گزاره defer در زبان برنامه‌نویسی Go — راهنمای کاربردی

1func main() {
2   defer func() {
3      println(`defer 1`)
4   }()
5   defer func() {
6      println(`defer 2`)
7   }()
8}
9defer 2 <- Last in, first to go out
10defer 1

1type _defer struct {
2   siz     int32
3   started bool
4   sp      uintptr
5   pc      uintptr
6   fn      *funcval
7   _panic  *_panic
8   link    *_defer // next deferred function to be executed
9}

1func newdefer(siz int32) *_defer {
2   var d *_defer
3   gp := getg() // get the current goroutine
4   [...]
5   // deferred list is now attached to the new _defer struct
6   d.link = gp._defer 
7   gp._defer = d // the new struct is now the first to be called
8   return d
9}

1func deferreturn(arg0 uintptr) {
2   gp := getg() // get the current goroutine
3   d:= gp._defer // copy the deferred function to a variable
4   if d == nil { // if there is not deferred func, just return
5      return
6   }
7   [...]
8   fn := d.fn // get the function to call
9   d.fn = nil // reset the function
10   gp._defer = d.link // attach the next one to the goroutine
11   freedefer(d) // freeing the _defer struc
12   jmpdefer(fn, uintptr(unsafe.Pointer(&arg0))) // call the func
13}

1// first deferred func
20x001d 00029 (main.go:6)   MOVL   $0, (SP)
30x0024 00036 (main.go:6)   PCDATA $2, $1
40x0024 00036 (main.go:6)   LEAQ   "".main.func1·f(SB), AX
50x002b 00043 (main.go:6)   PCDATA $2, $0
60x002b 00043 (main.go:6)   MOVQ   AX, 8(SP)
70x0030 00048 (main.go:6)   CALL   runtime.deferproc(SB)
80x0035 00053 (main.go:6)   TESTL  AX, AX
90x0037 00055 (main.go:6)   JNE    117
10// second deferred func
110x0039 00057 (main.go:10)  MOVL   $0, (SP)
120x0040 00064 (main.go:10)  PCDATA $2, $1
130x0040 00064 (main.go:10)  LEAQ   "".main.func2·f(SB), AX
140x0047 00071 (main.go:10)  PCDATA $2, $0
150x0047 00071 (main.go:10)  MOVQ   AX, 8(SP)
160x004c 00076 (main.go:10)  CALL   runtime.deferproc(SB)
170x0051 00081 (main.go:10)  TESTL  AX, AX
180x0053 00083 (main.go:10)  JNE    101
19// end of main func
200x0055 00085 (main.go:18)  XCHGL  AX, AX
210x0056 00086 (main.go:18)  CALL   runtime.deferreturn(SB)
220x005b 00091 (main.go:18)  MOVQ   16(SP), BP
230x0060 00096 (main.go:18)  ADDQ   $24, SP
240x0064 00100 (main.go:18)  RET
250x0065 00101 (main.go:10)  XCHGL  AX, AX
260x0066 00102 (main.go:10)  CALL   runtime.deferreturn(SB)
270x006b 00107 (main.go:10)  MOVQ   16(SP), BP
280x0070 00112 (main.go:10)  ADDQ   $24, SP
290x0074 00116 (main.go:10)  RET

1func main() {
2   fmt.Printf("with named param, x: %d\n", namedParam())
3   fmt.Printf("without named param, x: %d\n", notNamedParam())
4}
5func namedParam() (x int) {
6   x = 1
7   defer func() { x = 2 }()
8   return x
9}
10
11func notNamedParam() (int) {
12   x := 1
13   defer func() { x = 2 }()
14   return x
15}
16with named param, x: 2
17without named param, x: 1

1func gopanic(e interface{}) {
2   [...]
3   var p _panic
4   [...]
5   d := gp._defer // current attached defer on the goroutine
6   [...]
7   d._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
8   [...]
9}

10x0067 00103 (main.go:21)   CALL   runtime.gopanic(SB)
20x006c 00108 (main.go:21)  UNDEF
30x006e 00110 (main.go:16)  XCHGL  AX, AX
40x006f 00111 (main.go:16)  CALL   runtime.deferreturn(SB)

1func main() {
2   fmt.Printf("error from err1: %v\n", err1())
3   fmt.Printf("error from err2: %v\n", err2())
4}
5
6func err1() error {
7   var err error
8
9   defer func() {
10      if r := recover(); r != nil {
11         err = errors.New("recovered")
12      }
13   }()
14   panic(`foo`)
15
16   return err
17}
18
19func err2() (err error) {
20   defer func() {
21      if r := recover(); r != nil {
22         err = errors.New("recovered")
23      }
24   }()
25   panic(`foo`)
26
27   return err
28}
29error from err1: <nil>
30error from err2: recovered

1name         old time/op  new time/op  delta
2Defer-4      99.0ns ± 9%  52.4ns ± 5%  -47.04%  (p=0.000 n=9+10)
3Defer10-4    90.6ns ±13%  45.0ns ± 3%  -50.37%  (p=0.000 n=10+10)

1name     old time/op  new time/op  delta
2Defer-4  51.3ns ± 3%  45.8ns ± 1%  -10.72%  (p=0.000 n=10+10)