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Source file src/reflect/value.go

Documentation: reflect

  // 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 reflect
  
  import (
  	"math"
  	"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)
  	//	- 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 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)
  }
  
  // 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 {
  	if e.Kind == 0 {
  		return "reflect: call of " + e.Method + " on zero Value"
  	}
  	return "reflect: call of " + e.Method + " on " + e.Kind.String() + " 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
  }
  
  // nonEmptyInterface is the header for a interface value with methods.
  type nonEmptyInterface struct {
  	// see ../runtime/iface.go:/Itab
  	itab *struct {
  		ityp   *rtype // static interface type
  		typ    *rtype // dynamic concrete type
  		link   unsafe.Pointer
  		bad    int32
  		unused int32
  		fun    [100000]unsafe.Pointer // method table
  	}
  	word unsafe.Pointer
  }
  
  // mustBe panics if f's kind is not expected.
  // Making this a method on flag instead of on Value
  // (and embedding flag in Value) means that we can write
  // the very clear v.mustBe(Bool) and have it compile into
  // v.flag.mustBe(Bool), which will only bother to copy the
  // single important word for the receiver.
  func (f flag) mustBe(expected Kind) {
  	if f.kind() != expected {
  		panic(&ValueError{methodName(), f.kind()})
  	}
  }
  
  // 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")
  	}
  }
  
  // Addr returns a pointer value representing the address of v.
  // It panics if CanAddr() returns false.
  // Addr is typically used to obtain a pointer to a struct field
  // or slice element in order to call a method that requires a
  // pointer receiver.
  func (v Value) Addr() Value {
  	if v.flag&flagAddr == 0 {
  		panic("reflect.Value.Addr of unaddressable value")
  	}
  	return Value{v.typ.ptrTo(), v.ptr, (v.flag & flagRO) | flag(Ptr)}
  }
  
  // Bool returns v's underlying value.
  // It panics if v's kind is not Bool.
  func (v Value) Bool() bool {
  	v.mustBe(Bool)
  	return *(*bool)(v.ptr)
  }
  
  // Bytes returns v's underlying value.
  // It panics if v's underlying value is not a slice of bytes.
  func (v Value) Bytes() []byte {
  	v.mustBe(Slice)
  	if v.typ.Elem().Kind() != Uint8 {
  		panic("reflect.Value.Bytes of non-byte slice")
  	}
  	// Slice is always bigger than a word; assume flagIndir.
  	return *(*[]byte)(v.ptr)
  }
  
  // runes returns v's underlying value.
  // It panics if v's underlying value is not a slice of runes (int32s).
  func (v Value) runes() []rune {
  	v.mustBe(Slice)
  	if v.typ.Elem().Kind() != Int32 {
  		panic("reflect.Value.Bytes of non-rune slice")
  	}
  	// Slice is always bigger than a word; assume flagIndir.
  	return *(*[]rune)(v.ptr)
  }
  
  // CanAddr reports whether the value's address can be obtained with Addr.
  // Such values are called addressable. A value is addressable if it is
  // an element of a slice, an element of an addressable array,
  // a field of an addressable struct, or the result of dereferencing a pointer.
  // If CanAddr returns false, calling Addr will panic.
  func (v Value) CanAddr() bool {
  	return v.flag&flagAddr != 0
  }
  
  // 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
  }
  
  // Call calls the function v with the input arguments in.
  // For example, if len(in) == 3, v.Call(in) represents the Go call v(in[0], in[1], in[2]).
  // Call panics if v's Kind is not Func.
  // It returns the output results as Values.
  // As in Go, each input argument must be assignable to the
  // type of the function's corresponding input parameter.
  // If v is a variadic function, Call creates the variadic slice parameter
  // itself, copying in the corresponding values.
  func (v Value) Call(in []Value) []Value {
  	v.mustBe(Func)
  	v.mustBeExported()
  	return v.call("Call", in)
  }
  
  // CallSlice calls the variadic function v with the input arguments in,
  // assigning the slice in[len(in)-1] to v's final variadic argument.
  // For example, if len(in) == 3, v.CallSlice(in) represents the Go call v(in[0], in[1], in[2]...).
  // CallSlice panics if v's Kind is not Func or if v is not variadic.
  // It returns the output results as Values.
  // As in Go, each input argument must be assignable to the
  // type of the function's corresponding input parameter.
  func (v Value) CallSlice(in []Value) []Value {
  	v.mustBe(Func)
  	v.mustBeExported()
  	return v.call("CallSlice", in)
  }
  
  var callGC bool // for testing; see TestCallMethodJump
  
  func (v Value) call(op string, in []Value) []Value {
  	// Get function pointer, type.
  	t := v.typ
  	var (
  		fn       unsafe.Pointer
  		rcvr     Value
  		rcvrtype *rtype
  	)
  	if v.flag&flagMethod != 0 {
  		rcvr = v
  		rcvrtype, t, fn = methodReceiver(op, v, int(v.flag)>>flagMethodShift)
  	} else if v.flag&flagIndir != 0 {
  		fn = *(*unsafe.Pointer)(v.ptr)
  	} else {
  		fn = v.ptr
  	}
  
  	if fn == nil {
  		panic("reflect.Value.Call: call of nil function")
  	}
  
  	isSlice := op == "CallSlice"
  	n := t.NumIn()
  	if isSlice {
  		if !t.IsVariadic() {
  			panic("reflect: CallSlice of non-variadic function")
  		}
  		if len(in) < n {
  			panic("reflect: CallSlice with too few input arguments")
  		}
  		if len(in) > n {
  			panic("reflect: CallSlice with too many input arguments")
  		}
  	} else {
  		if t.IsVariadic() {
  			n--
  		}
  		if len(in) < n {
  			panic("reflect: Call with too few input arguments")
  		}
  		if !t.IsVariadic() && len(in) > n {
  			panic("reflect: Call with too many input arguments")
  		}
  	}
  	for _, x := range in {
  		if x.Kind() == Invalid {
  			panic("reflect: " + op + " using zero Value argument")
  		}
  	}
  	for i := 0; i < n; i++ {
  		if xt, targ := in[i].Type(), t.In(i); !xt.AssignableTo(targ) {
  			panic("reflect: " + op + " using " + xt.String() + " as type " + targ.String())
  		}
  	}
  	if !isSlice && t.IsVariadic() {
  		// prepare slice for remaining values
  		m := len(in) - n
  		slice := MakeSlice(t.In(n), m, m)
  		elem := t.In(n).Elem()
  		for i := 0; i < m; i++ {
  			x := in[n+i]
  			if xt := x.Type(); !xt.AssignableTo(elem) {
  				panic("reflect: cannot use " + xt.String() + " as type " + elem.String() + " in " + op)
  			}
  			slice.Index(i).Set(x)
  		}
  		origIn := in
  		in = make([]Value, n+1)
  		copy(in[:n], origIn)
  		in[n] = slice
  	}
  
  	nin := len(in)
  	if nin != t.NumIn() {
  		panic("reflect.Value.Call: wrong argument count")
  	}
  	nout := t.NumOut()
  
  	// Compute frame type.
  	frametype, _, retOffset, _, framePool := funcLayout(t, rcvrtype)
  
  	// Allocate a chunk of memory for frame.
  	var args unsafe.Pointer
  	if nout == 0 {
  		args = framePool.Get().(unsafe.Pointer)
  	} else {
  		// Can't use pool if the function has return values.
  		// We will leak pointer to args in ret, so its lifetime is not scoped.
  		args = unsafe_New(frametype)
  	}
  	off := uintptr(0)
  
  	// Copy inputs into args.
  	if rcvrtype != nil {
  		storeRcvr(rcvr, args)
  		off = ptrSize
  	}
  	for i, v := range in {
  		v.mustBeExported()
  		targ := t.In(i).(*rtype)
  		a := uintptr(targ.align)
  		off = (off + a - 1) &^ (a - 1)
  		n := targ.size
  		addr := unsafe.Pointer(uintptr(args) + off)
  		v = v.assignTo("reflect.Value.Call", targ, addr)
  		if v.flag&flagIndir != 0 {
  			typedmemmove(targ, addr, v.ptr)
  		} else {
  			*(*unsafe.Pointer)(addr) = v.ptr
  		}
  		off += n
  	}
  
  	// Call.
  	call(frametype, fn, args, uint32(frametype.size), uint32(retOffset))
  
  	// For testing; see TestCallMethodJump.
  	if callGC {
  		runtime.GC()
  	}
  
  	var ret []Value
  	if nout == 0 {
  		// This is untyped because the frame is really a
  		// stack, even though it's a heap object.
  		memclrNoHeapPointers(args, frametype.size)
  		framePool.Put(args)
  	} else {
  		// Zero the now unused input area of args,
  		// because the Values returned by this function contain pointers to the args object,
  		// and will thus keep the args object alive indefinitely.
  		memclrNoHeapPointers(args, retOffset)
  		// Wrap Values around return values in args.
  		ret = make([]Value, nout)
  		off = retOffset
  		for i := 0; i < nout; i++ {
  			tv := t.Out(i)
  			a := uintptr(tv.Align())
  			off = (off + a - 1) &^ (a - 1)
  			fl := flagIndir | flag(tv.Kind())
  			ret[i] = Value{tv.common(), unsafe.Pointer(uintptr(args) + off), fl}
  			off += tv.Size()
  		}
  	}
  
  	return ret
  }
  
  // callReflect is the call implementation used by a function
  // returned by MakeFunc. In many ways it is the opposite of the
  // method Value.call above. The method above converts a call using Values
  // into a call of a function with a concrete argument frame, while
  // callReflect converts a call of a function with a concrete argument
  // frame into a call using Values.
  // It is in this file so that it can be next to the call method above.
  // The remainder of the MakeFunc implementation is in makefunc.go.
  //
  // NOTE: This function must be marked as a "wrapper" in the generated code,
  // so that the linker can make it work correctly for panic and recover.
  // The gc compilers know to do that for the name "reflect.callReflect".
  func callReflect(ctxt *makeFuncImpl, frame unsafe.Pointer) {
  	ftyp := ctxt.typ
  	f := ctxt.fn
  
