Source file src/pkg/reflect/value.go

// 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 (
    "runtime"
    "unsafe"
)

const ptrSize = uintptr(unsafe.Sizeof((*byte)(nil)))
const cannotSet = "cannot set value obtained via unexported struct field"

type addr unsafe.Pointer

// TODO: This will have to go away when
// the new gc goes in.
func memmove(adst, asrc addr, n uintptr) {
    dst := uintptr(adst)
    src := uintptr(asrc)
    switch {
    case src < dst && src+n > dst:
        // byte copy backward
        // careful: i is unsigned
        for i := n; i > 0; {
            i--
            *(*byte)(addr(dst + i)) = *(*byte)(addr(src + i))
        }
    case (n|src|dst)&(ptrSize-1) != 0:
        // byte copy forward
        for i := uintptr(0); i < n; i++ {
            *(*byte)(addr(dst + i)) = *(*byte)(addr(src + i))
        }
    default:
        // word copy forward
        for i := uintptr(0); i < n; i += ptrSize {
            *(*uintptr)(addr(dst + i)) = *(*uintptr)(addr(src + i))
        }
    }
}

// Value is the common interface to reflection values.
// The implementations of Value (e.g., ArrayValue, StructValue)
// have additional type-specific methods.
type Value interface {
    // Type returns the value's type.
    Type() Type

    // Interface returns the value as an interface{}.
    Interface() interface{}

    // CanSet returns whether the value can be changed.
    // Values obtained by the use of non-exported struct fields
    // can be used in Get but not Set.
    // If CanSet() returns false, calling the type-specific Set
    // will cause a crash.
    CanSet() bool

    // SetValue assigns v to the value; v must have the same type as the value.
    SetValue(v Value)

    // Addr returns a pointer to the underlying data.
    // It is for advanced clients that also
    // import the "unsafe" package.
    Addr() uintptr

    // Method returns a FuncValue corresponding to the value's i'th method.
    // The arguments to a Call on the returned FuncValue
    // should not include a receiver; the FuncValue will use
    // the value as the receiver.
    Method(i int) *FuncValue

    getAddr() addr
}

type value struct {
    typ    Type
    addr   addr
    canSet bool
}

func (v *value) Type() Type { return v.typ }

func (v *value) Addr() uintptr { return uintptr(v.addr) }

func (v *value) getAddr() addr { return v.addr }

func (v *value) Interface() interface{} {
    if typ, ok := v.typ.(*InterfaceType); ok {
        // There are two different representations of interface values,
        // one if the interface type has methods and one if it doesn't.
        // These two representations require different expressions
        // to extract correctly.
        if typ.NumMethod() == 0 {
            // Extract as interface value without methods.
            return *(*interface{})(v.addr)
        }
        // Extract from v.addr as interface value with methods.
        return *(*interface {
            m()
        })(v.addr)
    }
    return unsafe.Unreflect(v.typ, unsafe.Pointer(v.addr))
}

func (v *value) CanSet() bool { return v.canSet }

/*
 * basic types
 */

// BoolValue represents a bool value.
type BoolValue struct {
    value
}

// Get returns the underlying bool value.
func (v *BoolValue) Get() bool { return *(*bool)(v.addr) }

// Set sets v to the value x.
func (v *BoolValue) Set(x bool) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*bool)(v.addr) = x
}

// Set sets v to the value x.
func (v *BoolValue) SetValue(x Value) { v.Set(x.(*BoolValue).Get()) }

// FloatValue represents a float value.
type FloatValue struct {
    value
}

// Get returns the underlying float value.
func (v *FloatValue) Get() float { return *(*float)(v.addr) }

// Set sets v to the value x.
func (v *FloatValue) Set(x float) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*float)(v.addr) = x
}

// Set sets v to the value x.
func (v *FloatValue) SetValue(x Value) { v.Set(x.(*FloatValue).Get()) }

// Float32Value represents a float32 value.
type Float32Value struct {
    value
}

// Get returns the underlying float32 value.
func (v *Float32Value) Get() float32 { return *(*float32)(v.addr) }

// Set sets v to the value x.
func (v *Float32Value) Set(x float32) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*float32)(v.addr) = x
}

// Set sets v to the value x.
func (v *Float32Value) SetValue(x Value) { v.Set(x.(*Float32Value).Get()) }

// Float64Value represents a float64 value.
type Float64Value struct {
    value
}

// Get returns the underlying float64 value.
func (v *Float64Value) Get() float64 { return *(*float64)(v.addr) }

// Set sets v to the value x.
func (v *Float64Value) Set(x float64) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*float64)(v.addr) = x
}

// Set sets v to the value x.
func (v *Float64Value) SetValue(x Value) { v.Set(x.(*Float64Value).Get()) }

