Golang 空map和未初始化map的注意事项说明

var m1 map[string]string
fmt.Println(m1["1"])

var m1 map[string]string
m1["1"] = "1"

a := make(map[string]int)
a["a"] = 1
a["b"] = 2
// clear all
a = make(map[string]int)

package main
func main() {
        m := make(map[byte]int)
        m[1] = 1
        m[2] = 2
        for k := range m {
	        delete(m, k)
        }
}

go tool compile -S map_clear.go

go tool compile -l -N -S map_clear.go

// mapclear deletes all keys from a map.
func mapclear(t *maptype, h *hmap) {
	if raceenabled && h != nil {
		callerpc := getcallerpc()
		pc := funcPC(mapclear)
		racewritepc(unsafe.Pointer(h), callerpc, pc)
	}
	if h == nil || h.count == 0 {
		return
	}
	if h.flags&hashWriting != 0 {
		throw("concurrent map writes")
	}
	h.flags ^= hashWriting
	h.flags &^= sameSizeGrow
	h.oldbuckets = nil
	h.nevacuate = 0
	h.noverflow = 0
	h.count = 0
	// Keep the mapextra allocation but clear any extra information.
	if h.extra != nil {
		*h.extra = mapextra{}
	}
	// makeBucketArray clears the memory pointed to by h.buckets
	// and recovers any overflow buckets by generating them
	// as if h.buckets was newly alloced.
	_, nextOverflow := makeBucketArray(t, h.B, h.buckets)
	if nextOverflow != nil {
		// If overflow buckets are created then h.extra
		// will have been allocated during initial bucket creation.
		h.extra.nextOverflow = nextOverflow
	}
	if h.flags&hashWriting == 0 {
		throw("concurrent map writes")
	}
	h.flags &^= hashWriting
}

func BenchmarkMakeNewMap(b *testing.B) {
	tmpMap := make(map[string]string, 10000)
	for i := 0; i < b.N; i++ {
		for j := 0; j < 10000; j++ {
			tmpMap["tmp"+strconv.Itoa(j)] = "tmp"
		}
		tmpMap = make(map[string]string, 10000)
	}
}
func BenchmarkDeleteMap(b *testing.B) {
	tmpMap := make(map[string]string, 10000)
	for i := 0; i < b.N; i++ {
		for j := 0; j < 10000; j++ {
			tmpMap["tmp"+strconv.Itoa(j)] = "tmp"
		}
		for k := range tmpMap {
			delete(tmpMap, k)
		}
	}
}

func TestMakeNewMap(t *testing.T) {
   debug.SetGCPercent(-1)
   var m runtime.MemStats
   tmpMap := make(map[string]string, 1000000)
   for j := 0; j < 1000000; j++ {
      tmpMap["tmp"+strconv.Itoa(j)] = "tmp"
   }
   start := time.Now()
   tmpMap = make(map[string]string, 1000000)
   fmt.Println(time.Since(start).Microseconds())
   runtime.ReadMemStats(&m)
   fmt.Printf("%d Kb\n", m.Alloc/1024)
}
func TestDeleteMap(t *testing.T) {
   debug.SetGCPercent(-1)
   var m runtime.MemStats
   tmpMap2 := make(map[string]string, 1000000)
   for j := 0; j < 1000000; j++ {
      tmpMap2["tmp"+strconv.Itoa(j)] = "tmp"
   }
   start := time.Now()
   for k := range tmpMap2 {
      delete(tmpMap2, k)
   }
   fmt.Println(time.Since(start).Microseconds())
   runtime.ReadMemStats(&m)
   fmt.Printf("%d Kb\n", m.Alloc/1024)
}

tmpMap = make(map[string]string)

tmpMap = make(map[string]string, 10000)