  	// Copy argument frame into Values.
  	ptr := frame
  	off := uintptr(0)
  	in := make([]Value, 0, int(ftyp.inCount))
  	for _, typ := range ftyp.in() {
  		off += -off & uintptr(typ.align-1)
  		addr := unsafe.Pointer(uintptr(ptr) + off)
  		v := Value{typ, nil, flag(typ.Kind())}
  		if ifaceIndir(typ) {
  			// value cannot be inlined in interface data.
  			// Must make a copy, because f might keep a reference to it,
  			// and we cannot let f keep a reference to the stack frame
  			// after this function returns, not even a read-only reference.
  			v.ptr = unsafe_New(typ)
  			typedmemmove(typ, v.ptr, addr)
  			v.flag |= flagIndir
  		} else {
  			v.ptr = *(*unsafe.Pointer)(addr)
  		}
  		in = append(in, v)
  		off += typ.size
  	}
  
  	// Call underlying function.
  	out := f(in)
  	numOut := ftyp.NumOut()
  	if len(out) != numOut {
  		panic("reflect: wrong return count from function created by MakeFunc")
  	}
  
  	// Copy results back into argument frame.
  	if numOut > 0 {
  		off += -off & (ptrSize - 1)
  		if runtime.GOARCH == "amd64p32" {
  			off = align(off, 8)
  		}
  		for i, typ := range ftyp.out() {
  			v := out[i]
  			if v.typ != typ {
  				panic("reflect: function created by MakeFunc using " + funcName(f) +
  					" returned wrong type: have " +
  					out[i].typ.String() + " for " + typ.String())
  			}
  			if v.flag&flagRO != 0 {
  				panic("reflect: function created by MakeFunc using " + funcName(f) +
  					" returned value obtained from unexported field")
  			}
  			off += -off & uintptr(typ.align-1)
  			addr := unsafe.Pointer(uintptr(ptr) + off)
  			if v.flag&flagIndir != 0 {
  				typedmemmove(typ, addr, v.ptr)
  			} else {
  				*(*unsafe.Pointer)(addr) = v.ptr
  			}
  			off += typ.size
  		}
  	}
  
  	// runtime.getArgInfo expects to be able to find ctxt on the
  	// stack when it finds our caller, makeFuncStub. Make sure it
  	// doesn't get garbage collected.
  	runtime.KeepAlive(ctxt)
  }
  
  // methodReceiver returns information about the receiver
  // described by v. The Value v may or may not have the
  // flagMethod bit set, so the kind cached in v.flag should
  // not be used.
  // The return value rcvrtype gives the method's actual receiver type.
  // The return value t gives the method type signature (without the receiver).
  // The return value fn is a pointer to the method code.
  func methodReceiver(op string, v Value, methodIndex int) (rcvrtype, t *rtype, fn unsafe.Pointer) {
  	i := methodIndex
  	if v.typ.Kind() == Interface {
  		tt := (*interfaceType)(unsafe.Pointer(v.typ))
  		if uint(i) >= uint(len(tt.methods)) {
  			panic("reflect: internal error: invalid method index")
  		}
  		m := &tt.methods[i]
  		if !tt.nameOff(m.name).isExported() {
  			panic("reflect: " + op + " of unexported method")
  		}
  		iface := (*nonEmptyInterface)(v.ptr)
  		if iface.itab == nil {
  			panic("reflect: " + op + " of method on nil interface value")
  		}
  		rcvrtype = iface.itab.typ
  		fn = unsafe.Pointer(&iface.itab.fun[i])
  		t = tt.typeOff(m.typ)
  	} else {
  		rcvrtype = v.typ
  		ut := v.typ.uncommon()
  		if ut == nil || uint(i) >= uint(ut.mcount) {
  			panic("reflect: internal error: invalid method index")
  		}
  		m := ut.methods()[i]
  		if !v.typ.nameOff(m.name).isExported() {
  			panic("reflect: " + op + " of unexported method")
  		}
  		ifn := v.typ.textOff(m.ifn)
  		fn = unsafe.Pointer(&ifn)
  		t = v.typ.typeOff(m.mtyp)
  	}
  	return
  }
  
  // v is a method receiver. Store at p the word which is used to
  // encode that receiver at the start of the argument list.
  // Reflect uses the "interface" calling convention for
  // methods, which always uses one word to record the receiver.
  func storeRcvr(v Value, p unsafe.Pointer) {
  	t := v.typ
  	if t.Kind() == Interface {
  		// the interface data word becomes the receiver word
  		iface := (*nonEmptyInterface)(v.ptr)
  		*(*unsafe.Pointer)(p) = iface.word
  	} else if v.flag&flagIndir != 0 && !ifaceIndir(t) {
  		*(*unsafe.Pointer)(p) = *(*unsafe.Pointer)(v.ptr)
  	} else {
  		*(*unsafe.Pointer)(p) = v.ptr
  	}
  }
  
  // align returns the result of rounding x up to a multiple of n.
  // n must be a power of two.
  func align(x, n uintptr) uintptr {
  	return (x + n - 1) &^ (n - 1)
  }
  
  // callMethod is the call implementation used by a function returned
  // by makeMethodValue (used by v.Method(i).Interface()).
  // It is a streamlined version of the usual reflect call: the caller has
  // already laid out the argument frame for us, so we don't have
  // to deal with individual Values for each argument.
  // It is in this file so that it can be next to the two similar functions above.
  // The remainder of the makeMethodValue implementation is in makefunc.go.
  //
  // NOTE: This function must be marked as a "wrapper" in the generated code,
  // so that the linker can make it work correctly for panic and recover.
  // The gc compilers know to do that for the name "reflect.callMethod".
  func callMethod(ctxt *methodValue, frame unsafe.Pointer) {
  	rcvr := ctxt.rcvr
  	rcvrtype, t, fn := methodReceiver("call", rcvr, ctxt.method)
  	frametype, argSize, retOffset, _, framePool := funcLayout(t, rcvrtype)
  
  	// Make a new frame that is one word bigger so we can store the receiver.
  	args := framePool.Get().(unsafe.Pointer)
  
  	// Copy in receiver and rest of args.
  	// Avoid constructing out-of-bounds pointers if there are no args.
  	storeRcvr(rcvr, args)
  	if argSize-ptrSize > 0 {
  		typedmemmovepartial(frametype, unsafe.Pointer(uintptr(args)+ptrSize), frame, ptrSize, argSize-ptrSize)
  	}
  
  	// Call.
  	call(frametype, fn, args, uint32(frametype.size), uint32(retOffset))
  
  	// Copy return values. On amd64p32, the beginning of return values
  	// is 64-bit aligned, so the caller's frame layout (which doesn't have
  	// a receiver) is different from the layout of the fn call, which has
  	// a receiver.
  	// Ignore any changes to args and just copy return values.
  	// Avoid constructing out-of-bounds pointers if there are no return values.
  	if frametype.size-retOffset > 0 {
  		callerRetOffset := retOffset - ptrSize
  		if runtime.GOARCH == "amd64p32" {
  			callerRetOffset = align(argSize-ptrSize, 8)
  		}
  		typedmemmovepartial(frametype,
  			unsafe.Pointer(uintptr(frame)+callerRetOffset),
  			unsafe.Pointer(uintptr(args)+retOffset),
  			retOffset,
  			frametype.size-retOffset)
  	}
  
  	// This is untyped because the frame is really a stack, even
  	// though it's a heap object.
  	memclrNoHeapPointers(args, frametype.size)
  	framePool.Put(args)
  
  	// See the comment in callReflect.
  	runtime.KeepAlive(ctxt)
  }
  
  // funcName returns the name of f, for use in error messages.
  func funcName(f func([]Value) []Value) string {
  	pc := *(*uintptr)(unsafe.Pointer(&f))
  	rf := runtime.FuncForPC(pc)
  	if rf != nil {
  		return rf.Name()
  	}
  	return "closure"
  }
  
  // Cap returns v's capacity.
  // It panics if v's Kind is not Array, Chan, or Slice.
  func (v Value) Cap() int {
  	k := v.kind()
  	switch k {
  	case Array:
  		return v.typ.Len()
  	case Chan:
  		return chancap(v.pointer())
  	case Slice:
  		// Slice is always bigger than a word; assume flagIndir.
  		return (*sliceHeader)(v.ptr).Cap
  	}
  	panic(&ValueError{"reflect.Value.Cap", v.kind()})
  }
  
  // Close closes the channel v.
  // It panics if v's Kind is not Chan.
  func (v Value) Close() {
  	v.mustBe(Chan)
  	v.mustBeExported()
  	chanclose(v.pointer())
  }
  
  // Complex returns v's underlying value, as a complex128.
  // It panics if v's Kind is not Complex64 or Complex128
  func (v Value) Complex() complex128 {
  	k := v.kind()
  	switch k {
  	case Complex64:
  		return complex128(*(*complex64)(v.ptr))
  	case Complex128:
  		return *(*complex128)(v.ptr)
  	}
  	panic(&ValueError{"reflect.Value.Complex", v.kind()})
  }
  
  // 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 & flagRO
  		}
  		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{"reflect.Value.Elem", v.kind()})
  }
  