//// ComplexValue represents a complex value.
//type ComplexValue struct {
//	value
//}
//
//// Get returns the underlying complex value.
//func (v *ComplexValue) Get() complex { return *(*complex)(v.addr) }
//
//// Set sets v to the value x.
//func (v *ComplexValue) Set(x complex) {
//	if !v.canSet {
//		panic(cannotSet)
//	}
//	*(*complex)(v.addr) = x
//}
//
//// Set sets v to the value x.
//func (v *ComplexValue) SetValue(x Value) { v.Set(x.(*ComplexValue).Get()) }
//
//// Complex64Value represents a complex64 value.
//type Complex64Value struct {
//	value
//}
//
//// Get returns the underlying complex64 value.
//func (v *Complex64Value) Get() complex64 { return *(*complex64)(v.addr) }
//
//// Set sets v to the value x.
//func (v *Complex64Value) Set(x complex64) {
//	if !v.canSet {
//		panic(cannotSet)
//	}
//	*(*complex64)(v.addr) = x
//}
//
//// Set sets v to the value x.
//func (v *Complex64Value) SetValue(x Value) { v.Set(x.(*Complex64Value).Get()) }
//
//// Complex128Value represents a complex128 value.
//type Complex128Value struct {
//	value
//}
//
//// Get returns the underlying complex128 value.
//func (v *Complex128Value) Get() complex128 { return *(*complex128)(v.addr) }
//
//// Set sets v to the value x.
//func (v *Complex128Value) Set(x complex128) {
//	if !v.canSet {
//		panic(cannotSet)
//	}
//	*(*complex128)(v.addr) = x
//}
//
//// Set sets v to the value x.
//func (v *Complex128Value) SetValue(x Value) { v.Set(x.(*Complex128Value).Get()) }

// IntValue represents an int value.
type IntValue struct {
    value
}

// Get returns the underlying int value.
func (v *IntValue) Get() int { return *(*int)(v.addr) }

// Set sets v to the value x.
func (v *IntValue) Set(x int) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*int)(v.addr) = x
}

// Set sets v to the value x.
func (v *IntValue) SetValue(x Value) { v.Set(x.(*IntValue).Get()) }

// Int8Value represents an int8 value.
type Int8Value struct {
    value
}

// Get returns the underlying int8 value.
func (v *Int8Value) Get() int8 { return *(*int8)(v.addr) }

// Set sets v to the value x.
func (v *Int8Value) Set(x int8) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*int8)(v.addr) = x
}

// Set sets v to the value x.
func (v *Int8Value) SetValue(x Value) { v.Set(x.(*Int8Value).Get()) }

// Int16Value represents an int16 value.
type Int16Value struct {
    value
}

// Get returns the underlying int16 value.
func (v *Int16Value) Get() int16 { return *(*int16)(v.addr) }

// Set sets v to the value x.
func (v *Int16Value) Set(x int16) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*int16)(v.addr) = x
}

// Set sets v to the value x.
func (v *Int16Value) SetValue(x Value) { v.Set(x.(*Int16Value).Get()) }

// Int32Value represents an int32 value.
type Int32Value struct {
    value
}

// Get returns the underlying int32 value.
func (v *Int32Value) Get() int32 { return *(*int32)(v.addr) }

// Set sets v to the value x.
func (v *Int32Value) Set(x int32) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*int32)(v.addr) = x
}

// Set sets v to the value x.
func (v *Int32Value) SetValue(x Value) { v.Set(x.(*Int32Value).Get()) }

// Int64Value represents an int64 value.
type Int64Value struct {
    value
}

// Get returns the underlying int64 value.
func (v *Int64Value) Get() int64 { return *(*int64)(v.addr) }

// Set sets v to the value x.
func (v *Int64Value) Set(x int64) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*int64)(v.addr) = x
}

// Set sets v to the value x.
func (v *Int64Value) SetValue(x Value) { v.Set(x.(*Int64Value).Get()) }

// StringHeader is the runtime representation of a string.
type StringHeader struct {
    Data uintptr
    Len  int
}

// StringValue represents a string value.
type StringValue struct {
    value
}

// Get returns the underlying string value.
func (v *StringValue) Get() string { return *(*string)(v.addr) }

// Set sets v to the value x.
func (v *StringValue) Set(x string) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*string)(v.addr) = x
}

// Set sets v to the value x.
func (v *StringValue) SetValue(x Value) { v.Set(x.(*StringValue).Get()) }

// UintValue represents a uint value.
type UintValue struct {
    value
}

// Get returns the underlying uint value.
func (v *UintValue) Get() uint { return *(*uint)(v.addr) }

// Set sets v to the value x.
func (v *UintValue) Set(x uint) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*uint)(v.addr) = x
}

// Set sets v to the value x.
func (v *UintValue) SetValue(x Value) { v.Set(x.(*UintValue).Get()) }

