Lambda表達(dá)式提供了匿名函數(shù)這個概念,可以使您在一個函數(shù)中書寫另一個匿名的函數(shù)體
先來看下如何在VC2010中書寫Lambda表達(dá)式
[捕捉項(xiàng)表](參數(shù)表)->返回類型{函數(shù)體}
捕捉項(xiàng)表中在變量前添加&操作符表示捕捉引用,添加=表示捕捉值
參數(shù)表與"->"和后面的返回類型是可選的編譯器會由函數(shù)體內(nèi)的return語句判斷返回類型,當(dāng)返回類型很復(fù)雜時編譯器無法判斷,則必須手動給出!
下面舉個簡單的例子:
1 #include <stdio.h>
2
3 int main()
4 {
5 int a = 100;
6 [a]{
7 printf("%d\n",a);
8 }();
9 return 0;
10 }
然后在VS2010提示中編譯它:cl /FAs a.cpp
/FAs參數(shù)指定了輸出匯編文件a.asm.
然后我們來看一下它生成的匯編代碼:
1 ; Listing generated by Microsoft (R) Optimizing Compiler Version 16.00.30319.01
2
3 TITLE D:\a.cpp
4 .686P
5 .XMM
6 include listing.inc
7 .model flat
8
9 INCLUDELIB LIBCMT
10 INCLUDELIB OLDNAMES
11
12 PUBLIC ??R<lambda0>@?A0x428ab923@@QBEXXZ ; `anonymous namespace'::<lambda0>::operator()
13 PUBLIC ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z ; `anonymous namespace'::<lambda0>::<lambda0>
14 PUBLIC _main
15 ; Function compile flags: /Odtp
16 ; File d:\a.cpp
17 _TEXT SEGMENT
18 $T3875 = -8 ; size = 4
19 _a$ = -4 ; size = 4
20 _main PROC
21
22 ; 4 : {
23
24 push ebp
25 mov ebp, esp
26 sub esp, 8
27
28 ; 5 : int a = 100;
29
30 mov DWORD PTR _a$[ebp], 100 ; 00000064H
31
32 ; 6 : [a]{
33 ; 7 : printf("%d\n",a);
34 ; 8 : }();
35
36 lea eax, DWORD PTR _a$[ebp]
37 push eax
38 lea ecx, DWORD PTR $T3875[ebp]
39 call ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z ; `anonymous namespace'::<lambda0>::<lambda0>
40 mov ecx, eax
41 call ??R<lambda0>@?A0x428ab923@@QBEXXZ ; `anonymous namespace'::<lambda0>::operator()
42
43 ; 9 : return 0;
44
45 xor eax, eax
46
47 ; 10 : }
48
49 mov esp, ebp
50 pop ebp
51 ret 0
52 _main ENDP
53 ; Function compile flags: /Odtp
54 _TEXT ENDS
55 ; COMDAT ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z
56 _TEXT SEGMENT
57 _this$ = -4 ; size = 4
58 __A$ = 8 ; size = 4
59 ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z PROC ; `anonymous namespace'::<lambda0>::<lambda0>, COMDAT
60 ; _this$ = ecx
61
62 ; 8 : }();
63
64 push ebp
65 mov ebp, esp
66 push ecx
67 mov DWORD PTR _this$[ebp], ecx
68 mov eax, DWORD PTR _this$[ebp]
69 mov ecx, DWORD PTR __A$[ebp]
70 mov edx, DWORD PTR [ecx]
71 mov DWORD PTR [eax], edx
72 mov eax, DWORD PTR _this$[ebp]
73 mov esp, ebp
74 pop ebp
75 ret 4
76 ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z ENDP ; `anonymous namespace'::<lambda0>::<lambda0>
77 _TEXT ENDS
78 PUBLIC ??_C@_03PMGGPEJJ@?$CFd?6?$AA@ ; `string'
79 EXTRN _printf:PROC
80 ; COMDAT ??_C@_03PMGGPEJJ@?$CFd?6?$AA@
81 CONST SEGMENT
82 ??_C@_03PMGGPEJJ@?$CFd?6?$AA@ DB '%d', 0aH, 00H ; `string'
83 ; Function compile flags: /Odtp
84 CONST ENDS
85 ; COMDAT ??R<lambda0>@?A0x428ab923@@QBEXXZ
86 _TEXT SEGMENT
87 _this$ = -4 ; size = 4
88 ??R<lambda0>@?A0x428ab923@@QBEXXZ PROC ; `anonymous namespace'::<lambda0>::operator(), COMDAT
89 ; _this$ = ecx
90
91 ; 6 : [a]{
92
93 push ebp
94 mov ebp, esp
95 push ecx
96 mov DWORD PTR _this$[ebp], ecx
97
98 ; 7 : printf("%d\n",a);
99
100 mov eax, DWORD PTR _this$[ebp]
101 mov ecx, DWORD PTR [eax]
102 push ecx
103 push OFFSET ??_C@_03PMGGPEJJ@?$CFd?6?$AA@
104 call _printf
105 add esp, 8
106
107 ; 8 : }();
108
109 mov esp, ebp
110 pop ebp
111 ret 0
112 ??R<lambda0>@?A0x428ab923@@QBEXXZ ENDP ; `anonymous namespace'::<lambda0>::operator()
113 _TEXT ENDS
114 END
115
由匯編代碼可以看到先為Lambda函數(shù)生成了一個匿名對象,然后調(diào)用了()的操作符重載函數(shù)調(diào)用這個匿名函數(shù).