  // Field returns the i'th field of the struct v.
  // It panics if v's Kind is not Struct or i is out of range.
  func (v Value) Field(i int) Value {
  	if v.kind() != Struct {
  		panic(&ValueError{"reflect.Value.Field", v.kind()})
  	}
  	tt := (*structType)(unsafe.Pointer(v.typ))
  	if uint(i) >= uint(len(tt.fields)) {
  		panic("reflect: Field index out of range")
  	}
  	field := &tt.fields[i]
  	typ := field.typ
  
  	// Inherit permission bits from v, but clear flagEmbedRO.
  	fl := v.flag&(flagStickyRO|flagIndir|flagAddr) | flag(typ.Kind())
  	// Using an unexported field forces flagRO.
  	if !field.name.isExported() {
  		if field.anon() {
  			fl |= flagEmbedRO
  		} else {
  			fl |= flagStickyRO
  		}
  	}
  	// Either flagIndir is set and v.ptr points at struct,
  	// or flagIndir is not set and v.ptr is the actual struct data.
  	// In the former case, we want v.ptr + offset.
  	// In the latter case, we must have field.offset = 0,
  	// so v.ptr + field.offset is still okay.
  	ptr := unsafe.Pointer(uintptr(v.ptr) + field.offset())
  	return Value{typ, ptr, fl}
  }
  
  // FieldByIndex returns the nested field corresponding to index.
  // It panics if v's Kind is not struct.
  func (v Value) FieldByIndex(index []int) Value {
  	if len(index) == 1 {
  		return v.Field(index[0])
  	}
  	v.mustBe(Struct)
  	for i, x := range index {
  		if i > 0 {
  			if v.Kind() == Ptr && v.typ.Elem().Kind() == Struct {
  				if v.IsNil() {
  					panic("reflect: indirection through nil pointer to embedded struct")
  				}
  				v = v.Elem()
  			}
  		}
  		v = v.Field(x)
  	}
  	return v
  }
  
  // FieldByName returns the struct field with the given name.
  // It returns the zero Value if no field was found.
  // It panics if v's Kind is not struct.
  func (v Value) FieldByName(name string) Value {
  	v.mustBe(Struct)
  	if f, ok := v.typ.FieldByName(name); ok {
  		return v.FieldByIndex(f.Index)
  	}
  	return Value{}
  }
  
  // FieldByNameFunc returns the struct field with a name
  // that satisfies the match function.
  // It panics if v's Kind is not struct.
  // It returns the zero Value if no field was found.
  func (v Value) FieldByNameFunc(match func(string) bool) Value {
  	if f, ok := v.typ.FieldByNameFunc(match); ok {
  		return v.FieldByIndex(f.Index)
  	}
  	return Value{}
  }
  
  // Float returns v's underlying value, as a float64.
  // It panics if v's Kind is not Float32 or Float64
  func (v Value) Float() float64 {
  	k := v.kind()
  	switch k {
  	case Float32:
  		return float64(*(*float32)(v.ptr))
  	case Float64:
  		return *(*float64)(v.ptr)
  	}
  	panic(&ValueError{"reflect.Value.Float", v.kind()})
  }
  
  var uint8Type = TypeOf(uint8(0)).(*rtype)
  
  // Index returns v's i'th element.
  // It panics if v's Kind is not Array, Slice, or String or i is out of range.
  func (v Value) Index(i int) Value {
  	switch v.kind() {
  	case Array:
  		tt := (*arrayType)(unsafe.Pointer(v.typ))
  		if uint(i) >= uint(tt.len) {
  			panic("reflect: array index out of range")
  		}
  		typ := tt.elem
  		offset := uintptr(i) * typ.size
  
  		// Either flagIndir is set and v.ptr points at array,
  		// or flagIndir is not set and v.ptr is the actual array data.
  		// In the former case, we want v.ptr + offset.
  		// In the latter case, we must be doing Index(0), so offset = 0,
  		// so v.ptr + offset is still okay.
  		val := unsafe.Pointer(uintptr(v.ptr) + offset)
  		fl := v.flag&(flagRO|flagIndir|flagAddr) | flag(typ.Kind()) // bits same as overall array
  		return Value{typ, val, fl}
  
  	case Slice:
  		// Element flag same as Elem of Ptr.
  		// Addressable, indirect, possibly read-only.
  		s := (*sliceHeader)(v.ptr)
  		if uint(i) >= uint(s.Len) {
  			panic("reflect: slice index out of range")
  		}
  		tt := (*sliceType)(unsafe.Pointer(v.typ))
  		typ := tt.elem
  		val := arrayAt(s.Data, i, typ.size)
  		fl := flagAddr | flagIndir | v.flag&flagRO | flag(typ.Kind())
  		return Value{typ, val, fl}
  
  	case String:
  		s := (*stringHeader)(v.ptr)
  		if uint(i) >= uint(s.Len) {
  			panic("reflect: string index out of range")
  		}
  		p := arrayAt(s.Data, i, 1)
  		fl := v.flag&flagRO | flag(Uint8) | flagIndir
  		return Value{uint8Type, p, fl}
  	}
  	panic(&ValueError{"reflect.Value.Index", v.kind()})
  }
  
  // Int returns v's underlying value, as an int64.
  // It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64.
  func (v Value) Int() int64 {
  	k := v.kind()
  	p := v.ptr
  	switch k {
  	case Int:
  		return int64(*(*int)(p))
  	case Int8:
  		return int64(*(*int8)(p))
  	case Int16:
  		return int64(*(*int16)(p))
  	case Int32:
  		return int64(*(*int32)(p))
  	case Int64:
  		return *(*int64)(p)
  	}
  	panic(&ValueError{"reflect.Value.Int", v.kind()})
  }
  
  // CanInterface reports whether Interface can be used without panicking.
  func (v Value) CanInterface() bool {
  	if v.flag == 0 {
  		panic(&ValueError{"reflect.Value.CanInterface", Invalid})
  	}
  	return v.flag&flagRO == 0
  }
  
  // Interface returns v's current value as an interface{}.
  // It is equivalent to:
  //	var i interface{} = (v's underlying value)
  // It panics if the Value was obtained by accessing
  // unexported struct fields.
  func (v Value) Interface() (i interface{}) {
  	return valueInterface(v, true)
  }
  
  func valueInterface(v Value, safe bool) interface{} {
  	if v.flag == 0 {
  		panic(&ValueError{"reflect.Value.Interface", 0})
  	}
  	if safe && v.flag&flagRO != 0 {
  		// Do not allow access to unexported values via Interface,
  		// because they might be pointers that should not be
  		// writable or methods or function that should not be callable.
  		panic("reflect.Value.Interface: cannot return value obtained from unexported field or method")
  	}
  	if v.flag&flagMethod != 0 {
  		v = makeMethodValue("Interface", v)
  	}
  
  	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)
  }
  
  // InterfaceData returns the interface v's value as a uintptr pair.
  // It panics if v's Kind is not Interface.
  func (v Value) InterfaceData() [2]uintptr {
  	// TODO: deprecate this
  	v.mustBe(Interface)
  	// We treat this as a read operation, so we allow
  	// it even for unexported data, because the caller
  	// has to import "unsafe" to turn it into something
  	// that can be abused.
  	// Interface value is always bigger than a word; assume flagIndir.
  	return *(*[2]uintptr)(v.ptr)
  }
  
  // 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:
  		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{"reflect.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()
  }
  
  // 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()})
  }
  
  // MapIndex returns the value associated with key in the map v.
  // It panics if v's Kind is not Map.
  // It returns the zero Value if key is not found in the map or if v represents a nil map.
  // As in Go, the key's value must be assignable to the map's key type.
  func (v Value) MapIndex(key Value) Value {
  	v.mustBe(Map)
  	tt := (*mapType)(unsafe.Pointer(v.typ))
  
  	// Do not require key to be exported, so that DeepEqual
  	// and other programs can use all the keys returned by
  	// MapKeys as arguments to MapIndex. If either the map
  	// or the key is unexported, though, the result will be
  	// considered unexported. This is consistent with the
  	// behavior for structs, which allow read but not write
  	// of unexported fields.
  	key = key.assignTo("reflect.Value.MapIndex", tt.key, nil)
  
  	var k unsafe.Pointer
  	if key.flag&flagIndir != 0 {
  		k = key.ptr
  	} else {
  		k = unsafe.Pointer(&key.ptr)
  	}
  	e := mapaccess(v.typ, v.pointer(), k)
  	if e == nil {
  		return Value{}
  	}
  	typ := tt.elem
  	fl := (v.flag | key.flag) & flagRO
  	fl |= flag(typ.Kind())
  	if ifaceIndir(typ) {
  		// Copy result so future changes to the map
  		// won't change the underlying value.
  		c := unsafe_New(typ)
  		typedmemmove(typ, c, e)
  		return Value{typ, c, fl | flagIndir}
  	} else {
  		return Value{typ, *(*unsafe.Pointer)(e), fl}
  	}
  }
  
