在go Reflection包中，调用Value.Kind（）是Value.Type（）。Kind（）的语法糖吗？

Question

Both of the reflect.Type interface and reflect.Value type implement the same Kind() method signature, suppose that we have some value object v := reflect.ValueOf(x)

Is v.Kind() just call v.Type().Kind() ?

Answer 1

They contain the same value, but do not seem to refer to the same thing:

• type.go source
• value.go source

A Type is usually implemented by unexported struct rtype (via TypeOf), while the Value contains a *rtype and extends flag, which is itself a reduced form of the Kind:

// flag holds metadata about the value.
// The lowest bits are flag bits:
//  - flagRO: obtained via unexported field, so read-only
//  - flagIndir: val holds a pointer to the data
//  - flagAddr: v.CanAddr is true (implies flagIndir)
//  - flagMethod: v is a method value.
// The next five bits give the Kind of the value.
// This repeats typ.Kind() except for method values.
// The remaining 23+ bits give a method number for method values.
// If flag.kind() != Func, code can assume that flagMethod is unset.
// If typ.size > ptrSize, code can assume that flagIndir is set.

When getting the ValueOf something:

// ValueOf returns a new Value initialized to the concrete value
// stored in the interface i.  ValueOf(nil) returns the zero Value.
func ValueOf(i interface{}) Value {
    [...]
    // For an interface value with the noAddr bit set,
    // the representation is identical to an empty interface.
    eface := *(*emptyInterface)(unsafe.Pointer(&i))
    typ := eface.typ


    /** Flag is built from the type, then kept separate (my comment) */

    fl := flag(typ.Kind()) << flagKindShift
    if typ.size > ptrSize {
        fl |= flagIndir
    }
    return Value{typ, unsafe.Pointer(eface.word), fl}
}

And so when you get the kind of a Value (remember it extends its flag):

func (v Value) Kind() Kind {
    return v.kind()
}

func (f flag) kind() Kind {
    return Kind((f >> flagKindShift) & flagKindMask)
}

While getting the kind of a type: (Type is an interface, usually implemented by *rtype)

func (t *rtype) Kind() Kind { return Kind(t.kind & kindMask) }

So although they seem to be equal in most of the cases, v.Kind() is not v.Type().Kind()