src/internal/reflectlite/value.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package reflectlite

 import (
 	"runtime"
 	"unsafe"
 )

 const ptrSize = 4 << (^uintptr(0) >> 63) // unsafe.Sizeof(uintptr(0)) but an ideal const

 // Value is the reflection interface to a Go value.
 //
 // Not all methods apply to all kinds of values. Restrictions,
 // if any, are noted in the documentation for each method.
 // Use the Kind method to find out the kind of value before
 // calling kind-specific methods. Calling a method
 // inappropriate to the kind of type causes a run time panic.
 //
 // The zero Value represents no value.
 // Its IsValid method returns false, its Kind method returns Invalid,
 // its String method returns "<invalid Value>", and all other methods panic.
 // Most functions and methods never return an invalid value.
 // If one does, its documentation states the conditions explicitly.
 //
 // A Value can be used concurrently by multiple goroutines provided that
 // the underlying Go value can be used concurrently for the equivalent
 // direct operations.
 //
 // To compare two Values, compare the results of the Interface method.
 // Using == on two Values does not compare the underlying values
 // they represent.
 type Value struct {
 	// typ holds the type of the value represented by a Value.
 	typ *rtype

 	// Pointer-valued data or, if flagIndir is set, pointer to data.
 	// Valid when either flagIndir is set or typ.pointers() is true.
 	ptr unsafe.Pointer

 	// flag holds metadata about the value.
 	// The lowest bits are flag bits:
 	//	- flagStickyRO: obtained via unexported not embedded field, so read-only
 	//	- flagEmbedRO: obtained via unexported embedded field, so read-only
 	//	- flagIndir: val holds a pointer to the data
 	//	- flagAddr: v.CanAddr is true (implies flagIndir)
 	// Value cannot represent method values.
 	// 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 ifaceIndir(typ), code can assume that flagIndir is set.
 	flag

 	// A method value represents a curried method invocation
 	// like r.Read for some receiver r. The typ+val+flag bits describe
 	// the receiver r, but the flag's Kind bits say Func (methods are
 	// functions), and the top bits of the flag give the method number
 	// in r's type's method table.
 }

 type flag uintptr

 const (
 	flagKindWidth        = 5 // there are 27 kinds
 	flagKindMask    flag = 1<<flagKindWidth - 1
 	flagStickyRO    flag = 1 << 5
 	flagEmbedRO     flag = 1 << 6
 	flagIndir       flag = 1 << 7
 	flagAddr        flag = 1 << 8
 	flagMethod      flag = 1 << 9
 	flagMethodShift      = 10
 	flagRO          flag = flagStickyRO | flagEmbedRO
 )

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

 func (f flag) ro() flag {
 	if f&flagRO != 0 {
 		return flagStickyRO
 	}
 	return 0
 }

 // pointer returns the underlying pointer represented by v.
 // v.Kind() must be Ptr, Map, Chan, Func, or UnsafePointer
 func (v Value) pointer() unsafe.Pointer {
 	if v.typ.size != ptrSize || !v.typ.pointers() {
 		panic("can't call pointer on a non-pointer Value")
 	}
 	if v.flag&flagIndir != 0 {
 		return *(*unsafe.Pointer)(v.ptr)
 	}
 	return v.ptr
 }

 // packEface converts v to the empty interface.
 func packEface(v Value) interface{} {
 	t := v.typ
 	var i interface{}
 	e := (*emptyInterface)(unsafe.Pointer(&i))
 	// First, fill in the data portion of the interface.
 	switch {
 	case ifaceIndir(t):
 		if v.flag&flagIndir == 0 {
 			panic("bad indir")
 		}
 		// Value is indirect, and so is the interface we're making.
 		ptr := v.ptr
 		if v.flag&flagAddr != 0 {
 			// TODO: pass safe boolean from valueInterface so
 			// we don't need to copy if safe==true?
 			c := unsafe_New(t)
 			typedmemmove(t, c, ptr)
 			ptr = c
 		}
 		e.word = ptr
 	case v.flag&flagIndir != 0:
 		// Value is indirect, but interface is direct. We need
 		// to load the data at v.ptr into the interface data word.
 		e.word = *(*unsafe.Pointer)(v.ptr)
 	default:
 		// Value is direct, and so is the interface.
 		e.word = v.ptr
 	}
 	// Now, fill in the type portion. We're very careful here not
 	// to have any operation between the e.word and e.typ assignments
 	// that would let the garbage collector observe the partially-built
 	// interface value.
 	e.typ = t
 	return i
 }

