《the way to go》一处关于go匿名函数的“勘误”

2019独角兽企业重金招聘Python工程师标准>>>

《the way to go》是go的一门经典入门书籍，但是由于书的编纂比较早，有些知识点已经过时了。

在6.8章节中有这么一处代码，

package mainimport "fmt"func main() {f() } func f() {for i := 0; i < 4; i++ {g := func(i int) { fmt.Printf("%d", i) }g(i)fmt.Printf(" - g is of type %T and has value %v\n", g, g)} }

输出

0 - g is of type func(int) and has value 0x681a80 1 - g is of type func(int) and has value 0x681b00 2 - g is of type func(int) and has value 0x681ac0 3 - g is of type func(int) and has value 0x681400

但是，当你自己去运行这个代码的时候，你就发现，输出四次的函数内存地址是一样的，这里我们写一个简易的代码，然后把它转成汇编，看一下

package mainfunc main() {for i := 0; i < 4; i++ {g := func() {}_ = g} }

执行命令

go tool compile -S main.go

得到汇编代码

"".main t=1 size=18 args=0x0 locals=0x00x0000 00000 (main.go:3) TEXT "".main(SB), $0-00x0000 00000 (main.go:3) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)0x0000 00000 (main.go:3) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)0x0000 00000 (main.go:4) MOVQ $0, AX0x0002 00002 (main.go:4) CMPQ AX, $40x0006 00006 (main.go:4) JGE $0, 170x0008 00008 (main.go:4) INCQ AX0x000b 00011 (main.go:4) CMPQ AX, $40x000f 00015 (main.go:4) JLT $0, 80x0011 00017 (main.go:8) RET0x0000 31 c0 48 83 f8 04 7d 09 48 ff c0 48 83 f8 04 7c 1.H...}.H..H...|0x0010 f7 c3 .. "".main.func1 t=1 size=1 args=0x0 locals=0x00x0000 00000 (main.go:5) TEXT "".main.func1(SB), $0-00x0000 00000 (main.go:5) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)0x0000 00000 (main.go:5) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)0x0000 00000 (main.go:5) RET0x0000 c3 . "".init t=1 size=79 args=0x0 locals=0x80x0000 00000 (main.go:9) TEXT "".init(SB), $8-00x0000 00000 (main.go:9) MOVQ (TLS), CX0x0009 00009 (main.go:9) CMPQ SP, 16(CX)0x000d 00013 (main.go:9) JLS 720x000f 00015 (main.go:9) SUBQ $8, SP0x0013 00019 (main.go:9) MOVQ BP, (SP)0x0017 00023 (main.go:9) LEAQ (SP), BP0x001b 00027 (main.go:9) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)0x001b 00027 (main.go:9) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)0x001b 00027 (main.go:9) MOVBLZX "".initdone·(SB), AX0x0022 00034 (main.go:9) CMPB AL, $10x0024 00036 (main.go:9) JLS $0, 470x0026 00038 (main.go:9) MOVQ (SP), BP0x002a 00042 (main.go:9) ADDQ $8, SP0x002e 00046 (main.go:9) RET0x002f 00047 (main.go:9) JNE $0, 560x0031 00049 (main.go:9) PCDATA $0, $00x0031 00049 (main.go:9) CALL runtime.throwinit(SB)0x0036 00054 (main.go:9) UNDEF0x0038 00056 (main.go:9) MOVB $2, "".initdone·(SB)0x003f 00063 (main.go:9) MOVQ (SP), BP0x0043 00067 (main.go:9) ADDQ $8, SP0x0047 00071 (main.go:9) RET0x0048 00072 (main.go:9) NOP0x0048 00072 (main.go:9) PCDATA $0, $-10x0048 00072 (main.go:9) CALL runtime.morestack_noctxt(SB)0x004d 00077 (main.go:9) JMP 00x0000 65 48 8b 0c 25 00 00 00 00 48 3b 61 10 76 39 48 eH..%....H;a.v9H0x0010 83 ec 08 48 89 2c 24 48 8d 2c 24 0f b6 05 00 00 ...H.,$H.,$.....0x0020 00 00 3c 01 76 09 48 8b 2c 24 48 83 c4 08 c3 75 ..<.v.H.,$H....u0x0030 07 e8 00 00 00 00 0f 0b c6 05 00 00 00 00 02 48 ...............H0x0040 8b 2c 24 48 83 c4 08 c3 e8 00 00 00 00 eb b1 .,$H...........rel 5+4 t=16 TLS+0rel 30+4 t=15 "".initdone·+0rel 50+4 t=8 runtime.throwinit+0rel 58+4 t=15 "".initdone·+-1rel 73+4 t=8 runtime.morestack_noctxt+0 gclocals·33cdeccccebe80329f1fdbee7f5874cb t=8 dupok size=80x0000 01 00 00 00 00 00 00 00 ........ go.info."".main t=45 size=270x0000 02 22 22 2e 6d 61 69 6e 00 00 00 00 00 00 00 00 ."".main........0x0010 00 00 00 00 00 00 00 00 00 01 00 ...........rel 9+8 t=1 "".main+0rel 17+8 t=1 "".main+18 go.info."".main.func1 t=45 size=330x0000 02 22 22 2e 6d 61 69 6e 2e 66 75 6e 63 31 00 00 ."".main.func1..0x0010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 ................0x0020 00 .rel 15+8 t=1 "".main.func1+0rel 23+8 t=1 "".main.func1+1 go.info."".init t=45 size=270x0000 02 22 22 2e 69 6e 69 74 00 00 00 00 00 00 00 00 ."".init........0x0010 00 00 00 00 00 00 00 00 00 01 00 ...........rel 9+8 t=1 "".init+0rel 17+8 t=1 "".init+79 "".initdone· t=32 size=1 "".main.func1·f t=8 dupok size=80x0000 00 00 00 00 00 00 00 00 ........rel 0+8 t=1 "".main.func1+0

请注意这里

"".main.func1 t=1 size=1 args=0x0 locals=0x00x0000 00000 (main.go:5) TEXT "".main.func1(SB), $0-00x0000 00000 (main.go:5) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)0x0000 00000 (main.go:5) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)0x0000 00000 (main.go:5) RET0x0000 c3

这里就是生成匿名函数的汇编代码，可以看到，整个代码只生成一次匿名函数。

我测试了1.2-1.8版本，都是一样的。

个人推测，《the way to go》编写的2012的时候，go1.0刚刚发布，编译器并没有对这种代码做优化，所以出现了书中描述的现象，

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转载于:https://my.oschina.net/u/222608/blog/1073901

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