// Uint8Value represents a uint8 value.
type Uint8Value struct {
    value
}

// Get returns the underlying uint8 value.
func (v *Uint8Value) Get() uint8 { return *(*uint8)(v.addr) }

// Set sets v to the value x.
func (v *Uint8Value) Set(x uint8) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*uint8)(v.addr) = x
}

// Set sets v to the value x.
func (v *Uint8Value) SetValue(x Value) { v.Set(x.(*Uint8Value).Get()) }

// Uint16Value represents a uint16 value.
type Uint16Value struct {
    value
}

// Get returns the underlying uint16 value.
func (v *Uint16Value) Get() uint16 { return *(*uint16)(v.addr) }

// Set sets v to the value x.
func (v *Uint16Value) Set(x uint16) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*uint16)(v.addr) = x
}

// Set sets v to the value x.
func (v *Uint16Value) SetValue(x Value) { v.Set(x.(*Uint16Value).Get()) }

// Uint32Value represents a uint32 value.
type Uint32Value struct {
    value
}

// Get returns the underlying uint32 value.
func (v *Uint32Value) Get() uint32 { return *(*uint32)(v.addr) }

// Set sets v to the value x.
func (v *Uint32Value) Set(x uint32) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*uint32)(v.addr) = x
}

// Set sets v to the value x.
func (v *Uint32Value) SetValue(x Value) { v.Set(x.(*Uint32Value).Get()) }

// Uint64Value represents a uint64 value.
type Uint64Value struct {
    value
}

// Get returns the underlying uint64 value.
func (v *Uint64Value) Get() uint64 { return *(*uint64)(v.addr) }

// Set sets v to the value x.
func (v *Uint64Value) Set(x uint64) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*uint64)(v.addr) = x
}

// Set sets v to the value x.
func (v *Uint64Value) SetValue(x Value) { v.Set(x.(*Uint64Value).Get()) }

// UintptrValue represents a uintptr value.
type UintptrValue struct {
    value
}

// Get returns the underlying uintptr value.
func (v *UintptrValue) Get() uintptr { return *(*uintptr)(v.addr) }

// Set sets v to the value x.
func (v *UintptrValue) Set(x uintptr) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*uintptr)(v.addr) = x
}

// Set sets v to the value x.
func (v *UintptrValue) SetValue(x Value) { v.Set(x.(*UintptrValue).Get()) }

// UnsafePointerValue represents an unsafe.Pointer value.
type UnsafePointerValue struct {
    value
}

// Get returns the underlying uintptr value.
// Get returns uintptr, not unsafe.Pointer, so that
// programs that do not import "unsafe" cannot
// obtain a value of unsafe.Pointer type from "reflect".
func (v *UnsafePointerValue) Get() uintptr { return uintptr(*(*unsafe.Pointer)(v.addr)) }

// Set sets v to the value x.
func (v *UnsafePointerValue) Set(x unsafe.Pointer) {
    if !v.canSet {
        panic(cannotSet)
    }
    *(*unsafe.Pointer)(v.addr) = x
}

// Set sets v to the value x.
func (v *UnsafePointerValue) SetValue(x Value) {
    v.Set(unsafe.Pointer(x.(*UnsafePointerValue).Get()))
}

func typesMustMatch(t1, t2 Type) {
    if t1 != t2 {
        panicln("type mismatch:", t1.String(), "!=", t2.String())
    }
}

/*
 * array
 */

// ArrayOrSliceValue is the common interface
// implemented by both ArrayValue and SliceValue.
type ArrayOrSliceValue interface {
    Value
    Len() int
    Cap() int
    Elem(i int) Value
    addr() addr
}

// ArrayCopy 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.
// The arrays dst and src must have the same element type.
func ArrayCopy(dst, src ArrayOrSliceValue) int {
    // TODO: This will have to move into the runtime
    // once the real gc goes in.
    de := dst.Type().(ArrayOrSliceType).Elem()
    se := src.Type().(ArrayOrSliceType).Elem()
    typesMustMatch(de, se)
    n := dst.Len()
    if xn := src.Len(); n > xn {
        n = xn
    }
    memmove(dst.addr(), src.addr(), uintptr(n)*de.Size())
    return n
}

// An ArrayValue represents an array.
type ArrayValue struct {
    value
}

// Len returns the length of the array.
func (v *ArrayValue) Len() int { return v.typ.(*ArrayType).Len() }

// Cap returns the capacity of the array (equal to Len()).
func (v *ArrayValue) Cap() int { return v.typ.(*ArrayType).Len() }

// addr returns the base address of the data in the array.
func (v *ArrayValue) addr() addr { return v.value.addr }

// Set assigns x to v.
// The new value x must have the same type as v.
func (v *ArrayValue) Set(x *ArrayValue) {
    if !v.canSet {
        panic(cannotSet)
    }
    typesMustMatch(v.typ, x.typ)
    ArrayCopy(v, x)
}