由此我們得出結(jié)論:調(diào)用一個匿名函數(shù)并沒有調(diào)用一個存在的函數(shù)或是一個類的成員函數(shù)快,只是書寫更方便罷了.
然后我們修改一下代碼,將匿名函數(shù)先賦給一個匿名函數(shù)指針,然后調(diào)用.
1 #include <stdio.h>
2
3 int main()
4 {
5 int a = 100;
6 auto ptr = [a]{
7 printf("%d\n",a);
8 };
9 ptr();
10 return 0;
11 }
編譯后得到代碼:
1 ; Listing generated by Microsoft (R) Optimizing Compiler Version 16.00.30319.01
2
3 TITLE D:\a.cpp
4 .686P
5 .XMM
6 include listing.inc
7 .model flat
8
9 INCLUDELIB LIBCMT
10 INCLUDELIB OLDNAMES
11
12 PUBLIC ??R<lambda0>@?A0x428ab923@@QBEXXZ ; `anonymous namespace'::<lambda0>::operator()
13 PUBLIC ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z ; `anonymous namespace'::<lambda0>::<lambda0>
14 PUBLIC _main
15 ; Function compile flags: /Odtp
16 ; File d:\a.cpp
17 _TEXT SEGMENT
18 _ptr$ = -8 ; size = 4
19 _a$ = -4 ; size = 4
20 _main PROC
21
22 ; 4 : {
23
24 push ebp
25 mov ebp, esp
26 sub esp, 8
27
28 ; 5 : int a = 100;
29
30 mov DWORD PTR _a$[ebp], 100 ; 00000064H
31
32 ; 6 : auto ptr = [a]{
33 ; 7 : printf("%d\n",a);
34 ; 8 : };
35
36 lea eax, DWORD PTR _a$[ebp]
37 push eax
38 lea ecx, DWORD PTR _ptr$[ebp]
39 call ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z ; `anonymous namespace'::<lambda0>::<lambda0>
40
41 ; 9 : ptr();
42
43 lea ecx, DWORD PTR _ptr$[ebp]
44 call ??R<lambda0>@?A0x428ab923@@QBEXXZ ; `anonymous namespace'::<lambda0>::operator()
45
46 ; 10 : return 0;
47
48 xor eax, eax
49
50 ; 11 : }
51
52 mov esp, ebp
53 pop ebp
54 ret 0
55 _main ENDP
56 ; Function compile flags: /Odtp
57 _TEXT ENDS
58 ; COMDAT ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z
59 _TEXT SEGMENT
60 _this$ = -4 ; size = 4
61 __A$ = 8 ; size = 4
62 ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z PROC ; `anonymous namespace'::<lambda0>::<lambda0>, COMDAT
63 ; _this$ = ecx
64
65 ; 8 : };
66
67 push ebp
68 mov ebp, esp
69 push ecx
70 mov DWORD PTR _this$[ebp], ecx
71 mov eax, DWORD PTR _this$[ebp]
72 mov ecx, DWORD PTR __A$[ebp]
73 mov edx, DWORD PTR [ecx]
74 mov DWORD PTR [eax], edx
75 mov eax, DWORD PTR _this$[ebp]
76 mov esp, ebp
77 pop ebp
78 ret 4
79 ??0<lambda0>@?A0x428ab923@@QAE@ABH@Z ENDP ; `anonymous namespace'::<lambda0>::<lambda0>
80 _TEXT ENDS
81 PUBLIC ??_C@_03PMGGPEJJ@?$CFd?6?$AA@ ; `string'
82 EXTRN _printf:PROC
83 ; COMDAT ??_C@_03PMGGPEJJ@?$CFd?6?$AA@
84 CONST SEGMENT
85 ??_C@_03PMGGPEJJ@?$CFd?6?$AA@ DB '%d', 0aH, 00H ; `string'
86 ; Function compile flags: /Odtp
87 CONST ENDS
88 ; COMDAT ??R<lambda0>@?A0x428ab923@@QBEXXZ
89 _TEXT SEGMENT
90 _this$ = -4 ; size = 4
91 ??R<lambda0>@?A0x428ab923@@QBEXXZ PROC ; `anonymous namespace'::<lambda0>::operator(), COMDAT
92 ; _this$ = ecx
93
94 ; 6 : auto ptr = [a]{
95
96 push ebp
97 mov ebp, esp
98 push ecx
99 mov DWORD PTR _this$[ebp], ecx
100
101 ; 7 : printf("%d\n",a);
102
103 mov eax, DWORD PTR _this$[ebp]
104 mov ecx, DWORD PTR [eax]
105 push ecx
106 push OFFSET ??_C@_03PMGGPEJJ@?$CFd?6?$AA@
107 call _printf
108 add esp, 8
109
110 ; 8 : };
111
112 mov esp, ebp
113 pop ebp
114 ret 0
115 ??R<lambda0>@?A0x428ab923@@QBEXXZ ENDP ; `anonymous namespace'::<lambda0>::operator()
116 _TEXT ENDS
117 END
118
和上面的直接調(diào)用并無差別,因?yàn)橐粋€匿名函數(shù)含有一個隱藏的對象.
應(yīng)此__asm call ptr;這樣調(diào)用是錯誤的!因?yàn)槟銦o法用call指令來調(diào)用一個對象的成員函數(shù).
posted on 2011-02-17 16:32
lwch 閱讀(2694)
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