 // unpackEface converts the empty interface i to a Value.
 func unpackEface(i interface{}) Value {
 	e := (*emptyInterface)(unsafe.Pointer(&i))
 	// NOTE: don't read e.word until we know whether it is really a pointer or not.
 	t := e.typ
 	if t == nil {
 		return Value{}
 	}
 	f := flag(t.Kind())
 	if ifaceIndir(t) {
 		f |= flagIndir
 	}
 	return Value{t, e.word, f}
 }

 // A ValueError occurs when a Value method is invoked on
 // a Value that does not support it. Such cases are documented
 // in the description of each method.
 type ValueError struct {
 	Method string
 	Kind   Kind
 }

 func (e *ValueError) Error() string {
 	return "reflect: call of " + e.Method + " on zero Value"
 }

 // methodName returns the name of the calling method,
 // assumed to be two stack frames above.
 func methodName() string {
 	pc, _, _, _ := runtime.Caller(2)
 	f := runtime.FuncForPC(pc)
 	if f == nil {
 		return "unknown method"
 	}
 	return f.Name()
 }

 // emptyInterface is the header for an interface{} value.
 type emptyInterface struct {
 	typ  *rtype
 	word unsafe.Pointer
 }

 // mustBeExported panics if f records that the value was obtained using
 // an unexported field.
 func (f flag) mustBeExported() {
 	if f == 0 {
 		panic(&ValueError{methodName(), 0})
 	}
 	if f&flagRO != 0 {
 		panic("reflect: " + methodName() + " using value obtained using unexported field")
 	}
 }

 // mustBeAssignable panics if f records that the value is not assignable,
 // which is to say that either it was obtained using an unexported field
 // or it is not addressable.
 func (f flag) mustBeAssignable() {
 	if f == 0 {
 		panic(&ValueError{methodName(), Invalid})
 	}
 	// Assignable if addressable and not read-only.
 	if f&flagRO != 0 {
 		panic("reflect: " + methodName() + " using value obtained using unexported field")
 	}
 	if f&flagAddr == 0 {
 		panic("reflect: " + methodName() + " using unaddressable value")
 	}
 }

 // CanSet reports whether the value of v can be changed.
 // A Value can be changed only if it is addressable and was not
 // obtained by the use of unexported struct fields.
 // If CanSet returns false, calling Set or any type-specific
 // setter (e.g., SetBool, SetInt) will panic.
 func (v Value) CanSet() bool {
 	return v.flag&(flagAddr|flagRO) == flagAddr
 }

 // Elem returns the value that the interface v contains
 // or that the pointer v points to.
 // It panics if v's Kind is not Interface or Ptr.
 // It returns the zero Value if v is nil.
 func (v Value) Elem() Value {
 	k := v.kind()
 	switch k {
 	case Interface:
 		var eface interface{}
 		if v.typ.NumMethod() == 0 {
 			eface = *(*interface{})(v.ptr)
 		} else {
 			eface = (interface{})(*(*interface {
 				M()
 			})(v.ptr))
 		}
 		x := unpackEface(eface)
 		if x.flag != 0 {
 			x.flag |= v.flag.ro()
 		}
 		return x
 	case Ptr:
 		ptr := v.ptr
 		if v.flag&flagIndir != 0 {
 			ptr = *(*unsafe.Pointer)(ptr)
 		}
 		// The returned value's address is v's value.
 		if ptr == nil {
 			return Value{}
 		}
 		tt := (*ptrType)(unsafe.Pointer(v.typ))
 		typ := tt.elem
 		fl := v.flag&flagRO | flagIndir | flagAddr
 		fl |= flag(typ.Kind())
 		return Value{typ, ptr, fl}
 	}
 	panic(&ValueError{"reflectlite.Value.Elem", v.kind()})
 }

 func valueInterface(v Value) interface{} {
 	if v.flag == 0 {
 		panic(&ValueError{"reflectlite.Value.Interface", 0})
 	}

 	if v.kind() == Interface {
 		// Special case: return the element inside the interface.
 		// Empty interface has one layout, all interfaces with
 		// methods have a second layout.
 		if v.numMethod() == 0 {
 			return *(*interface{})(v.ptr)
 		}
 		return *(*interface {
 			M()
 		})(v.ptr)
 	}