// Set sets v to the value x.
func (v *ArrayValue) SetValue(x Value) { v.Set(x.(*ArrayValue)) }

// Elem returns the i'th element of v.
func (v *ArrayValue) Elem(i int) Value {
    typ := v.typ.(*ArrayType).Elem()
    n := v.Len()
    if i < 0 || i >= n {
        panic("index", i, "in array len", n)
    }
    p := addr(uintptr(v.addr()) + uintptr(i)*typ.Size())
    return newValue(typ, p, v.canSet)
}

/*
 * slice
 */

// runtime representation of slice
type SliceHeader struct {
    Data uintptr
    Len  int
    Cap  int
}

// A SliceValue represents a slice.
type SliceValue struct {
    value
}

func (v *SliceValue) slice() *SliceHeader { return (*SliceHeader)(v.value.addr) }

// IsNil returns whether v is a nil slice.
func (v *SliceValue) IsNil() bool { return v.slice().Data == 0 }

// Len returns the length of the slice.
func (v *SliceValue) Len() int { return int(v.slice().Len) }

// Cap returns the capacity of the slice.
func (v *SliceValue) Cap() int { return int(v.slice().Cap) }

// addr returns the base address of the data in the slice.
func (v *SliceValue) addr() addr { return addr(v.slice().Data) }

// SetLen changes the length of v.
// The new length n must be between 0 and the capacity, inclusive.
func (v *SliceValue) SetLen(n int) {
    s := v.slice()
    if n < 0 || n > int(s.Cap) {
        panicln("SetLen", n, "with capacity", s.Cap)
    }
    s.Len = n
}

// Set assigns x to v.
// The new value x must have the same type as v.
func (v *SliceValue) Set(x *SliceValue) {
    if !v.canSet {
        panic(cannotSet)
    }
    typesMustMatch(v.typ, x.typ)
    *v.slice() = *x.slice()
}

// Set sets v to the value x.
func (v *SliceValue) SetValue(x Value) { v.Set(x.(*SliceValue)) }

// Get returns the uintptr address of the v.Cap()'th element.  This gives
// the same result for all slices of the same array.
// It is mainly useful for printing.
func (v *SliceValue) Get() uintptr {
    typ := v.typ.(*SliceType)
    return uintptr(v.addr()) + uintptr(v.Cap())*typ.Elem().Size()
}

// Slice returns a sub-slice of the slice v.
func (v *SliceValue) Slice(beg, end int) *SliceValue {
    cap := v.Cap()
    if beg < 0 || end < beg || end > cap {
        panic("slice bounds [", beg, ":", end, "] with capacity ", cap)
    }
    typ := v.typ.(*SliceType)
    s := new(SliceHeader)
    s.Data = uintptr(v.addr()) + uintptr(beg)*typ.Elem().Size()
    s.Len = end - beg
    s.Cap = cap - beg
    return newValue(typ, addr(s), v.canSet).(*SliceValue)
}

// Elem returns the i'th element of v.
func (v *SliceValue) Elem(i int) Value {
    typ := v.typ.(*SliceType).Elem()
    n := v.Len()
    if i < 0 || i >= n {
        panicln("index", i, "in array of length", n)
    }
    p := addr(uintptr(v.addr()) + uintptr(i)*typ.Size())
    return newValue(typ, p, v.canSet)
}

// MakeSlice creates a new zero-initialized slice value
// for the specified slice type, length, and capacity.
func MakeSlice(typ *SliceType, len, cap int) *SliceValue {
    s := &SliceHeader{
        Data: uintptr(unsafe.NewArray(typ.Elem(), cap)),
        Len:  len,
        Cap:  cap,
    }
    return newValue(typ, addr(s), true).(*SliceValue)
}

/*
 * chan
 */

// A ChanValue represents a chan.
type ChanValue struct {
    value
}

// IsNil returns whether v is a nil channel.
func (v *ChanValue) IsNil() bool { return *(*uintptr)(v.addr) == 0 }

// Set assigns x to v.
// The new value x must have the same type as v.
func (v *ChanValue) Set(x *ChanValue) {
    if !v.canSet {
        panic(cannotSet)
    }
    typesMustMatch(v.typ, x.typ)
    *(*uintptr)(v.addr) = *(*uintptr)(x.addr)
}

// Set sets v to the value x.
func (v *ChanValue) SetValue(x Value) { v.Set(x.(*ChanValue)) }

// Get returns the uintptr value of v.
// It is mainly useful for printing.
func (v *ChanValue) Get() uintptr { return *(*uintptr)(v.addr) }

// implemented in ../pkg/runtime/reflect.cgo
func makechan(typ *runtime.ChanType, size uint32) (ch *byte)
func chansend(ch, val *byte, pres *bool)
func chanrecv(ch, val *byte, pres *bool)
func chanclosed(ch *byte) bool
func chanclose(ch *byte)
func chanlen(ch *byte) int32
func chancap(ch *byte) int32