  // MapKeys returns a slice containing all the keys present in the map,
  // in unspecified order.
  // It panics if v's Kind is not Map.
  // It returns an empty slice if v represents a nil map.
  func (v Value) MapKeys() []Value {
  	v.mustBe(Map)
  	tt := (*mapType)(unsafe.Pointer(v.typ))
  	keyType := tt.key
  
  	fl := v.flag&flagRO | flag(keyType.Kind())
  
  	m := v.pointer()
  	mlen := int(0)
  	if m != nil {
  		mlen = maplen(m)
  	}
  	it := mapiterinit(v.typ, m)
  	a := make([]Value, mlen)
  	var i int
  	for i = 0; i < len(a); i++ {
  		key := mapiterkey(it)
  		if key == nil {
  			// Someone deleted an entry from the map since we
  			// called maplen above. It's a data race, but nothing
  			// we can do about it.
  			break
  		}
  		if ifaceIndir(keyType) {
  			// Copy result so future changes to the map
  			// won't change the underlying value.
  			c := unsafe_New(keyType)
  			typedmemmove(keyType, c, key)
  			a[i] = Value{keyType, c, fl | flagIndir}
  		} else {
  			a[i] = Value{keyType, *(*unsafe.Pointer)(key), fl}
  		}
  		mapiternext(it)
  	}
  	return a[:i]
  }
  
  // Method returns a function value corresponding to v's i'th method.
  // The arguments to a Call on the returned function should not include
  // a receiver; the returned function will always use v as the receiver.
  // Method panics if i is out of range or if v is a nil interface value.
  func (v Value) Method(i int) Value {
  	if v.typ == nil {
  		panic(&ValueError{"reflect.Value.Method", Invalid})
  	}
  	if v.flag&flagMethod != 0 || uint(i) >= uint(v.typ.NumMethod()) {
  		panic("reflect: Method index out of range")
  	}
  	if v.typ.Kind() == Interface && v.IsNil() {
  		panic("reflect: Method on nil interface value")
  	}
  	fl := v.flag & (flagStickyRO | flagIndir) // Clear flagEmbedRO
  	fl |= flag(Func)
  	fl |= flag(i)<<flagMethodShift | flagMethod
  	return Value{v.typ, v.ptr, fl}
  }
  
  // NumMethod returns the number of exported methods in the value's method set.
  func (v Value) NumMethod() int {
  	if v.typ == nil {
  		panic(&ValueError{"reflect.Value.NumMethod", Invalid})
  	}
  	if v.flag&flagMethod != 0 {
  		return 0
  	}
  	return v.typ.NumMethod()
  }
  
  // MethodByName returns a function value corresponding to the method
  // of v with the given name.
  // The arguments to a Call on the returned function should not include
  // a receiver; the returned function will always use v as the receiver.
  // It returns the zero Value if no method was found.
  func (v Value) MethodByName(name string) Value {
  	if v.typ == nil {
  		panic(&ValueError{"reflect.Value.MethodByName", Invalid})
  	}
  	if v.flag&flagMethod != 0 {
  		return Value{}
  	}
  	m, ok := v.typ.MethodByName(name)
  	if !ok {
  		return Value{}
  	}
  	return v.Method(m.Index)
  }
  
  // NumField returns the number of fields in the struct v.
  // It panics if v's Kind is not Struct.
  func (v Value) NumField() int {
  	v.mustBe(Struct)
  	tt := (*structType)(unsafe.Pointer(v.typ))
  	return len(tt.fields)
  }
  
  // OverflowComplex reports whether the complex128 x cannot be represented by v's type.
  // It panics if v's Kind is not Complex64 or Complex128.
  func (v Value) OverflowComplex(x complex128) bool {
  	k := v.kind()
  	switch k {
  	case Complex64:
  		return overflowFloat32(real(x)) || overflowFloat32(imag(x))
  	case Complex128:
  		return false
  	}
  	panic(&ValueError{"reflect.Value.OverflowComplex", v.kind()})
  }
  
  // OverflowFloat reports whether the float64 x cannot be represented by v's type.
  // It panics if v's Kind is not Float32 or Float64.
  func (v Value) OverflowFloat(x float64) bool {
  	k := v.kind()
  	switch k {
  	case Float32:
  		return overflowFloat32(x)
  	case Float64:
  		return false
  	}
  	panic(&ValueError{"reflect.Value.OverflowFloat", v.kind()})
  }
  
  func overflowFloat32(x float64) bool {
  	if x < 0 {
  		x = -x
  	}
  	return math.MaxFloat32 < x && x <= math.MaxFloat64
  }
  
  // OverflowInt reports whether the int64 x cannot be represented by v's type.
  // It panics if v's Kind is not Int, Int8, int16, Int32, or Int64.
  func (v Value) OverflowInt(x int64) bool {
  	k := v.kind()
  	switch k {
  	case Int, Int8, Int16, Int32, Int64:
  		bitSize := v.typ.size * 8
  		trunc := (x << (64 - bitSize)) >> (64 - bitSize)
  		return x != trunc
  	}
  	panic(&ValueError{"reflect.Value.OverflowInt", v.kind()})
  }
  
  // OverflowUint reports whether the uint64 x cannot be represented by v's type.
  // It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
  func (v Value) OverflowUint(x uint64) bool {
  	k := v.kind()
  	switch k {
  	case Uint, Uintptr, Uint8, Uint16, Uint32, Uint64:
  		bitSize := v.typ.size * 8
  		trunc := (x << (64 - bitSize)) >> (64 - bitSize)
  		return x != trunc
  	}
  	panic(&ValueError{"reflect.Value.OverflowUint", v.kind()})
  }
  
  // Pointer returns v's value as a uintptr.
  // It returns uintptr instead of unsafe.Pointer so that
  // code using reflect cannot obtain unsafe.Pointers
  // without importing the unsafe package explicitly.
  // It panics if v's Kind is not Chan, Func, Map, Ptr, Slice, or UnsafePointer.
  //
  // If v's Kind is Func, the returned pointer is an underlying
  // code pointer, but not necessarily enough to identify a
  // single function uniquely. The only guarantee is that the
  // result is zero if and only if v is a nil func Value.
  //
  // If v's Kind is Slice, the returned pointer is to the first
  // element of the slice. If the slice is nil the returned value
  // is 0.  If the slice is empty but non-nil the return value is non-zero.
  func (v Value) Pointer() uintptr {
  	// TODO: deprecate
  	k := v.kind()
  	switch k {
  	case Chan, Map, Ptr, UnsafePointer:
  		return uintptr(v.pointer())
  	case Func:
  		if v.flag&flagMethod != 0 {
  			// As the doc comment says, the returned pointer is an
  			// underlying code pointer but not necessarily enough to
  			// identify a single function uniquely. All method expressions
  			// created via reflect have the same underlying code pointer,
  			// so their Pointers are equal. The function used here must
  			// match the one used in makeMethodValue.
  			f := methodValueCall
  			return **(**uintptr)(unsafe.Pointer(&f))
  		}
  		p := v.pointer()
  		// Non-nil func value points at data block.
  		// First word of data block is actual code.
  		if p != nil {
  			p = *(*unsafe.Pointer)(p)
  		}
  		return uintptr(p)
  
  	case Slice:
  		return (*SliceHeader)(v.ptr).Data
  	}
  	panic(&ValueError{"reflect.Value.Pointer", v.kind()})
  }
  
  // Recv receives and returns a value from the channel v.
  // It panics if v's Kind is not Chan.
  // The receive blocks until a value is ready.
  // The boolean value ok is true if the value x corresponds to a send
  // on the channel, false if it is a zero value received because the channel is closed.
  func (v Value) Recv() (x Value, ok bool) {
  	v.mustBe(Chan)
  	v.mustBeExported()
  	return v.recv(false)
  }
  
  // internal recv, possibly non-blocking (nb).
  // v is known to be a channel.
  func (v Value) recv(nb bool) (val Value, ok bool) {
  	tt := (*chanType)(unsafe.Pointer(v.typ))
  	if ChanDir(tt.dir)&RecvDir == 0 {
  		panic("reflect: recv on send-only channel")
  	}
  	t := tt.elem
  	val = Value{t, nil, flag(t.Kind())}
  	var p unsafe.Pointer
  	if ifaceIndir(t) {
  		p = unsafe_New(t)
  		val.ptr = p
  		val.flag |= flagIndir
  	} else {
  		p = unsafe.Pointer(&val.ptr)
  	}
  	selected, ok := chanrecv(v.pointer(), nb, p)
  	if !selected {
  		val = Value{}
  	}
  	return
  }
  
  // Send sends x on the channel v.
  // It panics if v's kind is not Chan or if x's type is not the same type as v's element type.
  // As in Go, x's value must be assignable to the channel's element type.
  func (v Value) Send(x Value) {
  	v.mustBe(Chan)
  	v.mustBeExported()
  	v.send(x, false)
  }
  
  // internal send, possibly non-blocking.
  // v is known to be a channel.
  func (v Value) send(x Value, nb bool) (selected bool) {
  	tt := (*chanType)(unsafe.Pointer(v.typ))
  	if ChanDir(tt.dir)&SendDir == 0 {
  		panic("reflect: send on recv-only channel")
  	}
  	x.mustBeExported()
  	x = x.assignTo("reflect.Value.Send", tt.elem, nil)
  	var p unsafe.Pointer
  	if x.flag&flagIndir != 0 {
  		p = x.ptr
  	} else {
  		p = unsafe.Pointer(&x.ptr)
  	}
  	return chansend(v.pointer(), p, nb)
  }
  
  // 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("reflect.Set", v.typ, target)
  	if x.flag&flagIndir != 0 {
  		typedmemmove(v.typ, v.ptr, x.ptr)
  	} else {
  		*(*unsafe.Pointer)(v.ptr) = x.ptr
  	}
  }
  
  // SetBool sets v's underlying value.
  // It panics if v's Kind is not Bool or if CanSet() is false.
  func (v Value) SetBool(x bool) {
  	v.mustBeAssignable()
  	v.mustBe(Bool)
  	*(*bool)(v.ptr) = x
  }
  