 	// TODO: pass safe to packEface so we don't need to copy if safe==true?
 	return packEface(v)
 }

 // IsNil reports whether its argument v is nil. The argument must be
 // a chan, func, interface, map, pointer, or slice value; if it is
 // not, IsNil panics. Note that IsNil is not always equivalent to a
 // regular comparison with nil in Go. For example, if v was created
 // by calling ValueOf with an uninitialized interface variable i,
 // i==nil will be true but v.IsNil will panic as v will be the zero
 // Value.
 func (v Value) IsNil() bool {
 	k := v.kind()
 	switch k {
 	case Chan, Func, Map, Ptr, UnsafePointer:
 		// if v.flag&flagMethod != 0 {
 		// 	return false
 		// }
 		ptr := v.ptr
 		if v.flag&flagIndir != 0 {
 			ptr = *(*unsafe.Pointer)(ptr)
 		}
 		return ptr == nil
 	case Interface, Slice:
 		// Both interface and slice are nil if first word is 0.
 		// Both are always bigger than a word; assume flagIndir.
 		return *(*unsafe.Pointer)(v.ptr) == nil
 	}
 	panic(&ValueError{"reflectlite.Value.IsNil", v.kind()})
 }

 // IsValid reports whether v represents a value.
 // It returns false if v is the zero Value.
 // If IsValid returns false, all other methods except String panic.
 // Most functions and methods never return an invalid Value.
 // If one does, its documentation states the conditions explicitly.
 func (v Value) IsValid() bool {
 	return v.flag != 0
 }

 // Kind returns v's Kind.
 // If v is the zero Value (IsValid returns false), Kind returns Invalid.
 func (v Value) Kind() Kind {
 	return v.kind()
 }

 // implemented in runtime:
 func chanlen(unsafe.Pointer) int
 func maplen(unsafe.Pointer) int

 // Len returns v's length.
 // It panics if v's Kind is not Array, Chan, Map, Slice, or String.
 func (v Value) Len() int {
 	k := v.kind()
 	switch k {
 	case Array:
 		tt := (*arrayType)(unsafe.Pointer(v.typ))
 		return int(tt.len)
 	case Chan:
 		return chanlen(v.pointer())
 	case Map:
 		return maplen(v.pointer())
 	case Slice:
 		// Slice is bigger than a word; assume flagIndir.
 		return (*sliceHeader)(v.ptr).Len
 	case String:
 		// String is bigger than a word; assume flagIndir.
 		return (*stringHeader)(v.ptr).Len
 	}
 	panic(&ValueError{"reflect.Value.Len", v.kind()})
 }

 // NumMethod returns the number of exported methods in the value's method set.
 func (v Value) numMethod() int {
 	if v.typ == nil {
 		panic(&ValueError{"reflectlite.Value.NumMethod", Invalid})
 	}
 	return v.typ.NumMethod()
 }

 // Set assigns x to the value v.
 // It panics if CanSet returns false.
 // As in Go, x's value must be assignable to v's type.
 func (v Value) Set(x Value) {
 	v.mustBeAssignable()
 	x.mustBeExported() // do not let unexported x leak
 	var target unsafe.Pointer
 	if v.kind() == Interface {
 		target = v.ptr
 	}
 	x = x.assignTo("reflectlite.Set", v.typ, target)
 	if x.flag&flagIndir != 0 {
 		typedmemmove(v.typ, v.ptr, x.ptr)
 	} else {
 		*(*unsafe.Pointer)(v.ptr) = x.ptr
 	}
 }

 // Type returns v's type.
 func (v Value) Type() Type {
 	f := v.flag
 	if f == 0 {
 		panic(&ValueError{"reflectlite.Value.Type", Invalid})
 	}
 	// Method values not supported.
 	return v.typ
 }

 // stringHeader is a safe version of StringHeader used within this package.
 type stringHeader struct {
 	Data unsafe.Pointer
 	Len  int
 }

 // sliceHeader is a safe version of SliceHeader used within this package.
 type sliceHeader struct {
 	Data unsafe.Pointer
 	Len  int
 	Cap  int
 }

 /*
  * constructors
  */

 // implemented in package runtime
 func unsafe_New(*rtype) unsafe.Pointer

 // 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 {
 	if i == nil {
 		return Value{}
 	}

