为什么地图类型的类型报告为53而不是21？

Question

显示代码：

package main

import (
    "fmt"
    "unsafe"
)

// copy form src/runtime/type.go
type _type struct {
    size       uintptr
    ptrdata    uintptr // size of memory prefix holding all pointers
    hash       uint32
    tflag      tflag
    align      uint8
    fieldAlign uint8
    kind       uint8
    equal      func(unsafe.Pointer, unsafe.Pointer) bool
    gcdata     *byte
    str        nameOff
    ptrToThis  typeOff
}
type tflag uint8
type nameOff int32
type typeOff int32

// copy form src/runtime/runtime2.go
type eface struct {
    _type *maptype
    data  unsafe.Pointer
}

// copy form src/runtime/type.go
type maptype struct {
    typ    _type
    key    *_type
    elem   *_type
    bucket *_type // internal type representing a hash bucket
    // function for hashing keys (ptr to key, seed) -> hash
    hasher     func(unsafe.Pointer, uintptr) uintptr
    keysize    uint8  // size of key slot
    elemsize   uint8  // size of elem slot
    bucketsize uint16 // size of bucket
    flags      uint32
}

func main() {
    var t interface{} = map[int]int{1: 1}

    p := (*eface)(unsafe.Pointer(&t))

    fmt.Println(p._type.typ.kind) // 53
}

打印53，

但您可以在src/运行时/ find kind.go中找到

const (
    kindBool = 1 + iota
    kindInt
    kindInt8
    kindInt16
    kindInt32
    kindInt64
    kindUint
    kindUint8
    kindUint16
    kindUint32
    kindUint64
    kindUintptr
    kindFloat32
    kindFloat64
    kindComplex64
    kindComplex128
    kindArray
    kindChan
    kindFunc
    kindInterface
    kindMap // 21
    kindPtr
    kindSlice
    kindString
    kindStruct
    kindUnsafePointer

    kindDirectIface = 1 << 5
    kindGCProg      = 1 << 6
    kindMask        = (1 << 5) - 1
)

地图类型为const 21。同样，chan是50岁，而不是kindChan(18)。为什么？

Answer 1

如果您查看src/运行时/typekind.go kind.go，将有一个函数来检查该值是否直接存储在接口值中，这适用于您的情况，因为您正在创建t of interface{}，但要将映射类型存储到它。

// isDirectIface reports whether t is stored directly in an interface value.
func isDirectIface(t *_type) bool {
    return t.kind&kindDirectIface != 0
}

当使用p作为isDirectIface(&p._type.typ)的值时，它返回true，因为设置了与接口类型相对应的底层位值(kindDirectIface = 1 << 5，32)。

因此，实际上，该值53表示(十进制21+32)作为接口类型32 (kindDirectIface)存储的映射类型21 (kindMap)。