  // SetBytes sets v's underlying value.
  // It panics if v's underlying value is not a slice of bytes.
  func (v Value) SetBytes(x []byte) {
  	v.mustBeAssignable()
  	v.mustBe(Slice)
  	if v.typ.Elem().Kind() != Uint8 {
  		panic("reflect.Value.SetBytes of non-byte slice")
  	}
  	*(*[]byte)(v.ptr) = x
  }
  
  // setRunes sets v's underlying value.
  // It panics if v's underlying value is not a slice of runes (int32s).
  func (v Value) setRunes(x []rune) {
  	v.mustBeAssignable()
  	v.mustBe(Slice)
  	if v.typ.Elem().Kind() != Int32 {
  		panic("reflect.Value.setRunes of non-rune slice")
  	}
  	*(*[]rune)(v.ptr) = x
  }
  
  // SetComplex sets v's underlying value to x.
  // It panics if v's Kind is not Complex64 or Complex128, or if CanSet() is false.
  func (v Value) SetComplex(x complex128) {
  	v.mustBeAssignable()
  	switch k := v.kind(); k {
  	default:
  		panic(&ValueError{"reflect.Value.SetComplex", v.kind()})
  	case Complex64:
  		*(*complex64)(v.ptr) = complex64(x)
  	case Complex128:
  		*(*complex128)(v.ptr) = x
  	}
  }
  
  // SetFloat sets v's underlying value to x.
  // It panics if v's Kind is not Float32 or Float64, or if CanSet() is false.
  func (v Value) SetFloat(x float64) {
  	v.mustBeAssignable()
  	switch k := v.kind(); k {
  	default:
  		panic(&ValueError{"reflect.Value.SetFloat", v.kind()})
  	case Float32:
  		*(*float32)(v.ptr) = float32(x)
  	case Float64:
  		*(*float64)(v.ptr) = x
  	}
  }
  
  // SetInt sets v's underlying value to x.
  // It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64, or if CanSet() is false.
  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
  	}
  }
  
  // SetLen sets v's length to n.
  // It panics if v's Kind is not Slice or if n is negative or
  // greater than the capacity of the slice.
  func (v Value) SetLen(n int) {
  	v.mustBeAssignable()
  	v.mustBe(Slice)
  	s := (*sliceHeader)(v.ptr)
  	if uint(n) > uint(s.Cap) {
  		panic("reflect: slice length out of range in SetLen")
  	}
  	s.Len = n
  }
  
  // SetCap sets v's capacity to n.
  // It panics if v's Kind is not Slice or if n is smaller than the length or
  // greater than the capacity of the slice.
  func (v Value) SetCap(n int) {
  	v.mustBeAssignable()
  	v.mustBe(Slice)
  	s := (*sliceHeader)(v.ptr)
  	if n < s.Len || n > s.Cap {
  		panic("reflect: slice capacity out of range in SetCap")
  	}
  	s.Cap = n
  }
  
  // SetMapIndex sets the value associated with key in the map v to val.
  // It panics if v's Kind is not Map.
  // If val is the zero Value, SetMapIndex deletes the key from the map.
  // Otherwise if v holds a nil map, SetMapIndex will panic.
  // As in Go, key's value must be assignable to the map's key type,
  // and val's value must be assignable to the map's value type.
  func (v Value) SetMapIndex(key, val Value) {
  	v.mustBe(Map)
  	v.mustBeExported()
  	key.mustBeExported()
  	tt := (*mapType)(unsafe.Pointer(v.typ))
  	key = key.assignTo("reflect.Value.SetMapIndex", tt.key, nil)
  	var k unsafe.Pointer
  	if key.flag&flagIndir != 0 {
  		k = key.ptr
  	} else {
  		k = unsafe.Pointer(&key.ptr)
  	}
  	if val.typ == nil {
  		mapdelete(v.typ, v.pointer(), k)
  		return
  	}
  	val.mustBeExported()
  	val = val.assignTo("reflect.Value.SetMapIndex", tt.elem, nil)
  	var e unsafe.Pointer
  	if val.flag&flagIndir != 0 {
  		e = val.ptr
  	} else {
  		e = unsafe.Pointer(&val.ptr)
  	}
  	mapassign(v.typ, v.pointer(), k, e)
  }
  
  // SetUint sets v's underlying value to x.
  // It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64, or if CanSet() is false.
  func (v Value) SetUint(x uint64) {
  	v.mustBeAssignable()
  	switch k := v.kind(); k {
  	default:
  		panic(&ValueError{"reflect.Value.SetUint", v.kind()})
  	case Uint:
  		*(*uint)(v.ptr) = uint(x)
  	case Uint8:
  		*(*uint8)(v.ptr) = uint8(x)
  	case Uint16:
  		*(*uint16)(v.ptr) = uint16(x)
  	case Uint32:
  		*(*uint32)(v.ptr) = uint32(x)
  	case Uint64:
  		*(*uint64)(v.ptr) = x
  	case Uintptr:
  		*(*uintptr)(v.ptr) = uintptr(x)
  	}
  }
  
  // SetPointer sets the unsafe.Pointer value v to x.
  // It panics if v's Kind is not UnsafePointer.
  func (v Value) SetPointer(x unsafe.Pointer) {
  	v.mustBeAssignable()
  	v.mustBe(UnsafePointer)
  	*(*unsafe.Pointer)(v.ptr) = x
  }
  
  // SetString sets v's underlying value to x.
  // It panics if v's Kind is not String or if CanSet() is false.
  func (v Value) SetString(x string) {
  	v.mustBeAssignable()
  	v.mustBe(String)
  	*(*string)(v.ptr) = x
  }
  
  // Slice returns v[i:j].
  // It panics if v's Kind is not Array, Slice or String, or if v is an unaddressable array,
  // or if the indexes are out of bounds.
  func (v Value) Slice(i, j int) Value {
  	var (
  		cap  int
  		typ  *sliceType
  		base unsafe.Pointer
  	)
  	switch kind := v.kind(); kind {
  	default:
  		panic(&ValueError{"reflect.Value.Slice", v.kind()})
  
  	case Array:
  		if v.flag&flagAddr == 0 {
  			panic("reflect.Value.Slice: slice of unaddressable array")
  		}
  		tt := (*arrayType)(unsafe.Pointer(v.typ))
  		cap = int(tt.len)
  		typ = (*sliceType)(unsafe.Pointer(tt.slice))
  		base = v.ptr
  
  	case Slice:
  		typ = (*sliceType)(unsafe.Pointer(v.typ))
  		s := (*sliceHeader)(v.ptr)
  		base = s.Data
  		cap = s.Cap
  
  	case String:
  		s := (*stringHeader)(v.ptr)
  		if i < 0 || j < i || j > s.Len {
  			panic("reflect.Value.Slice: string slice index out of bounds")
  		}
  		t := stringHeader{arrayAt(s.Data, i, 1), j - i}
  		return Value{v.typ, unsafe.Pointer(&t), v.flag}
  	}
  
  	if i < 0 || j < i || j > cap {
  		panic("reflect.Value.Slice: slice index out of bounds")
  	}
  
  	// Declare slice so that gc can see the base pointer in it.
  	var x []unsafe.Pointer
  
  	// Reinterpret as *sliceHeader to edit.
  	s := (*sliceHeader)(unsafe.Pointer(&x))
  	s.Len = j - i
  	s.Cap = cap - i
  	if cap-i > 0 {
  		s.Data = arrayAt(base, i, typ.elem.Size())
  	} else {
  		// do not advance pointer, to avoid pointing beyond end of slice
  		s.Data = base
  	}
  
  	fl := v.flag&flagRO | flagIndir | flag(Slice)
  	return Value{typ.common(), unsafe.Pointer(&x), fl}
  }
  
  // Slice3 is the 3-index form of the slice operation: it returns v[i:j:k].
  // It panics if v's Kind is not Array or Slice, or if v is an unaddressable array,
  // or if the indexes are out of bounds.
  func (v Value) Slice3(i, j, k int) Value {
  	var (
  		cap  int
  		typ  *sliceType
  		base unsafe.Pointer
  	)
  	switch kind := v.kind(); kind {
  	default:
  		panic(&ValueError{"reflect.Value.Slice3", v.kind()})
  
  	case Array:
  		if v.flag&flagAddr == 0 {
  			panic("reflect.Value.Slice3: slice of unaddressable array")
  		}
  		tt := (*arrayType)(unsafe.Pointer(v.typ))
  		cap = int(tt.len)
  		typ = (*sliceType)(unsafe.Pointer(tt.slice))
  		base = v.ptr
  
  	case Slice:
  		typ = (*sliceType)(unsafe.Pointer(v.typ))
  		s := (*sliceHeader)(v.ptr)
  		base = s.Data
  		cap = s.Cap
  	}
  
  	if i < 0 || j < i || k < j || k > cap {
  		panic("reflect.Value.Slice3: slice index out of bounds")
  	}
  
  	// Declare slice so that the garbage collector
  	// can see the base pointer in it.
  	var x []unsafe.Pointer
  
  	// Reinterpret as *sliceHeader to edit.
  	s := (*sliceHeader)(unsafe.Pointer(&x))
  	s.Len = j - i
  	s.Cap = k - i
  	if k-i > 0 {
  		s.Data = arrayAt(base, i, typ.elem.Size())
  	} else {
  		// do not advance pointer, to avoid pointing beyond end of slice
  		s.Data = base
  	}
  
  	fl := v.flag&flagRO | flagIndir | flag(Slice)
  	return Value{typ.common(), unsafe.Pointer(&x), fl}
  }
  
