Goreflect反射原理示例详解

type emptyInterface struct {
    typ  *rtype
    Word unsafe.Pointer  
}

package main
import "fmt"
func main() {
	s := 1024
	var a interface{} = &s
	fmt.Println(a)
}

go tool compile -N -S .\main.go

0x0057 00087 (.\main.go:7)      MOVQ    "".&s+104(SP), AX
0x005c 00092 (.\main.go:7)      MOVQ    AX, ""..autotmp_9+88(SP)
0x0061 00097 (.\main.go:7)      LEAQ    type.*int(SB), CX
0x0068 00104 (.\main.go:7)      MOVQ    CX, "".a+144(SP)
0x0070 00112 (.\main.go:7)      MOVQ    AX, "".a+152(SP)

type EmptyInterface struct {
	typ  unsafe.Pointer
	word unsafe.Pointer
}
func getWordPtr(i interface{})  unsafe.Pointer {
	eface := *(*EmptyInterface)(unsafe.Pointer(&i))
	return eface.word
}
func Test_GetWordPtr(t *testing.T) {
	str := "Hello, KeSan"
	strPtr := &str
	//此处由编译器做了类型转换 *string -> interface{}
	wordPtr := getWordPtr(strPtr)
	t.Logf("String Ptr: %p",  strPtr)
	t.Logf("Word Ptr: %p", wordPtr)
}

func TypeOf(i interface{}) Type {
	eface := *(*emptyInterface)(unsafe.Pointer(&i))
	return toType(eface.typ)
}
// 将 *rtype 转成接口类型的Type
func toType(t *rtype) Type {
	if t == nil {
		return nil
	}
	return t
}

type rtype struct {
	size       uintptr
	ptrdata    uintptr // number of bytes in the type that can contain pointers
	hash       uint32  // hash of type; avoids computation in hash tables
	tflag      tflag   // extra type information flags
	align      uint8   // alignment of variable with this type
	fieldAlign uint8   // alignment of struct field with this type
	kind       uint8   // enumeration for C
	// function for comparing objects of this type
	// (ptr to object A, ptr to object B) -> ==?
	equal     func(unsafe.Pointer, unsafe.Pointer) bool
	GCdata    *byte   // garbage collection data
	str       nameOff // string form
	ptrToThis typeOff // type for pointer to this type, may be zero
}

// 结构体的类型信息
type structType struct {
	rtype
	pkgPath name
	fields  []structField // sorted by offset
}
// channel 的类型信息
type chanType struct {
	rtype
	elem *rtype  // channel element type
	dir  uintptr // channel direction (ChanDir)
}

type Coordinate struct {
    X int64
    Y int64
    Z int64
}

func (v Value) Field(i int) Value {
	if v.kind() != Struct {
		panic(&ValueError{"reflect.Value.Field", v.kind()})
	}
	// 获取类型信息
	tt := (*structType)(unsafe.Pointer(v.typ))
	if uint(i) >= uint(len(tt.fields)) {
		panic("reflect: Field index out of range")
	}
	// 获取字段信息
	field := &tt.fields[i]
	typ := field.typ
	// 继承结构体的部分flag信息
	fl := v.flag&(flagStickyRO|flagIndir|flagAddr) | flag(typ.Kind())
	if !field.name.isExported() {
		if field.embedded() {
			fl |= flagEmbedRO
		} else {
			fl |= flagStickyRO
		}
	}
	// 根据偏移地址计 + 结构体的首地址 计算出 字段在内存中的地址, 并返回Value对象
	ptr := add(v.ptr, field.offset(), "same as non-reflect &v.field")
	return Value{typ, ptr, fl}
}

func (v Value) SetInt(x int64) {
	v.mustBeAssignable()
	switch k := v.kind(); k {
	default:
		panic(&ValueError{"reflect.Value.SetInt", v.kind()})
	case Int:
		*(*int)(v.ptr) = int(x)
	case Int8:
		*(*int8)(v.ptr) = int8(x)
	case Int16:
		*(*int16)(v.ptr) = int16(x)
	case Int32:
		*(*int32)(v.ptr) = int32(x)
	case Int64:
		*(*int64)(v.ptr) = x
	}
}

// 切片头
type SliceHeader struct {
	Data uintptr // 数组的指针地址
	Len  int     // 数组长度
	Cap  int     // 数组容量
}
// 字符串头
type StringHeader struct {
	Data uintptr // 字节数组的指针地址
	Len  int     // 字节数组的长度
}