 	// TODO: Maybe allow contents of a Value to live on the stack.
 	// For now we make the contents always escape to the heap. It
 	// makes life easier in a few places (see chanrecv/mapassign
 	// comment below).
 	escapes(i)

 	return unpackEface(i)
 }

 // assignTo returns a value v that can be assigned directly to typ.
 // It panics if v is not assignable to typ.
 // For a conversion to an interface type, target is a suggested scratch space to use.
 func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value {
 	// if v.flag&flagMethod != 0 {
 	// 	v = makeMethodValue(context, v)
 	// }

 	switch {
 	case directlyAssignable(dst, v.typ):
 		// Overwrite type so that they match.
 		// Same memory layout, so no harm done.
 		fl := v.flag&(flagAddr|flagIndir) | v.flag.ro()
 		fl |= flag(dst.Kind())
 		return Value{dst, v.ptr, fl}

 	case implements(dst, v.typ):
 		if target == nil {
 			target = unsafe_New(dst)
 		}
 		if v.Kind() == Interface && v.IsNil() {
 			// A nil ReadWriter passed to nil Reader is OK,
 			// but using ifaceE2I below will panic.
 			// Avoid the panic by returning a nil dst (e.g., Reader) explicitly.
 			return Value{dst, nil, flag(Interface)}
 		}
 		x := valueInterface(v)
 		if dst.NumMethod() == 0 {
 			*(*interface{})(target) = x
 		} else {
 			ifaceE2I(dst, x, target)
 		}
 		return Value{dst, target, flagIndir | flag(Interface)}
 	}

 	// Failed.
 	panic(context + ": value of type " + v.typ.String() + " is not assignable to type " + dst.String())
 }

 // arrayAt returns the i-th element of p,
 // an array whose elements are eltSize bytes wide.
 // The array pointed at by p must have at least i+1 elements:
 // it is invalid (but impossible to check here) to pass i >= len,
 // because then the result will point outside the array.
 // whySafe must explain why i < len. (Passing "i < len" is fine;
 // the benefit is to surface this assumption at the call site.)
 func arrayAt(p unsafe.Pointer, i int, eltSize uintptr, whySafe string) unsafe.Pointer {
 	return add(p, uintptr(i)*eltSize, "i < len")
 }

 func ifaceE2I(t *rtype, src interface{}, dst unsafe.Pointer)

 // typedmemmove copies a value of type t to dst from src.
 //go:noescape
 func typedmemmove(t *rtype, dst, src unsafe.Pointer)

 // Dummy annotation marking that the value x escapes,
 // for use in cases where the reflect code is so clever that
 // the compiler cannot follow.
 func escapes(x interface{}) {
 	if dummy.b {
 		dummy.x = x
 	}
 }

 var dummy struct {
 	b bool
 	x interface{}
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package reflectlite

	import (
	"runtime"
	"unsafe"
	)

	const ptrSize = 4 << (^uintptr(0) >> 63) // unsafe.Sizeof(uintptr(0)) but an ideal const

	// Value is the reflection interface to a Go value.
	//
	// Not all methods apply to all kinds of values. Restrictions,
	// if any, are noted in the documentation for each method.
	// Use the Kind method to find out the kind of value before
	// calling kind-specific methods. Calling a method
	// inappropriate to the kind of type causes a run time panic.
	//
	// The zero Value represents no value.
	// Its IsValid method returns false, its Kind method returns Invalid,
	// its String method returns "<invalid Value>", and all other methods panic.
	// Most functions and methods never return an invalid value.
	// If one does, its documentation states the conditions explicitly.
	//
	// A Value can be used concurrently by multiple goroutines provided that
	// the underlying Go value can be used concurrently for the equivalent
	// direct operations.
	//
	// To compare two Values, compare the results of the Interface method.
	// Using == on two Values does not compare the underlying values
	// they represent.
	type Value struct {
	// typ holds the type of the value represented by a Value.
	typ *rtype

	// Pointer-valued data or, if flagIndir is set, pointer to data.
	// Valid when either flagIndir is set or typ.pointers() is true.
	ptr unsafe.Pointer

	// flag holds metadata about the value.
	// The lowest bits are flag bits:
	// - flagStickyRO: obtained via unexported not embedded field, so read-only
	// - flagEmbedRO: obtained via unexported embedded field, so read-only
	// - flagIndir: val holds a pointer to the data
	// - flagAddr: v.CanAddr is true (implies flagIndir)
	// Value cannot represent method values.
	// 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 ifaceIndir(typ), code can assume that flagIndir is set.
	flag