// Closed returns the result of closed(c) on the underlying channel.
func (v *ChanValue) Closed() bool {
    ch := *(**byte)(v.addr)
    return chanclosed(ch)
}

// Close closes the channel.
func (v *ChanValue) Close() {
    ch := *(**byte)(v.addr)
    chanclose(ch)
}

func (v *ChanValue) Len() int {
    ch := *(**byte)(v.addr)
    return int(chanlen(ch))
}

func (v *ChanValue) Cap() int {
    ch := *(**byte)(v.addr)
    return int(chancap(ch))
}

// internal send; non-blocking if b != nil
func (v *ChanValue) send(x Value, b *bool) {
    t := v.Type().(*ChanType)
    if t.Dir()&SendDir == 0 {
        panic("send on recv-only channel")
    }
    typesMustMatch(t.Elem(), x.Type())
    ch := *(**byte)(v.addr)
    chansend(ch, (*byte)(x.getAddr()), b)
}

// internal recv; non-blocking if b != nil
func (v *ChanValue) recv(b *bool) Value {
    t := v.Type().(*ChanType)
    if t.Dir()&RecvDir == 0 {
        panic("recv on send-only channel")
    }
    ch := *(**byte)(v.addr)
    x := MakeZero(t.Elem())
    chanrecv(ch, (*byte)(x.getAddr()), b)
    return x
}

// Send sends x on the channel v.
func (v *ChanValue) Send(x Value) { v.send(x, nil) }

// Recv receives and returns a value from the channel v.
func (v *ChanValue) Recv() Value { return v.recv(nil) }

// TrySend attempts to sends x on the channel v but will not block.
// It returns true if the value was sent, false otherwise.
func (v *ChanValue) TrySend(x Value) bool {
    var ok bool
    v.send(x, &ok)
    return ok
}

// TryRecv attempts to receive a value from the channel v but will not block.
// It returns the value if one is received, nil otherwise.
func (v *ChanValue) TryRecv() Value {
    var ok bool
    x := v.recv(&ok)
    if !ok {
        return nil
    }
    return x
}

// MakeChan creates a new channel with the specified type and buffer size.
func MakeChan(typ *ChanType, buffer int) *ChanValue {
    if buffer < 0 {
        panic("MakeChan: negative buffer size")
    }
    if typ.Dir() != BothDir {
        panic("MakeChan: unidirectional channel type")
    }
    v := MakeZero(typ).(*ChanValue)
    *(**byte)(v.addr) = makechan((*runtime.ChanType)(unsafe.Pointer(typ)), uint32(buffer))
    return v
}

/*
 * func
 */

// A FuncValue represents a function value.
type FuncValue struct {
    value
    first       *value
    isInterface bool
}

// IsNil returns whether v is a nil function.
func (v *FuncValue) IsNil() bool { return *(*uintptr)(v.addr) == 0 }

// Get returns the uintptr value of v.
// It is mainly useful for printing.
func (v *FuncValue) Get() uintptr { return *(*uintptr)(v.addr) }

// Set assigns x to v.
// The new value x must have the same type as v.
func (v *FuncValue) Set(x *FuncValue) {
    if !v.canSet {
        panic(cannotSet)
    }
    typesMustMatch(v.typ, x.typ)
    *(*uintptr)(v.addr) = *(*uintptr)(x.addr)
}

// Set sets v to the value x.
func (v *FuncValue) SetValue(x Value) { v.Set(x.(*FuncValue)) }

// Method returns a FuncValue corresponding to v's i'th method.
// The arguments to a Call on the returned FuncValue
// should not include a receiver; the FuncValue will use v
// as the receiver.
func (v *value) Method(i int) *FuncValue {
    t := v.Type().uncommon()
    if t == nil || i < 0 || i >= len(t.methods) {
        return nil
    }
    p := &t.methods[i]
    fn := p.tfn
    fv := &FuncValue{value: value{toType(*p.typ), addr(&fn), true}, first: v, isInterface: false}
    return fv
}

// implemented in ../pkg/runtime/*/asm.s
func call(fn, arg *byte, n uint32)

type tiny struct {
    b byte
}

// Call calls the function v with input parameters in.
// It returns the function's output parameters as Values.
func (fv *FuncValue) Call(in []Value) []Value {
    var structAlign = Typeof((*tiny)(nil)).(*PtrType).Elem().Size()

    t := fv.Type().(*FuncType)
    nin := len(in)
    if fv.first != nil && !fv.isInterface {
        nin++
    }
    if nin != t.NumIn() {
        panic("FuncValue: wrong argument count")
    }
    nout := t.NumOut()

