
32

test.func02(10.001, 20.002);
33

test.func03(10,10);
34

printf("\n");
35

printf("(FUNC)(static_cast< double (*)(double, double) > (Test02))\n");
37

test.pFunc = (FUNC)(static_cast< double (*)(double, double) > (Test02));
38

printf("%p, %p, %p\n", test.func01, test.func02, test.func03);
39

test.func01(10,10);
41

double xx = test.func02(10.001, 20.002);
42

test.func03(10,10);
43

printf("\n");
44

printf("(FUNC)(static_cast< int (*)(int, int) > (Test03))\n");
46

test.pFunc = (FUNC)(static_cast< int (*)(int, int) > (Test03));
47

printf("%p, %p, %p\n", test.func01, test.func02, test.func03);
48

test.func01(10,10);
50

double y01 = test.func02(10.001, 20.002);
51

int yy = test.func03(10,10);
52

return 0;
54

}
55

void Test01(int x, int y) {
57

printf("hehe, test it. x=%d, y=%d\n", x, y);
58

}
59

double Test02(double x, double y) {
61

double sum =x+y;
62

printf("sum double: %10.10f+%10.10f=%10.10f\nx address:%p\ny address:%p\n",x,y,sum, &x, &y);
63

return sum;
64

}
65

int Test03(int x, int y) {
67

int sum =x+y;
68

printf("sum int: %d+%d=%d\n",x,y,sum);
69

return sum;
70

}

輸出結(jié)果如下：

此程序模仿了MFC中的消息函數(shù)分配機(jī)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
12

y address:0012FEB8
13

sum double: 10.0010000000+20.0020000000=30.0030000000
14

x address:0012FED8
15

y address:0012FEE0
16

sum double: 0.0000000000+10.0010000000=10.0010000000
17

x address:0012FED0
18

y address:0012FED8
19

(FUNC)(static_cast< int (*)(int, int) > (Test03))
21

00401070, 00401070, 00401070
22

sum int: 10+10=20
23

sum int: 309237645+1076101251=1385338896
24

sum int: 10+10=20

從輸出結(jié)果可以看出，union的每個(gè)成員的內(nèi)存地址都是一樣的。這樣就好理解union MessageMapFunctions的真實(shí)作用了。

Feedback

# re: union初探回復(fù) 更多評(píng)論

2007-11-25 10:36 by aspmaster

不錯(cuò)，雖然候Sir在深入淺出中說(shuō)了，但當(dāng)時(shí)還是沒(méi)有看明白

# re: union初探回復(fù) 更多評(píng)論

2012-07-09 10:34 by HooperStephanie21

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# re: union初探 回復(fù) 更多評(píng)論

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