	// A method value represents a curried method invocation
	// like r.Read for some receiver r. The typ+val+flag bits describe
	// the receiver r, but the flag's Kind bits say Func (methods are
	// functions), and the top bits of the flag give the method number
	// in r's type's method table.
	}

	type flag uintptr

	const (
	flagKindWidth = 5 // there are 27 kinds
	flagKindMask flag = 1<<flagKindWidth - 1
	flagStickyRO flag = 1 << 5
	flagEmbedRO flag = 1 << 6
	flagIndir flag = 1 << 7
	flagAddr flag = 1 << 8
	flagMethod flag = 1 << 9
	flagMethodShift = 10
	flagRO flag = flagStickyRO \| flagEmbedRO
	)

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

	func (f flag) ro() flag {
	if f&flagRO != 0 {
	return flagStickyRO
	}
	return 0
	}

	// pointer returns the underlying pointer represented by v.
	// v.Kind() must be Ptr, Map, Chan, Func, or UnsafePointer
	func (v Value) pointer() unsafe.Pointer {
	if v.typ.size != ptrSize \|\| !v.typ.pointers() {
	panic("can't call pointer on a non-pointer Value")
	}
	if v.flag&flagIndir != 0 {
	return (unsafe.Pointer)(v.ptr)
	}
	return v.ptr
	}

	// packEface converts v to the empty interface.
	func packEface(v Value) interface{} {
	t := v.typ
	var i interface{}
	e := (*emptyInterface)(unsafe.Pointer(&i))
	// First, fill in the data portion of the interface.
	switch {
	case ifaceIndir(t):
	if v.flag&flagIndir == 0 {
	panic("bad indir")
	}
	// Value is indirect, and so is the interface we're making.
	ptr := v.ptr
	if v.flag&flagAddr != 0 {
	// TODO: pass safe boolean from valueInterface so
	// we don't need to copy if safe==true?
	c := unsafe_New(t)
	typedmemmove(t, c, ptr)
	ptr = c
	}
	e.word = ptr
	case v.flag&flagIndir != 0:
	// Value is indirect, but interface is direct. We need
	// to load the data at v.ptr into the interface data word.
	e.word = (unsafe.Pointer)(v.ptr)
	default:
	// Value is direct, and so is the interface.
	e.word = v.ptr
	}
	// Now, fill in the type portion. We're very careful here not
	// to have any operation between the e.word and e.typ assignments
	// that would let the garbage collector observe the partially-built
	// interface value.
	e.typ = t
	return i
	}

	// unpackEface converts the empty interface i to a Value.
	func unpackEface(i interface{}) Value {
	e := (*emptyInterface)(unsafe.Pointer(&i))
	// NOTE: don't read e.word until we know whether it is really a pointer or not.
	t := e.typ
	if t == nil {
	return Value{}
	}
	f := flag(t.Kind())
	if ifaceIndir(t) {
	f \|= flagIndir
	}
	return Value{t, e.word, f}
	}

	// A ValueError occurs when a Value method is invoked on
	// a Value that does not support it. Such cases are documented
	// in the description of each method.
	type ValueError struct {
	Method string
	Kind Kind
	}

	func (e *ValueError) Error() string {
	return "reflect: call of " + e.Method + " on zero Value"
	}

	// methodName returns the name of the calling method,
	// assumed to be two stack frames above.
	func methodName() string {
	pc, _, _, _ := runtime.Caller(2)
	f := runtime.FuncForPC(pc)
	if f == nil {
	return "unknown method"
	}
	return f.Name()
	}

	// emptyInterface is the header for an interface{} value.
	type emptyInterface struct {
	typ *rtype
	word unsafe.Pointer
	}

	// mustBeExported panics if f records that the value was obtained using
	// an unexported field.
	func (f flag) mustBeExported() {
	if f == 0 {
	panic(&ValueError{methodName(), 0})
	}
	if f&flagRO != 0 {
	panic("reflect: " + methodName() + " using value obtained using unexported field")
	}
	}

	// mustBeAssignable panics if f records that the value is not assignable,
	// which is to say that either it was obtained using an unexported field
	// or it is not addressable.
	func (f flag) mustBeAssignable() {
	if f == 0 {
	panic(&ValueError{methodName(), Invalid})
	}
	// Assignable if addressable and not read-only.
	if f&flagRO != 0 {
	panic("reflect: " + methodName() + " using value obtained using unexported field")
	}
	if f&flagAddr == 0 {
	panic("reflect: " + methodName() + " using unaddressable value")
	}
	}