    // Compute arg size & allocate.
    // This computation is 6g/8g-dependent
    // and probably wrong for gccgo, but so
    // is most of this function.
    size := uintptr(0)
    if fv.isInterface {
        // extra word for interface value
        size += ptrSize
    }
    for i := 0; i < nin; i++ {
        tv := t.In(i)
        a := uintptr(tv.Align())
        size = (size + a - 1) &^ (a - 1)
        size += tv.Size()
    }
    size = (size + structAlign - 1) &^ (structAlign - 1)
    for i := 0; i < nout; i++ {
        tv := t.Out(i)
        a := uintptr(tv.Align())
        size = (size + a - 1) &^ (a - 1)
        size += tv.Size()
    }

    // size must be > 0 in order for &args[0] to be valid.
    // the argument copying is going to round it up to
    // a multiple of 8 anyway, so make it 8 to begin with.
    if size < 8 {
        size = 8
    }

    // round to pointer size
    size = (size + ptrSize - 1) &^ (ptrSize - 1)

    // Copy into args.
    //
    // TODO(rsc): revisit when reference counting happens.
    // The values are holding up the in references for us,
    // but something must be done for the out references.
    // For now make everything look like a pointer by pretending
    // to allocate a []*int.
    args := make([]*int, size/ptrSize)
    ptr := uintptr(unsafe.Pointer(&args[0]))
    off := uintptr(0)
    delta := 0
    if v := fv.first; v != nil {
        // Hard-wired first argument.
        if fv.isInterface {
            // v is a single uninterpreted word
            memmove(addr(ptr), v.getAddr(), ptrSize)
            off = ptrSize
        } else {
            // v is a real value
            tv := v.Type()
            typesMustMatch(t.In(0), tv)
            n := tv.Size()
            memmove(addr(ptr), v.getAddr(), n)
            off = n
            delta = 1
        }
    }
    for i, v := range in {
        tv := v.Type()
        typesMustMatch(t.In(i+delta), tv)
        a := uintptr(tv.Align())
        off = (off + a - 1) &^ (a - 1)
        n := tv.Size()
        memmove(addr(ptr+off), v.getAddr(), n)
        off += n
    }
    off = (off + structAlign - 1) &^ (structAlign - 1)

    // Call
    call(*(**byte)(fv.addr), (*byte)(addr(ptr)), uint32(size))

    // Copy return values out of args.
    //
    // TODO(rsc): revisit like above.
    ret := make([]Value, nout)
    for i := 0; i < nout; i++ {
        tv := t.Out(i)
        a := uintptr(tv.Align())
        off = (off + a - 1) &^ (a - 1)
        v := MakeZero(tv)
        n := tv.Size()
        memmove(v.getAddr(), addr(ptr+off), n)
        ret[i] = v
        off += n
    }

    return ret
}

/*
 * interface
 */

// An InterfaceValue represents an interface value.
type InterfaceValue struct {
    value
}

// No Get because v.Interface() is available.

// IsNil returns whether v is a nil interface value.
func (v *InterfaceValue) IsNil() bool { return v.Interface() == nil }

// Elem returns the concrete value stored in the interface value v.
func (v *InterfaceValue) Elem() Value { return NewValue(v.Interface()) }

// ../runtime/reflect.cgo
func setiface(typ *InterfaceType, x *interface{}, addr addr)

// Set assigns x to v.
func (v *InterfaceValue) Set(x Value) {
    var i interface{}
    if x != nil {
        i = x.Interface()
    }
    if !v.canSet {
        panic(cannotSet)
    }
    // Two different representations; see comment in Get.
    // Empty interface is easy.
    t := v.typ.(*InterfaceType)
    if t.NumMethod() == 0 {
        *(*interface{})(v.addr) = i
        return
    }

    // Non-empty interface requires a runtime check.
    setiface(t, &i, v.addr)
}

// Set sets v to the value x.
func (v *InterfaceValue) SetValue(x Value) { v.Set(x) }

// Method returns a FuncValue corresponding to v's i'th method.
// The arguments to a Call on the returned FuncValue
// should not include a receiver; the FuncValue will use v
// as the receiver.
func (v *InterfaceValue) Method(i int) *FuncValue {
    t := v.Type().(*InterfaceType)
    if t == nil || i < 0 || i >= len(t.methods) {
        return nil
    }
    p := &t.methods[i]

    // Interface is two words: itable, data.
    tab := *(**runtime.Itable)(v.addr)
    data := &value{Typeof((*byte)(nil)), addr(uintptr(v.addr) + ptrSize), true}

    // Function pointer is at p.perm in the table.
    fn := tab.Fn[i]
    fv := &FuncValue{value: value{toType(*p.typ), addr(&fn), true}, first: data, isInterface: true}
    return fv
}

/*
 * map
 */

// A MapValue represents a map value.
type MapValue struct {
    value
}

// IsNil returns whether v is a nil map value.
func (v *MapValue) IsNil() bool { return *(*uintptr)(v.addr) == 0 }

