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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package reflectlite
import (
"runtime"
"unsafe"
)
const ptrSize = 4 << (^uintptr(0) >> 63) // unsafe.Sizeof(uintptr(0)) but an ideal const
// Value is the reflection interface to a Go value.
//
// Not all methods apply to all kinds of values. Restrictions,
// if any, are noted in the documentation for each method.
// Use the Kind method to find out the kind of value before
// calling kind-specific methods. Calling a method
// inappropriate to the kind of type causes a run time panic.
//
// The zero Value represents no value.
// Its IsValid method returns false, its Kind method returns Invalid,
// its String method returns "<invalid Value>", and all other methods panic.
// Most functions and methods never return an invalid value.
// If one does, its documentation states the conditions explicitly.
//
// A Value can be used concurrently by multiple goroutines provided that
// the underlying Go value can be used concurrently for the equivalent
// direct operations.
//
// To compare two Values, compare the results of the Interface method.
// Using == on two Values does not compare the underlying values
// they represent.
type Value struct {
// typ holds the type of the value represented by a Value.
typ *rtype
// Pointer-valued data or, if flagIndir is set, pointer to data.
// Valid when either flagIndir is set or typ.pointers() is true.
ptr unsafe.Pointer
// flag holds metadata about the value.
// The lowest bits are flag bits:
// - flagStickyRO: obtained via unexported not embedded field, so read-only
// - flagEmbedRO: obtained via unexported embedded field, so read-only
// - flagIndir: val holds a pointer to the data
// - flagAddr: v.CanAddr is true (implies flagIndir)
// Value cannot represent method values.
// The next five bits give the Kind of the value.
// This repeats typ.Kind() except for method values.
// The remaining 23+ bits give a method number for method values.
// If flag.kind() != Func, code can assume that flagMethod is unset.
// If ifaceIndir(typ), code can assume that flagIndir is set.
flag
// A method value represents a curried method invocation
// like r.Read for some receiver r. The typ+val+flag bits describe
// the receiver r, but the flag's Kind bits say Func (methods are
// functions), and the top bits of the flag give the method number
// in r's type's method table.
}
type flag uintptr
const (
flagKindWidth = 5 // there are 27 kinds
flagKindMask flag = 1<<flagKindWidth - 1
flagStickyRO flag = 1 << 5
flagEmbedRO flag = 1 << 6
flagIndir flag = 1 << 7
flagAddr flag = 1 << 8
flagMethod flag = 1 << 9
flagMethodShift = 10
flagRO flag = flagStickyRO | flagEmbedRO
)
func (f flag) kind() Kind {
return Kind(f & flagKindMask)
}
func (f flag) ro() flag {
if f&flagRO != 0 {
return flagStickyRO
}
return 0
}
// pointer returns the underlying pointer represented by v.
// v.Kind() must be Ptr, Map, Chan, Func, or UnsafePointer
func (v Value) pointer() unsafe.Pointer {
if v.typ.size != ptrSize || !v.typ.pointers() {
panic("can't call pointer on a non-pointer Value")
}
if v.flag&flagIndir != 0 {
return *(*unsafe.Pointer)(v.ptr)
}
return v.ptr
}
// packEface converts v to the empty interface.
func packEface(v Value) interface{} {
t := v.typ
var i interface{}
e := (*emptyInterface)(unsafe.Pointer(&i))
// First, fill in the data portion of the interface.
switch {
case ifaceIndir(t):
if v.flag&flagIndir == 0 {
panic("bad indir")
}
// Value is indirect, and so is the interface we're making.
ptr := v.ptr
if v.flag&flagAddr != 0 {
// TODO: pass safe boolean from valueInterface so
// we don't need to copy if safe==true?
c := unsafe_New(t)
typedmemmove(t, c, ptr)
ptr = c
}
e.word = ptr
case v.flag&flagIndir != 0:
// Value is indirect, but interface is direct. We need
// to load the data at v.ptr into the interface data word.
e.word = *(*unsafe.Pointer)(v.ptr)
default:
// Value is direct, and so is the interface.
e.word = v.ptr
}
// Now, fill in the type portion. We're very careful here not
// to have any operation between the e.word and e.typ assignments
// that would let the garbage collector observe the partially-built
// interface value.
e.typ = t
return i
}
// unpackEface converts the empty interface i to a Value.
func unpackEface(i interface{}) Value {
e := (*emptyInterface)(unsafe.Pointer(&i))
// NOTE: don't read e.word until we know whether it is really a pointer or not.
t := e.typ
if t == nil {
return Value{}
}
f := flag(t.Kind())
if ifaceIndir(t) {
f |= flagIndir
}
return Value{t, e.word, f}
}
// A ValueError occurs when a Value method is invoked on
// a Value that does not support it. Such cases are documented
// in the description of each method.