	// CanSet reports whether the value of v can be changed.
	// A Value can be changed only if it is addressable and was not
	// obtained by the use of unexported struct fields.
	// If CanSet returns false, calling Set or any type-specific
	// setter (e.g., SetBool, SetInt) will panic.
	func (v Value) CanSet() bool {
	return v.flag&(flagAddr\|flagRO) == flagAddr
	}

	// Elem returns the value that the interface v contains
	// or that the pointer v points to.
	// It panics if v's Kind is not Interface or Ptr.
	// It returns the zero Value if v is nil.
	func (v Value) Elem() Value {
	k := v.kind()
	switch k {
	case Interface:
	var eface interface{}
	if v.typ.NumMethod() == 0 {
	eface = (interface{})(v.ptr)
	} else {
	eface = (interface{})((interface {
	M()
	})(v.ptr))
	}
	x := unpackEface(eface)
	if x.flag != 0 {
	x.flag \|= v.flag.ro()
	}
	return x
	case Ptr:
	ptr := v.ptr
	if v.flag&flagIndir != 0 {
	ptr = (unsafe.Pointer)(ptr)
	}
	// The returned value's address is v's value.
	if ptr == nil {
	return Value{}
	}
	tt := (*ptrType)(unsafe.Pointer(v.typ))
	typ := tt.elem
	fl := v.flag&flagRO \| flagIndir \| flagAddr
	fl \|= flag(typ.Kind())
	return Value{typ, ptr, fl}
	}
	panic(&ValueError{"reflectlite.Value.Elem", v.kind()})
	}

	func valueInterface(v Value) interface{} {
	if v.flag == 0 {
	panic(&ValueError{"reflectlite.Value.Interface", 0})
	}

	if v.kind() == Interface {
	// Special case: return the element inside the interface.
	// Empty interface has one layout, all interfaces with
	// methods have a second layout.
	if v.numMethod() == 0 {
	return (interface{})(v.ptr)
	}
	return (interface {
	M()
	})(v.ptr)
	}

	// TODO: pass safe to packEface so we don't need to copy if safe==true?
	return packEface(v)
	}

	// IsNil reports whether its argument v is nil. The argument must be
	// a chan, func, interface, map, pointer, or slice value; if it is
	// not, IsNil panics. Note that IsNil is not always equivalent to a
	// regular comparison with nil in Go. For example, if v was created
	// by calling ValueOf with an uninitialized interface variable i,
	// i==nil will be true but v.IsNil will panic as v will be the zero
	// Value.
	func (v Value) IsNil() bool {
	k := v.kind()
	switch k {
	case Chan, Func, Map, Ptr, UnsafePointer:
	// if v.flag&flagMethod != 0 {
	// return false
	// }
	ptr := v.ptr
	if v.flag&flagIndir != 0 {
	ptr = (unsafe.Pointer)(ptr)
	}
	return ptr == nil
	case Interface, Slice:
	// Both interface and slice are nil if first word is 0.
	// Both are always bigger than a word; assume flagIndir.
	return (unsafe.Pointer)(v.ptr) == nil
	}
	panic(&ValueError{"reflectlite.Value.IsNil", v.kind()})
	}

	// IsValid reports whether v represents a value.
	// It returns false if v is the zero Value.
	// If IsValid returns false, all other methods except String panic.
	// Most functions and methods never return an invalid Value.
	// If one does, its documentation states the conditions explicitly.
	func (v Value) IsValid() bool {
	return v.flag != 0
	}

	// Kind returns v's Kind.
	// If v is the zero Value (IsValid returns false), Kind returns Invalid.
	func (v Value) Kind() Kind {
	return v.kind()
	}

	// implemented in runtime:
	func chanlen(unsafe.Pointer) int
	func maplen(unsafe.Pointer) int

	// Len returns v's length.
	// It panics if v's Kind is not Array, Chan, Map, Slice, or String.
	func (v Value) Len() int {
	k := v.kind()
	switch k {
	case Array:
	tt := (*arrayType)(unsafe.Pointer(v.typ))
	return int(tt.len)
	case Chan:
	return chanlen(v.pointer())
	case Map:
	return maplen(v.pointer())
	case Slice:
	// Slice is bigger than a word; assume flagIndir.
	return (*sliceHeader)(v.ptr).Len
	case String:
	// String is bigger than a word; assume flagIndir.
	return (*stringHeader)(v.ptr).Len
	}
	panic(&ValueError{"reflect.Value.Len", v.kind()})
	}