  // String returns the string v's underlying value, as a string.
  // String is a special case because of Go's String method convention.
  // Unlike the other getters, it does not panic if v's Kind is not String.
  // Instead, it returns a string of the form "<T value>" where T is v's type.
  // The fmt package treats Values specially. It does not call their String
  // method implicitly but instead prints the concrete values they hold.
  func (v Value) String() string {
  	switch k := v.kind(); k {
  	case Invalid:
  		return "<invalid Value>"
  	case String:
  		return *(*string)(v.ptr)
  	}
  	// If you call String on a reflect.Value of other type, it's better to
  	// print something than to panic. Useful in debugging.
  	return "<" + v.Type().String() + " Value>"
  }
  
  // TryRecv attempts to receive a value from the channel v but will not block.
  // It panics if v's Kind is not Chan.
  // If the receive delivers a value, x is the transferred value and ok is true.
  // If the receive cannot finish without blocking, x is the zero Value and ok is false.
  // If the channel is closed, x is the zero value for the channel's element type and ok is false.
  func (v Value) TryRecv() (x Value, ok bool) {
  	v.mustBe(Chan)
  	v.mustBeExported()
  	return v.recv(true)
  }
  
  // TrySend attempts to send x on the channel v but will not block.
  // It panics if v's Kind is not Chan.
  // It reports whether the value was sent.
  // As in Go, x's value must be assignable to the channel's element type.
  func (v Value) TrySend(x Value) bool {
  	v.mustBe(Chan)
  	v.mustBeExported()
  	return v.send(x, true)
  }
  
  // Type returns v's type.
  func (v Value) Type() Type {
  	f := v.flag
  	if f == 0 {
  		panic(&ValueError{"reflect.Value.Type", Invalid})
  	}
  	if f&flagMethod == 0 {
  		// Easy case
  		return v.typ
  	}
  
  	// Method value.
  	// v.typ describes the receiver, not the method type.
  	i := int(v.flag) >> flagMethodShift
  	if v.typ.Kind() == Interface {
  		// Method on interface.
  		tt := (*interfaceType)(unsafe.Pointer(v.typ))
  		if uint(i) >= uint(len(tt.methods)) {
  			panic("reflect: internal error: invalid method index")
  		}
  		m := &tt.methods[i]
  		return v.typ.typeOff(m.typ)
  	}
  	// Method on concrete type.
  	ut := v.typ.uncommon()
  	if ut == nil || uint(i) >= uint(ut.mcount) {
  		panic("reflect: internal error: invalid method index")
  	}
  	m := ut.methods()[i]
  	return v.typ.typeOff(m.mtyp)
  }
  
  // Uint returns v's underlying value, as a uint64.
  // It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
  func (v Value) Uint() uint64 {
  	k := v.kind()
  	p := v.ptr
  	switch k {
  	case Uint:
  		return uint64(*(*uint)(p))
  	case Uint8:
  		return uint64(*(*uint8)(p))
  	case Uint16:
  		return uint64(*(*uint16)(p))
  	case Uint32:
  		return uint64(*(*uint32)(p))
  	case Uint64:
  		return *(*uint64)(p)
  	case Uintptr:
  		return uint64(*(*uintptr)(p))
  	}
  	panic(&ValueError{"reflect.Value.Uint", v.kind()})
  }
  
  // UnsafeAddr returns a pointer to v's data.
  // It is for advanced clients that also import the "unsafe" package.
  // It panics if v is not addressable.
  func (v Value) UnsafeAddr() uintptr {
  	// TODO: deprecate
  	if v.typ == nil {
  		panic(&ValueError{"reflect.Value.UnsafeAddr", Invalid})
  	}
  	if v.flag&flagAddr == 0 {
  		panic("reflect.Value.UnsafeAddr of unaddressable value")
  	}
  	return uintptr(v.ptr)
  }
  
  // StringHeader is the runtime representation of a string.
  // It cannot be used safely or portably and its representation may
  // change in a later release.
  // Moreover, the Data field is not sufficient to guarantee the data
  // it references will not be garbage collected, so programs must keep
  // a separate, correctly typed pointer to the underlying data.
  type StringHeader struct {
  	Data uintptr
  	Len  int
  }
  
  // stringHeader is a safe version of StringHeader used within this package.
  type stringHeader struct {
  	Data unsafe.Pointer
  	Len  int
  }
  
  // SliceHeader is the runtime representation of a slice.
  // It cannot be used safely or portably and its representation may
  // change in a later release.
  // Moreover, the Data field is not sufficient to guarantee the data
  // it references will not be garbage collected, so programs must keep
  // a separate, correctly typed pointer to the underlying data.
  type SliceHeader struct {
  	Data uintptr
  	Len  int
  	Cap  int
  }
  
  // sliceHeader is a safe version of SliceHeader used within this package.
  type sliceHeader struct {
  	Data unsafe.Pointer
  	Len  int
  	Cap  int
  }
  
  func typesMustMatch(what string, t1, t2 Type) {
  	if t1 != t2 {
  		panic(what + ": " + t1.String() + " != " + t2.String())
  	}
  }
  
  // arrayAt returns the i-th element of p, a C-array whose elements are
  // eltSize wide (in bytes).
  func arrayAt(p unsafe.Pointer, i int, eltSize uintptr) unsafe.Pointer {
  	return unsafe.Pointer(uintptr(p) + uintptr(i)*eltSize)
  }
  
  // grow grows the slice s so that it can hold extra more values, allocating
  // more capacity if needed. It also returns the old and new slice lengths.
  func grow(s Value, extra int) (Value, int, int) {
  	i0 := s.Len()
  	i1 := i0 + extra
  	if i1 < i0 {
  		panic("reflect.Append: slice overflow")
  	}
  	m := s.Cap()
  	if i1 <= m {
  		return s.Slice(0, i1), i0, i1
  	}
  	if m == 0 {
  		m = extra
  	} else {
  		for m < i1 {
  			if i0 < 1024 {
  				m += m
  			} else {
  				m += m / 4
  			}
  		}
  	}
  	t := MakeSlice(s.Type(), i1, m)
  	Copy(t, s)
  	return t, i0, i1
  }
  
  // Append appends the values x to a slice s and returns the resulting slice.
  // As in Go, each x's value must be assignable to the slice's element type.
  func Append(s Value, x ...Value) Value {
  	s.mustBe(Slice)
  	s, i0, i1 := grow(s, len(x))
  	for i, j := i0, 0; i < i1; i, j = i+1, j+1 {
  		s.Index(i).Set(x[j])
  	}
  	return s
  }
  
  // AppendSlice appends a slice t to a slice s and returns the resulting slice.
  // The slices s and t must have the same element type.
  func AppendSlice(s, t Value) Value {
  	s.mustBe(Slice)
  	t.mustBe(Slice)
  	typesMustMatch("reflect.AppendSlice", s.Type().Elem(), t.Type().Elem())
  	s, i0, i1 := grow(s, t.Len())
  	Copy(s.Slice(i0, i1), t)
  	return s
  }
  
  // Copy copies the contents of src into dst until either
  // dst has been filled or src has been exhausted.
  // It returns the number of elements copied.
  // Dst and src each must have kind Slice or Array, and
  // dst and src must have the same element type.
  func Copy(dst, src Value) int {
  	dk := dst.kind()
  	if dk != Array && dk != Slice {
  		panic(&ValueError{"reflect.Copy", dk})
  	}
  	if dk == Array {
  		dst.mustBeAssignable()
  	}
  	dst.mustBeExported()
  
  	sk := src.kind()
  	if sk != Array && sk != Slice {
  		panic(&ValueError{"reflect.Copy", sk})
  	}
  	src.mustBeExported()
  
  	de := dst.typ.Elem()
  	se := src.typ.Elem()
  	typesMustMatch("reflect.Copy", de, se)
  
  	var ds, ss sliceHeader
  	if dk == Array {
  		ds.Data = dst.ptr
  		ds.Len = dst.Len()
  		ds.Cap = ds.Len
  	} else {
  		ds = *(*sliceHeader)(dst.ptr)
  	}
  	if sk == Array {
  		ss.Data = src.ptr
  		ss.Len = src.Len()
  		ss.Cap = ss.Len
  	} else {
  		ss = *(*sliceHeader)(src.ptr)
  	}
  
  	return typedslicecopy(de.common(), ds, ss)
  }
  
  // A runtimeSelect is a single case passed to rselect.
  // This must match ../runtime/select.go:/runtimeSelect
  type runtimeSelect struct {
  	dir SelectDir      // SelectSend, SelectRecv or SelectDefault
  	typ *rtype         // channel type
  	ch  unsafe.Pointer // channel
  	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
  }
  
  // rselect runs a select. It returns the index of the chosen case.
  // If the case was a receive, val is filled in with the received value.
  // The conventional OK bool indicates whether the receive corresponds
  // to a sent value.
  //go:noescape
  func rselect([]runtimeSelect) (chosen int, recvOK bool)
  
  // A SelectDir describes the communication direction of a select case.
  type SelectDir int
  
  // NOTE: These values must match ../runtime/select.go:/selectDir.
  
  const (
  	_             SelectDir = iota
  	SelectSend              // case Chan <- Send
  	SelectRecv              // case <-Chan:
  	SelectDefault           // default
  )
  
  // A SelectCase describes a single case in a select operation.
  // The kind of case depends on Dir, the communication direction.
  //
  // If Dir is SelectDefault, the case represents a default case.
  // Chan and Send must be zero Values.
  //
  // If Dir is SelectSend, the case represents a send operation.
  // Normally Chan's underlying value must be a channel, and Send's underlying value must be
  // assignable to the channel's element type. As a special case, if Chan is a zero Value,
  // then the case is ignored, and the field Send will also be ignored and may be either zero
  // or non-zero.
  //
  // If Dir is SelectRecv, the case represents a receive operation.
  // Normally Chan's underlying value must be a channel and Send must be a zero Value.
  // If Chan is a zero Value, then the case is ignored, but Send must still be a zero Value.
  // When a receive operation is selected, the received Value is returned by Select.
  //
  type SelectCase struct {
  	Dir  SelectDir // direction of case
  	Chan Value     // channel to use (for send or receive)
  	Send Value     // value to send (for send)
  }
  