// Set assigns x to v.
// The new value x must have the same type as v.
func (v *MapValue) Set(x *MapValue) {
    if !v.canSet {
        panic(cannotSet)
    }
    typesMustMatch(v.typ, x.typ)
    *(*uintptr)(v.addr) = *(*uintptr)(x.addr)
}

// Set sets v to the value x.
func (v *MapValue) SetValue(x Value) { v.Set(x.(*MapValue)) }

// Get returns the uintptr value of v.
// It is mainly useful for printing.
func (v *MapValue) Get() uintptr { return *(*uintptr)(v.addr) }

// implemented in ../pkg/runtime/reflect.cgo
func mapaccess(m, key, val *byte) bool
func mapassign(m, key, val *byte)
func maplen(m *byte) int32
func mapiterinit(m *byte) *byte
func mapiternext(it *byte)
func mapiterkey(it *byte, key *byte) bool
func makemap(t *runtime.MapType) *byte

// Elem returns the value associated with key in the map v.
// It returns nil if key is not found in the map.
func (v *MapValue) Elem(key Value) Value {
    t := v.Type().(*MapType)
    typesMustMatch(t.Key(), key.Type())
    m := *(**byte)(v.addr)
    if m == nil {
        return nil
    }
    newval := MakeZero(t.Elem())
    if !mapaccess(m, (*byte)(key.getAddr()), (*byte)(newval.getAddr())) {
        return nil
    }
    return newval
}

// SetElem sets the value associated with key in the map v to val.
// If val is nil, Put deletes the key from map.
func (v *MapValue) SetElem(key, val Value) {
    t := v.Type().(*MapType)
    typesMustMatch(t.Key(), key.Type())
    var vaddr *byte
    if val != nil {
        typesMustMatch(t.Elem(), val.Type())
        vaddr = (*byte)(val.getAddr())
    }
    m := *(**byte)(v.addr)
    mapassign(m, (*byte)(key.getAddr()), vaddr)
}

// Len returns the number of keys in the map v.
func (v *MapValue) Len() int {
    m := *(**byte)(v.addr)
    if m == nil {
        return 0
    }
    return int(maplen(m))
}

// Keys returns a slice containing all the keys present in the map,
// in unspecified order.
func (v *MapValue) Keys() []Value {
    tk := v.Type().(*MapType).Key()
    m := *(**byte)(v.addr)
    mlen := int32(0)
    if m != nil {
        mlen = maplen(m)
    }
    it := mapiterinit(m)
    a := make([]Value, mlen)
    var i int
    for i = 0; i < len(a); i++ {
        k := MakeZero(tk)
        if !mapiterkey(it, (*byte)(k.getAddr())) {
            break
        }
        a[i] = k
        mapiternext(it)
    }
    return a[0:i]
}

// MakeMap creates a new map of the specified type.
func MakeMap(typ *MapType) *MapValue {
    v := MakeZero(typ).(*MapValue)
    *(**byte)(v.addr) = makemap((*runtime.MapType)(unsafe.Pointer(typ)))
    return v
}

/*
 * ptr
 */

// A PtrValue represents a pointer.
type PtrValue struct {
    value
}

// IsNil returns whether v is a nil pointer.
func (v *PtrValue) IsNil() bool { return *(*uintptr)(v.addr) == 0 }

// Get returns the uintptr value of v.
// It is mainly useful for printing.
func (v *PtrValue) Get() uintptr { return *(*uintptr)(v.addr) }

// Set assigns x to v.
// The new value x must have the same type as v.
func (v *PtrValue) Set(x *PtrValue) {
    if !v.canSet {
        panic(cannotSet)
    }
    typesMustMatch(v.typ, x.typ)
    // TODO: This will have to move into the runtime
    // once the new gc goes in
    *(*uintptr)(v.addr) = *(*uintptr)(x.addr)
}

// Set sets v to the value x.
func (v *PtrValue) SetValue(x Value) { v.Set(x.(*PtrValue)) }

// PointTo changes v to point to x.
func (v *PtrValue) PointTo(x Value) {
    if !x.CanSet() {
        panic("cannot set x; cannot point to x")
    }
    typesMustMatch(v.typ.(*PtrType).Elem(), x.Type())
    // TODO: This will have to move into the runtime
    // once the new gc goes in.
    *(*uintptr)(v.addr) = x.Addr()
}

// Elem returns the value that v points to.
// If v is a nil pointer, Elem returns a nil Value.
func (v *PtrValue) Elem() Value {
    if v.IsNil() {
        return nil
    }
    return newValue(v.typ.(*PtrType).Elem(), *(*addr)(v.addr), v.canSet)
}

// Indirect returns the value that v points to.
// If v is a nil pointer, Indirect returns a nil Value.
// If v is not a pointer, Indirect returns v.
func Indirect(v Value) Value {
    if pv, ok := v.(*PtrValue); ok {
        return pv.Elem()
    }
    return v
}