type ValueError struct {
Method string
Kind Kind
}
func (e *ValueError) Error() string {
return "reflect: call of " + e.Method + " on zero Value"
}
// methodName returns the name of the calling method,
// assumed to be two stack frames above.
func methodName() string {
pc, _, _, _ := runtime.Caller(2)
f := runtime.FuncForPC(pc)
if f == nil {
return "unknown method"
}
return f.Name()
}
// emptyInterface is the header for an interface{} value.
type emptyInterface struct {
typ *rtype
word unsafe.Pointer
}
// mustBeExported panics if f records that the value was obtained using
// an unexported field.
func (f flag) mustBeExported() {
if f == 0 {
panic(&ValueError{methodName(), 0})
}
if f&flagRO != 0 {
panic("reflect: " + methodName() + " using value obtained using unexported field")
}
}
// mustBeAssignable panics if f records that the value is not assignable,
// which is to say that either it was obtained using an unexported field
// or it is not addressable.
func (f flag) mustBeAssignable() {
if f == 0 {
panic(&ValueError{methodName(), Invalid})
}
// Assignable if addressable and not read-only.
if f&flagRO != 0 {
panic("reflect: " + methodName() + " using value obtained using unexported field")
}
if f&flagAddr == 0 {
panic("reflect: " + methodName() + " using unaddressable value")
}
}
// CanSet reports whether the value of v can be changed.
// A Value can be changed only if it is addressable and was not
// obtained by the use of unexported struct fields.
// If CanSet returns false, calling Set or any type-specific
// setter (e.g., SetBool, SetInt) will panic.
func (v Value) CanSet() bool {
return v.flag&(flagAddr|flagRO) == flagAddr
}
// Elem returns the value that the interface v contains
// or that the pointer v points to.
// It panics if v's Kind is not Interface or Ptr.
// It returns the zero Value if v is nil.
func (v Value) Elem() Value {
k := v.kind()
switch k {
case Interface:
var eface interface{}
if v.typ.NumMethod() == 0 {
eface = *(*interface{})(v.ptr)
} else {
eface = (interface{})(*(*interface {
M()
})(v.ptr))
}
x := unpackEface(eface)
if x.flag != 0 {
x.flag |= v.flag.ro()
}
return x
case Ptr:
ptr := v.ptr
if v.flag&flagIndir != 0 {
ptr = *(*unsafe.Pointer)(ptr)
}
// The returned value's address is v's value.
if ptr == nil {
return Value{}
}
tt := (*ptrType)(unsafe.Pointer(v.typ))
typ := tt.elem
fl := v.flag&flagRO | flagIndir | flagAddr
fl |= flag(typ.Kind())
return Value{typ, ptr, fl}
}
panic(&ValueError{"reflectlite.Value.Elem", v.kind()})
}
func valueInterface(v Value) interface{} {
if v.flag == 0 {
panic(&ValueError{"reflectlite.Value.Interface", 0})
}
if v.kind() == Interface {
// Special case: return the element inside the interface.
// Empty interface has one layout, all interfaces with
// methods have a second layout.
if v.numMethod() == 0 {
return *(*interface{})(v.ptr)
}
return *(*interface {
M()
})(v.ptr)
}
// TODO: pass safe to packEface so we don't need to copy if safe==true?
return packEface(v)
}
// IsNil reports whether its argument v is nil. The argument must be
// a chan, func, interface, map, pointer, or slice value; if it is
// not, IsNil panics. Note that IsNil is not always equivalent to a
// regular comparison with nil in Go. For example, if v was created
// by calling ValueOf with an uninitialized interface variable i,
// i==nil will be true but v.IsNil will panic as v will be the zero
// Value.
func (v Value) IsNil() bool {
k := v.kind()
switch k {
case Chan, Func, Map, Ptr, UnsafePointer:
// if v.flag&flagMethod != 0 {
// return false
// }
ptr := v.ptr
if v.flag&flagIndir != 0 {
ptr = *(*unsafe.Pointer)(ptr)
}
return ptr == nil
case Interface, Slice:
// Both interface and slice are nil if first word is 0.
// Both are always bigger than a word; assume flagIndir.
return *(*unsafe.Pointer)(v.ptr) == nil
}
panic(&ValueError{"reflectlite.Value.IsNil", v.kind()})
}
// IsValid reports whether v represents a value.
// It returns false if v is the zero Value.
// If IsValid returns false, all other methods except String panic.
// Most functions and methods never return an invalid Value.
// If one does, its documentation states the conditions explicitly.
func (v Value) IsValid() bool {
return v.flag != 0
}
// Kind returns v's Kind.
// If v is the zero Value (IsValid returns false), Kind returns Invalid.
func (v Value) Kind() Kind {
return v.kind()
}
// implemented in runtime:
func chanlen(unsafe.Pointer) int
func maplen(unsafe.Pointer) int
// Len returns v's length.