  // Select executes a select operation described by the list of cases.
  // Like the Go select statement, it blocks until at least one of the cases
  // can proceed, makes a uniform pseudo-random choice,
  // and then executes that case. It returns the index of the chosen case
  // and, if that case was a receive operation, the value received and a
  // boolean indicating whether the value corresponds to a send on the channel
  // (as opposed to a zero value received because the channel is closed).
  func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool) {
  	// NOTE: Do not trust that caller is not modifying cases data underfoot.
  	// The range is safe because the caller cannot modify our copy of the len
  	// and each iteration makes its own copy of the value c.
  	runcases := make([]runtimeSelect, len(cases))
  	haveDefault := false
  	for i, c := range cases {
  		rc := &runcases[i]
  		rc.dir = c.Dir
  		switch c.Dir {
  		default:
  			panic("reflect.Select: invalid Dir")
  
  		case SelectDefault: // default
  			if haveDefault {
  				panic("reflect.Select: multiple default cases")
  			}
  			haveDefault = true
  			if c.Chan.IsValid() {
  				panic("reflect.Select: default case has Chan value")
  			}
  			if c.Send.IsValid() {
  				panic("reflect.Select: default case has Send value")
  			}
  
  		case SelectSend:
  			ch := c.Chan
  			if !ch.IsValid() {
  				break
  			}
  			ch.mustBe(Chan)
  			ch.mustBeExported()
  			tt := (*chanType)(unsafe.Pointer(ch.typ))
  			if ChanDir(tt.dir)&SendDir == 0 {
  				panic("reflect.Select: SendDir case using recv-only channel")
  			}
  			rc.ch = ch.pointer()
  			rc.typ = &tt.rtype
  			v := c.Send
  			if !v.IsValid() {
  				panic("reflect.Select: SendDir case missing Send value")
  			}
  			v.mustBeExported()
  			v = v.assignTo("reflect.Select", tt.elem, nil)
  			if v.flag&flagIndir != 0 {
  				rc.val = v.ptr
  			} else {
  				rc.val = unsafe.Pointer(&v.ptr)
  			}
  
  		case SelectRecv:
  			if c.Send.IsValid() {
  				panic("reflect.Select: RecvDir case has Send value")
  			}
  			ch := c.Chan
  			if !ch.IsValid() {
  				break
  			}
  			ch.mustBe(Chan)
  			ch.mustBeExported()
  			tt := (*chanType)(unsafe.Pointer(ch.typ))
  			if ChanDir(tt.dir)&RecvDir == 0 {
  				panic("reflect.Select: RecvDir case using send-only channel")
  			}
  			rc.ch = ch.pointer()
  			rc.typ = &tt.rtype
  			rc.val = unsafe_New(tt.elem)
  		}
  	}
  
  	chosen, recvOK = rselect(runcases)
  	if runcases[chosen].dir == SelectRecv {
  		tt := (*chanType)(unsafe.Pointer(runcases[chosen].typ))
  		t := tt.elem
  		p := runcases[chosen].val
  		fl := flag(t.Kind())
  		if ifaceIndir(t) {
  			recv = Value{t, p, fl | flagIndir}
  		} else {
  			recv = Value{t, *(*unsafe.Pointer)(p), fl}
  		}
  	}
  	return chosen, recv, recvOK
  }
  
  /*
   * constructors
   */
  
  // implemented in package runtime
  func unsafe_New(*rtype) unsafe.Pointer
  func unsafe_NewArray(*rtype, int) unsafe.Pointer
  
  // MakeSlice creates a new zero-initialized slice value
  // for the specified slice type, length, and capacity.
  func MakeSlice(typ Type, len, cap int) Value {
  	if typ.Kind() != Slice {
  		panic("reflect.MakeSlice of non-slice type")
  	}
  	if len < 0 {
  		panic("reflect.MakeSlice: negative len")
  	}
  	if cap < 0 {
  		panic("reflect.MakeSlice: negative cap")
  	}
  	if len > cap {
  		panic("reflect.MakeSlice: len > cap")
  	}
  
  	s := sliceHeader{unsafe_NewArray(typ.Elem().(*rtype), cap), len, cap}
  	return Value{typ.common(), unsafe.Pointer(&s), flagIndir | flag(Slice)}
  }
  
  // MakeChan creates a new channel with the specified type and buffer size.
  func MakeChan(typ Type, buffer int) Value {
  	if typ.Kind() != Chan {
  		panic("reflect.MakeChan of non-chan type")
  	}
  	if buffer < 0 {
  		panic("reflect.MakeChan: negative buffer size")
  	}
  	if typ.ChanDir() != BothDir {
  		panic("reflect.MakeChan: unidirectional channel type")
  	}
  	ch := makechan(typ.(*rtype), uint64(buffer))
  	return Value{typ.common(), ch, flag(Chan)}
  }
  
  // MakeMap creates a new map with the specified type.
  func MakeMap(typ Type) Value {
  	return MakeMapWithSize(typ, 0)
  }
  
  // MakeMapWithSize creates a new map with the specified type
  // and initial space for approximately n elements.
  func MakeMapWithSize(typ Type, n int) Value {
  	if typ.Kind() != Map {
  		panic("reflect.MakeMapWithSize of non-map type")
  	}
  	m := makemap(typ.(*rtype), n)
  	return Value{typ.common(), m, flag(Map)}
  }
  
  // Indirect returns the value that v points to.
  // If v is a nil pointer, Indirect returns a zero Value.
  // If v is not a pointer, Indirect returns v.
  func Indirect(v Value) Value {
  	if v.Kind() != Ptr {
  		return v
  	}
  	return v.Elem()
  }
  
  // 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)
  }
  
  // Zero returns a Value representing the zero value for the specified type.
  // The result is different from the zero value of the Value struct,
  // which represents no value at all.
  // For example, Zero(TypeOf(42)) returns a Value with Kind Int and value 0.
  // The returned value is neither addressable nor settable.
  func Zero(typ Type) Value {
  	if typ == nil {
  		panic("reflect: Zero(nil)")
  	}
  	t := typ.common()
  	fl := flag(t.Kind())
  	if ifaceIndir(t) {
  		return Value{t, unsafe_New(typ.(*rtype)), fl | flagIndir}
  	}
  	return Value{t, nil, fl}
  }
  
  // New returns a Value representing a pointer to a new zero value
  // for the specified type. That is, the returned Value's Type is PtrTo(typ).
  func New(typ Type) Value {
  	if typ == nil {
  		panic("reflect: New(nil)")
  	}
  	ptr := unsafe_New(typ.(*rtype))
  	fl := flag(Ptr)
  	return Value{typ.common().ptrTo(), ptr, fl}
  }
  
  // NewAt returns a Value representing a pointer to a value of the
  // specified type, using p as that pointer.
  func NewAt(typ Type, p unsafe.Pointer) Value {
  	fl := flag(Ptr)
  	return Value{typ.common().ptrTo(), p, fl}
  }
  
  // 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 & (flagRO | flagAddr | flagIndir)
  		fl |= flag(dst.Kind())
  		return Value{dst, v.ptr, fl}
  
  	case implements(dst, v.typ):
  		if target == nil {
  			target = unsafe_New(dst)
  		}
  		x := valueInterface(v, false)
  		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())
  }
  
  // Convert returns the value v converted to type t.
  // If the usual Go conversion rules do not allow conversion
  // of the value v to type t, Convert panics.
  func (v Value) Convert(t Type) Value {
  	if v.flag&flagMethod != 0 {
  		v = makeMethodValue("Convert", v)
  	}
  	op := convertOp(t.common(), v.typ)
  	if op == nil {
  		panic("reflect.Value.Convert: value of type " + v.typ.String() + " cannot be converted to type " + t.String())
  	}
  	return op(v, t)
  }
  
  // convertOp returns the function to convert a value of type src
  // to a value of type dst. If the conversion is illegal, convertOp returns nil.
  func convertOp(dst, src *rtype) func(Value, Type) Value {
  	switch src.Kind() {
  	case Int, Int8, Int16, Int32, Int64:
  		switch dst.Kind() {
  		case Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
  			return cvtInt
  		case Float32, Float64:
  			return cvtIntFloat
  		case String:
  			return cvtIntString
  		}
  
  	case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
  		switch dst.Kind() {
  		case Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
  			return cvtUint
  		case Float32, Float64:
  			return cvtUintFloat
  		case String:
  			return cvtUintString
  		}
  
  	case Float32, Float64:
  		switch dst.Kind() {
  		case Int, Int8, Int16, Int32, Int64:
  			return cvtFloatInt
  		case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
  			return cvtFloatUint
  		case Float32, Float64:
  			return cvtFloat
  		}
  
  	case Complex64, Complex128:
  		switch dst.Kind() {
  		case Complex64, Complex128:
  			return cvtComplex
  		}
  
  	case String:
  		if dst.Kind() == Slice && dst.Elem().PkgPath() == "" {
  			switch dst.Elem().Kind() {
  			case Uint8:
  				return cvtStringBytes
  			case Int32:
  				return cvtStringRunes
  			}
  		}
  
  	case Slice:
  		if dst.Kind() == String && src.Elem().PkgPath() == "" {
  			switch src.Elem().Kind() {
  			case Uint8:
  				return cvtBytesString
  			case Int32:
  				return cvtRunesString
  			}
  		}
  	}
  