/*
 * struct
 */

// A StructValue represents a struct value.
type StructValue struct {
    value
}

// Set assigns x to v.
// The new value x must have the same type as v.
func (v *StructValue) Set(x *StructValue) {
    // TODO: This will have to move into the runtime
    // once the gc goes in.
    if !v.canSet {
        panic(cannotSet)
    }
    typesMustMatch(v.typ, x.typ)
    memmove(v.addr, x.addr, v.typ.Size())
}

// Set sets v to the value x.
func (v *StructValue) SetValue(x Value) { v.Set(x.(*StructValue)) }

// Field returns the i'th field of the struct.
func (v *StructValue) Field(i int) Value {
    t := v.typ.(*StructType)
    if i < 0 || i >= t.NumField() {
        return nil
    }
    f := t.Field(i)
    return newValue(f.Type, addr(uintptr(v.addr)+f.Offset), v.canSet && f.PkgPath == "")
}

// FieldByIndex returns the nested field corresponding to index.
func (t *StructValue) FieldByIndex(index []int) (v Value) {
    v = t
    for i, x := range index {
        if i > 0 {
            if p, ok := v.(*PtrValue); ok {
                v = p.Elem()
            }
            if s, ok := v.(*StructValue); ok {
                t = s
            } else {
                v = nil
                return
            }
        }
        v = t.Field(x)
    }
    return
}

// FieldByName returns the struct field with the given name.
// The result is nil if no field was found.
func (t *StructValue) FieldByName(name string) Value {
    if f, ok := t.Type().(*StructType).FieldByName(name); ok {
        return t.FieldByIndex(f.Index)
    }
    return nil
}

// NumField returns the number of fields in the struct.
func (v *StructValue) NumField() int { return v.typ.(*StructType).NumField() }

/*
 * constructors
 */

// NewValue returns a new Value initialized to the concrete value
// stored in the interface i.  NewValue(nil) returns nil.
func NewValue(i interface{}) Value {
    if i == nil {
        return nil
    }
    t, a := unsafe.Reflect(i)
    return newValue(toType(t), addr(a), true)
}


func newFuncValue(typ Type, addr addr, canSet bool) *FuncValue {
    return &FuncValue{value: value{typ, addr, canSet}}
}

func newValue(typ Type, addr addr, canSet bool) Value {
    // FuncValue has a different layout;
    // it needs a extra space for the fixed receivers.
    if _, ok := typ.(*FuncType); ok {
        return newFuncValue(typ, addr, canSet)
    }

    // All values have same memory layout;
    // build once and convert.
    v := &struct{ value }{value{typ, addr, canSet}}
    switch typ.(type) {
    case *ArrayType:
        // TODO(rsc): Something must prevent
        // clients of the package from doing
        // this same kind of cast.
        // We should be allowed because
        // they're our types.
        // Something about implicit assignment
        // to struct fields.
        return (*ArrayValue)(v)
    case *BoolType:
        return (*BoolValue)(v)
    case *ChanType:
        return (*ChanValue)(v)
    case *FloatType:
        return (*FloatValue)(v)
    case *Float32Type:
        return (*Float32Value)(v)
    case *Float64Type:
        return (*Float64Value)(v)
        //	case *ComplexType:
        //		return (*ComplexValue)(v)
        //	case *Complex64Type:
        //		return (*Complex64Value)(v)
        //	case *Complex128Type:
        //		return (*Complex128Value)(v)
    case *IntType:
        return (*IntValue)(v)
    case *Int8Type:
        return (*Int8Value)(v)
    case *Int16Type:
        return (*Int16Value)(v)
    case *Int32Type:
        return (*Int32Value)(v)
    case *Int64Type:
        return (*Int64Value)(v)
    case *InterfaceType:
        return (*InterfaceValue)(v)
    case *MapType:
        return (*MapValue)(v)
    case *PtrType:
        return (*PtrValue)(v)
    case *SliceType:
        return (*SliceValue)(v)
    case *StringType:
        return (*StringValue)(v)
    case *StructType:
        return (*StructValue)(v)
    case *UintType:
        return (*UintValue)(v)
    case *Uint8Type:
        return (*Uint8Value)(v)
    case *Uint16Type:
        return (*Uint16Value)(v)
    case *Uint32Type:
        return (*Uint32Value)(v)
    case *Uint64Type:
        return (*Uint64Value)(v)
    case *UintptrType:
        return (*UintptrValue)(v)
    case *UnsafePointerType:
        return (*UnsafePointerValue)(v)
    }
    panicln("newValue", typ.String())
}

// MakeZero returns a zero Value for the specified Type.
func MakeZero(typ Type) Value {
    if typ == nil {
        return nil
    }
    return newValue(typ, addr(unsafe.New(typ)), true)
}