// It panics if v's Kind is not Array, Chan, Map, Slice, or String.
func (v Value) Len() int {
k := v.kind()
switch k {
case Array:
tt := (*arrayType)(unsafe.Pointer(v.typ))
return int(tt.len)
case Chan:
return chanlen(v.pointer())
case Map:
return maplen(v.pointer())
case Slice:
// Slice is bigger than a word; assume flagIndir.
return (*sliceHeader)(v.ptr).Len
case String:
// String is bigger than a word; assume flagIndir.
return (*stringHeader)(v.ptr).Len
}
panic(&ValueError{"reflect.Value.Len", v.kind()})
}
// NumMethod returns the number of exported methods in the value's method set.
func (v Value) numMethod() int {
if v.typ == nil {
panic(&ValueError{"reflectlite.Value.NumMethod", Invalid})
}
return v.typ.NumMethod()
}
// Set assigns x to the value v.
// It panics if CanSet returns false.
// As in Go, x's value must be assignable to v's type.
func (v Value) Set(x Value) {
v.mustBeAssignable()
x.mustBeExported() // do not let unexported x leak
var target unsafe.Pointer
if v.kind() == Interface {
target = v.ptr
}
x = x.assignTo("reflectlite.Set", v.typ, target)
if x.flag&flagIndir != 0 {
typedmemmove(v.typ, v.ptr, x.ptr)
} else {
*(*unsafe.Pointer)(v.ptr) = x.ptr
}
}
// Type returns v's type.
func (v Value) Type() Type {
f := v.flag
if f == 0 {
panic(&ValueError{"reflectlite.Value.Type", Invalid})
}
// Method values not supported.
return v.typ
}
// stringHeader is a safe version of StringHeader used within this package.
type stringHeader struct {
Data unsafe.Pointer
Len int
}
// sliceHeader is a safe version of SliceHeader used within this package.
type sliceHeader struct {
Data unsafe.Pointer
Len int
Cap int
}
/*
* constructors
*/
// implemented in package runtime
func unsafe_New(*rtype) unsafe.Pointer
// ValueOf returns a new Value initialized to the concrete value
// stored in the interface i. ValueOf(nil) returns the zero Value.
func ValueOf(i interface{}) Value {
if i == nil {
return Value{}
}
// TODO: Maybe allow contents of a Value to live on the stack.
// For now we make the contents always escape to the heap. It
// makes life easier in a few places (see chanrecv/mapassign
// comment below).
escapes(i)
return unpackEface(i)
}
// assignTo returns a value v that can be assigned directly to typ.
// It panics if v is not assignable to typ.
// For a conversion to an interface type, target is a suggested scratch space to use.
func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value {
// if v.flag&flagMethod != 0 {
// v = makeMethodValue(context, v)
// }
switch {
case directlyAssignable(dst, v.typ):
// Overwrite type so that they match.
// Same memory layout, so no harm done.
fl := v.flag&(flagAddr|flagIndir) | v.flag.ro()
fl |= flag(dst.Kind())
return Value{dst, v.ptr, fl}
case implements(dst, v.typ):
if target == nil {
target = unsafe_New(dst)
}
if v.Kind() == Interface && v.IsNil() {
// A nil ReadWriter passed to nil Reader is OK,
// but using ifaceE2I below will panic.
// Avoid the panic by returning a nil dst (e.g., Reader) explicitly.
return Value{dst, nil, flag(Interface)}
}
x := valueInterface(v)
if dst.NumMethod() == 0 {
*(*interface{})(target) = x
} else {
ifaceE2I(dst, x, target)
}
return Value{dst, target, flagIndir | flag(Interface)}
}
// Failed.
panic(context + ": value of type " + v.typ.String() + " is not assignable to type " + dst.String())
}
// arrayAt returns the i-th element of p,
// an array whose elements are eltSize bytes wide.
// The array pointed at by p must have at least i+1 elements:
// it is invalid (but impossible to check here) to pass i >= len,
// because then the result will point outside the array.
// whySafe must explain why i < len. (Passing "i < len" is fine;
// the benefit is to surface this assumption at the call site.)
func arrayAt(p unsafe.Pointer, i int, eltSize uintptr, whySafe string) unsafe.Pointer {
return add(p, uintptr(i)*eltSize, "i < len")
}
func ifaceE2I(t *rtype, src interface{}, dst unsafe.Pointer)
// typedmemmove copies a value of type t to dst from src.
//go:noescape
func typedmemmove(t *rtype, dst, src unsafe.Pointer)
// Dummy annotation marking that the value x escapes,
// for use in cases where the reflect code is so clever that
// the compiler cannot follow.
func escapes(x interface{}) {
if dummy.b {
dummy.x = x
}
}
var dummy struct {
b bool
x interface{}
}
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