	// NumMethod returns the number of exported methods in the value's method set.
	func (v Value) numMethod() int {
	if v.typ == nil {
	panic(&ValueError{"reflectlite.Value.NumMethod", Invalid})
	}
	return v.typ.NumMethod()
	}

	// Set assigns x to the value v.
	// It panics if CanSet returns false.
	// As in Go, x's value must be assignable to v's type.
	func (v Value) Set(x Value) {
	v.mustBeAssignable()
	x.mustBeExported() // do not let unexported x leak
	var target unsafe.Pointer
	if v.kind() == Interface {
	target = v.ptr
	}
	x = x.assignTo("reflectlite.Set", v.typ, target)
	if x.flag&flagIndir != 0 {
	typedmemmove(v.typ, v.ptr, x.ptr)
	} else {
	(unsafe.Pointer)(v.ptr) = x.ptr
	}
	}

	// Type returns v's type.
	func (v Value) Type() Type {
	f := v.flag
	if f == 0 {
	panic(&ValueError{"reflectlite.Value.Type", Invalid})
	}
	// Method values not supported.
	return v.typ
	}

	// stringHeader is a safe version of StringHeader used within this package.
	type stringHeader struct {
	Data unsafe.Pointer
	Len int
	}

	// sliceHeader is a safe version of SliceHeader used within this package.
	type sliceHeader struct {
	Data unsafe.Pointer
	Len int
	Cap int
	}

	/*
	* constructors
	*/

	// implemented in package runtime
	func unsafe_New(*rtype) unsafe.Pointer

	// 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 {
	if i == nil {
	return Value{}
	}

	// TODO: Maybe allow contents of a Value to live on the stack.
	// For now we make the contents always escape to the heap. It
	// makes life easier in a few places (see chanrecv/mapassign
	// comment below).
	escapes(i)

	return unpackEface(i)
	}

	// assignTo returns a value v that can be assigned directly to typ.
	// It panics if v is not assignable to typ.
	// For a conversion to an interface type, target is a suggested scratch space to use.
	func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value {
	// if v.flag&flagMethod != 0 {
	// v = makeMethodValue(context, v)
	// }

	switch {
	case directlyAssignable(dst, v.typ):
	// Overwrite type so that they match.
	// Same memory layout, so no harm done.
	fl := v.flag&(flagAddr\|flagIndir) \| v.flag.ro()
	fl \|= flag(dst.Kind())
	return Value{dst, v.ptr, fl}

	case implements(dst, v.typ):
	if target == nil {
	target = unsafe_New(dst)
	}
	if v.Kind() == Interface && v.IsNil() {
	// A nil ReadWriter passed to nil Reader is OK,
	// but using ifaceE2I below will panic.
	// Avoid the panic by returning a nil dst (e.g., Reader) explicitly.
	return Value{dst, nil, flag(Interface)}
	}
	x := valueInterface(v)
	if dst.NumMethod() == 0 {
	(interface{})(target) = x
	} else {
	ifaceE2I(dst, x, target)
	}
	return Value{dst, target, flagIndir \| flag(Interface)}
	}

	// Failed.
	panic(context + ": value of type " + v.typ.String() + " is not assignable to type " + dst.String())
	}

	// arrayAt returns the i-th element of p,
	// an array whose elements are eltSize bytes wide.
	// The array pointed at by p must have at least i+1 elements:
	// it is invalid (but impossible to check here) to pass i >= len,
	// because then the result will point outside the array.
	// whySafe must explain why i < len. (Passing "i < len" is fine;
	// the benefit is to surface this assumption at the call site.)
	func arrayAt(p unsafe.Pointer, i int, eltSize uintptr, whySafe string) unsafe.Pointer {
	return add(p, uintptr(i)*eltSize, "i < len")
	}

	func ifaceE2I(t *rtype, src interface{}, dst unsafe.Pointer)

	// typedmemmove copies a value of type t to dst from src.
	//go:noescape
	func typedmemmove(t *rtype, dst, src unsafe.Pointer)

	// Dummy annotation marking that the value x escapes,
	// for use in cases where the reflect code is so clever that
	// the compiler cannot follow.
	func escapes(x interface{}) {
	if dummy.b {
	dummy.x = x
	}
	}

	var dummy struct {
	b bool
	x interface{}
	}