  	// dst and src have same underlying type.
  	if haveIdenticalUnderlyingType(dst, src, false) {
  		return cvtDirect
  	}
  
  	// dst and src are unnamed pointer types with same underlying base type.
  	if dst.Kind() == Ptr && dst.Name() == "" &&
  		src.Kind() == Ptr && src.Name() == "" &&
  		haveIdenticalUnderlyingType(dst.Elem().common(), src.Elem().common(), false) {
  		return cvtDirect
  	}
  
  	if implements(dst, src) {
  		if src.Kind() == Interface {
  			return cvtI2I
  		}
  		return cvtT2I
  	}
  
  	return nil
  }
  
  // makeInt returns a Value of type t equal to bits (possibly truncated),
  // where t is a signed or unsigned int type.
  func makeInt(f flag, bits uint64, t Type) Value {
  	typ := t.common()
  	ptr := unsafe_New(typ)
  	switch typ.size {
  	case 1:
  		*(*uint8)(ptr) = uint8(bits)
  	case 2:
  		*(*uint16)(ptr) = uint16(bits)
  	case 4:
  		*(*uint32)(ptr) = uint32(bits)
  	case 8:
  		*(*uint64)(ptr) = bits
  	}
  	return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
  }
  
  // makeFloat returns a Value of type t equal to v (possibly truncated to float32),
  // where t is a float32 or float64 type.
  func makeFloat(f flag, v float64, t Type) Value {
  	typ := t.common()
  	ptr := unsafe_New(typ)
  	switch typ.size {
  	case 4:
  		*(*float32)(ptr) = float32(v)
  	case 8:
  		*(*float64)(ptr) = v
  	}
  	return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
  }
  
  // makeComplex returns a Value of type t equal to v (possibly truncated to complex64),
  // where t is a complex64 or complex128 type.
  func makeComplex(f flag, v complex128, t Type) Value {
  	typ := t.common()
  	ptr := unsafe_New(typ)
  	switch typ.size {
  	case 8:
  		*(*complex64)(ptr) = complex64(v)
  	case 16:
  		*(*complex128)(ptr) = v
  	}
  	return Value{typ, ptr, f | flagIndir | flag(typ.Kind())}
  }
  
  func makeString(f flag, v string, t Type) Value {
  	ret := New(t).Elem()
  	ret.SetString(v)
  	ret.flag = ret.flag&^flagAddr | f
  	return ret
  }
  
  func makeBytes(f flag, v []byte, t Type) Value {
  	ret := New(t).Elem()
  	ret.SetBytes(v)
  	ret.flag = ret.flag&^flagAddr | f
  	return ret
  }
  
  func makeRunes(f flag, v []rune, t Type) Value {
  	ret := New(t).Elem()
  	ret.setRunes(v)
  	ret.flag = ret.flag&^flagAddr | f
  	return ret
  }
  
  // These conversion functions are returned by convertOp
  // for classes of conversions. For example, the first function, cvtInt,
  // takes any value v of signed int type and returns the value converted
  // to type t, where t is any signed or unsigned int type.
  
  // convertOp: intXX -> [u]intXX
  func cvtInt(v Value, t Type) Value {
  	return makeInt(v.flag&flagRO, uint64(v.Int()), t)
  }
  
  // convertOp: uintXX -> [u]intXX
  func cvtUint(v Value, t Type) Value {
  	return makeInt(v.flag&flagRO, v.Uint(), t)
  }
  
  // convertOp: floatXX -> intXX
  func cvtFloatInt(v Value, t Type) Value {
  	return makeInt(v.flag&flagRO, uint64(int64(v.Float())), t)
  }
  
  // convertOp: floatXX -> uintXX
  func cvtFloatUint(v Value, t Type) Value {
  	return makeInt(v.flag&flagRO, uint64(v.Float()), t)
  }
  
  // convertOp: intXX -> floatXX
  func cvtIntFloat(v Value, t Type) Value {
  	return makeFloat(v.flag&flagRO, float64(v.Int()), t)
  }
  
  // convertOp: uintXX -> floatXX
  func cvtUintFloat(v Value, t Type) Value {
  	return makeFloat(v.flag&flagRO, float64(v.Uint()), t)
  }
  
  // convertOp: floatXX -> floatXX
  func cvtFloat(v Value, t Type) Value {
  	return makeFloat(v.flag&flagRO, v.Float(), t)
  }
  
  // convertOp: complexXX -> complexXX
  func cvtComplex(v Value, t Type) Value {
  	return makeComplex(v.flag&flagRO, v.Complex(), t)
  }
  
  // convertOp: intXX -> string
  func cvtIntString(v Value, t Type) Value {
  	return makeString(v.flag&flagRO, string(v.Int()), t)
  }
  
  // convertOp: uintXX -> string
  func cvtUintString(v Value, t Type) Value {
  	return makeString(v.flag&flagRO, string(v.Uint()), t)
  }
  
  // convertOp: []byte -> string
  func cvtBytesString(v Value, t Type) Value {
  	return makeString(v.flag&flagRO, string(v.Bytes()), t)
  }
  
  // convertOp: string -> []byte
  func cvtStringBytes(v Value, t Type) Value {
  	return makeBytes(v.flag&flagRO, []byte(v.String()), t)
  }
  
  // convertOp: []rune -> string
  func cvtRunesString(v Value, t Type) Value {
  	return makeString(v.flag&flagRO, string(v.runes()), t)
  }
  
  // convertOp: string -> []rune
  func cvtStringRunes(v Value, t Type) Value {
  	return makeRunes(v.flag&flagRO, []rune(v.String()), t)
  }
  
  // convertOp: direct copy
  func cvtDirect(v Value, typ Type) Value {
  	f := v.flag
  	t := typ.common()
  	ptr := v.ptr
  	if f&flagAddr != 0 {
  		// indirect, mutable word - make a copy
  		c := unsafe_New(t)
  		typedmemmove(t, c, ptr)
  		ptr = c
  		f &^= flagAddr
  	}
  	return Value{t, ptr, v.flag&flagRO | f} // v.flag&flagRO|f == f?
  }
  
  // convertOp: concrete -> interface
  func cvtT2I(v Value, typ Type) Value {
  	target := unsafe_New(typ.common())
  	x := valueInterface(v, false)
  	if typ.NumMethod() == 0 {
  		*(*interface{})(target) = x
  	} else {
  		ifaceE2I(typ.(*rtype), x, target)
  	}
  	return Value{typ.common(), target, v.flag&flagRO | flagIndir | flag(Interface)}
  }
  
  // convertOp: interface -> interface
  func cvtI2I(v Value, typ Type) Value {
  	if v.IsNil() {
  		ret := Zero(typ)
  		ret.flag |= v.flag & flagRO
  		return ret
  	}
  	return cvtT2I(v.Elem(), typ)
  }
  
  // implemented in ../runtime
  func chancap(ch unsafe.Pointer) int
  func chanclose(ch unsafe.Pointer)
  func chanlen(ch unsafe.Pointer) int
  
  // Note: some of the noescape annotations below are technically a lie,
  // but safe in the context of this package. Functions like chansend
  // and mapassign don't escape the referent, but may escape anything
  // the referent points to (they do shallow copies of the referent).
  // It is safe in this package because the referent may only point
  // to something a Value may point to, and that is always in the heap
  // (due to the escapes() call in ValueOf).
  
  //go:noescape
  func chanrecv(ch unsafe.Pointer, nb bool, val unsafe.Pointer) (selected, received bool)
  
  //go:noescape
  func chansend(ch unsafe.Pointer, val unsafe.Pointer, nb bool) bool
  
  func makechan(typ *rtype, size uint64) (ch unsafe.Pointer)
  func makemap(t *rtype, cap int) (m unsafe.Pointer)
  
  //go:noescape
  func mapaccess(t *rtype, m unsafe.Pointer, key unsafe.Pointer) (val unsafe.Pointer)
  
  //go:noescape
  func mapassign(t *rtype, m unsafe.Pointer, key, val unsafe.Pointer)
  
  //go:noescape
  func mapdelete(t *rtype, m unsafe.Pointer, key unsafe.Pointer)
  
  // m escapes into the return value, but the caller of mapiterinit
  // doesn't let the return value escape.
  //go:noescape
  func mapiterinit(t *rtype, m unsafe.Pointer) unsafe.Pointer
  
  //go:noescape
  func mapiterkey(it unsafe.Pointer) (key unsafe.Pointer)
  
  //go:noescape
  func mapiternext(it unsafe.Pointer)
  
  //go:noescape
  func maplen(m unsafe.Pointer) int
  
  // call calls fn with a copy of the n argument bytes pointed at by arg.
  // After fn returns, reflectcall copies n-retoffset result bytes
  // back into arg+retoffset before returning. If copying result bytes back,
  // the caller must pass the argument frame type as argtype, so that
  // call can execute appropriate write barriers during the copy.
  func call(argtype *rtype, fn, arg unsafe.Pointer, n uint32, retoffset uint32)
  
  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)
  
  // typedmemmovepartial is like typedmemmove but assumes that
  // dst and src point off bytes into the value and only copies size bytes.
  //go:noescape
  func typedmemmovepartial(t *rtype, dst, src unsafe.Pointer, off, size uintptr)
  
  // typedslicecopy copies a slice of elemType values from src to dst,
  // returning the number of elements copied.
  //go:noescape
  func typedslicecopy(elemType *rtype, dst, src sliceHeader) int
  
  //go:noescape
  func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)
  
  // 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{}
  }
  

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