reflect: add PtrTo, add Value.Addr (old Addr is now UnsafeAddr)
This change makes it possible to take the address of a
struct field or slice element in order to call a method that
requires a pointer receiver.
Existing code that uses the Value.Addr method will have
to change (as gob does in this CL) to call UnsafeAddr instead.
R=r, rog
CC=golang-dev
https://golang.org/cl/4239052
diff --git a/src/pkg/reflect/value.go b/src/pkg/reflect/value.go
index 4d7d872..0b70b17 100644
--- a/src/pkg/reflect/value.go
+++ b/src/pkg/reflect/value.go
@@ -11,7 +11,7 @@
)
const ptrSize = uintptr(unsafe.Sizeof((*byte)(nil)))
-const cannotSet = "cannot set value obtained via unexported struct field"
+const cannotSet = "cannot set value obtained from unexported struct field"
type addr unsafe.Pointer
@@ -51,20 +51,32 @@
// Interface returns the value as an interface{}.
Interface() interface{}
- // CanSet returns whether the value can be changed.
+ // CanSet returns true if 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.
+ // If CanSet returns false, calling the type-specific Set will panic.
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
+ // CanAddr returns true if 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, the result of dereferencing a pointer,
+ // or the result of a call to NewValue, MakeChan, MakeMap, or MakeZero.
+ // If CanAddr returns false, calling Addr will panic.
+ CanAddr() bool
+
+ // Addr returns the address of the value.
+ // If the value is not addressable, Addr panics.
+ // 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.
+ Addr() *PtrValue
+
+ // UnsafeAddr returns a pointer to the underlying data.
+ // It is for advanced clients that also import the "unsafe" package.
+ UnsafeAddr() uintptr
// Method returns a FuncValue corresponding to the value's i'th method.
// The arguments to a Call on the returned FuncValue
@@ -75,19 +87,42 @@
getAddr() addr
}
+// flags for value
+const (
+ canSet uint32 = 1 << iota // can set value (write to *v.addr)
+ canAddr // can take address of value
+ canStore // can store through value (write to **v.addr)
+)
+
// value is the common implementation of most values.
// It is embedded in other, public struct types, but always
// with a unique tag like "uint" or "float" so that the client cannot
// convert from, say, *UintValue to *FloatValue.
type value struct {
- typ Type
- addr addr
- canSet bool
+ typ Type
+ addr addr
+ flag uint32
}
func (v *value) Type() Type { return v.typ }
-func (v *value) Addr() uintptr { return uintptr(v.addr) }
+func (v *value) Addr() *PtrValue {
+ if !v.CanAddr() {
+ panic("reflect: cannot take address of value")
+ }
+ a := v.addr
+ flag := canSet
+ if v.CanSet() {
+ flag |= canStore
+ }
+ // We could safely set canAddr here too -
+ // the caller would get the address of a -
+ // but it doesn't match the Go model.
+ // The language doesn't let you say &&v.
+ return newValue(PtrTo(v.typ), addr(&a), flag).(*PtrValue)
+}
+
+func (v *value) UnsafeAddr() uintptr { return uintptr(v.addr) }
func (v *value) getAddr() addr { return v.addr }
@@ -109,7 +144,10 @@
return unsafe.Unreflect(v.typ, unsafe.Pointer(v.addr))
}
-func (v *value) CanSet() bool { return v.canSet }
+func (v *value) CanSet() bool { return v.flag&canSet != 0 }
+
+func (v *value) CanAddr() bool { return v.flag&canAddr != 0 }
+
/*
* basic types
@@ -125,7 +163,7 @@
// Set sets v to the value x.
func (v *BoolValue) Set(x bool) {
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
*(*bool)(v.addr) = x
@@ -152,7 +190,7 @@
// Set sets v to the value x.
func (v *FloatValue) Set(x float64) {
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
switch v.typ.Kind() {
@@ -197,7 +235,7 @@
// Set sets v to the value x.
func (v *ComplexValue) Set(x complex128) {
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
switch v.typ.Kind() {
@@ -237,7 +275,7 @@
// Set sets v to the value x.
func (v *IntValue) Set(x int64) {
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
switch v.typ.Kind() {
@@ -282,7 +320,7 @@
// Set sets v to the value x.
func (v *StringValue) Set(x string) {
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
*(*string)(v.addr) = x
@@ -317,7 +355,7 @@
// Set sets v to the value x.
func (v *UintValue) Set(x uint64) {
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
switch v.typ.Kind() {
@@ -361,7 +399,7 @@
// Set sets v to the value x.
func (v *UnsafePointerValue) Set(x unsafe.Pointer) {
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
*(*unsafe.Pointer)(v.addr) = x
@@ -473,7 +511,7 @@
// 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 {
+ if !v.CanSet() {
panic(cannotSet)
}
typesMustMatch(v.typ, x.typ)
@@ -491,7 +529,7 @@
panic("array index out of bounds")
}
p := addr(uintptr(v.addr()) + uintptr(i)*typ.Size())
- return newValue(typ, p, v.canSet)
+ return newValue(typ, p, v.flag)
}
/*
@@ -537,7 +575,7 @@
// 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 {
+ if !v.CanSet() {
panic(cannotSet)
}
typesMustMatch(v.typ, x.typ)
@@ -566,7 +604,14 @@
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)
+
+ // Like the result of Addr, we treat Slice as an
+ // unaddressable temporary, so don't set canAddr.
+ flag := canSet
+ if v.flag&canStore != 0 {
+ flag |= canStore
+ }
+ return newValue(typ, addr(s), flag).(*SliceValue)
}
// Elem returns the i'th element of v.
@@ -577,7 +622,11 @@
panic("reflect: slice index out of range")
}
p := addr(uintptr(v.addr()) + uintptr(i)*typ.Size())
- return newValue(typ, p, v.canSet)
+ flag := canAddr
+ if v.flag&canStore != 0 {
+ flag |= canSet | canStore
+ }
+ return newValue(typ, p, flag)
}
// MakeSlice creates a new zero-initialized slice value
@@ -588,7 +637,7 @@
Len: len,
Cap: cap,
}
- return newValue(typ, addr(s), true).(*SliceValue)
+ return newValue(typ, addr(s), canAddr|canSet|canStore).(*SliceValue)
}
/*
@@ -606,7 +655,7 @@
// 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 {
+ if !v.CanSet() {
panic(cannotSet)
}
typesMustMatch(v.typ, x.typ)
@@ -733,7 +782,7 @@
// 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 {
+ if !v.CanSet() {
panic(cannotSet)
}
typesMustMatch(v.typ, x.typ)
@@ -754,7 +803,7 @@
}
p := &t.methods[i]
fn := p.tfn
- fv := &FuncValue{value: value{toType(*p.typ), addr(&fn), true}, first: v, isInterface: false}
+ fv := &FuncValue{value: value{toType(*p.typ), addr(&fn), 0}, first: v, isInterface: false}
return fv
}
@@ -765,6 +814,17 @@
b byte
}
+// Interface returns the fv as an interface value.
+// If fv is a method obtained by invoking Value.Method
+// (as opposed to Type.Method), Interface cannot return an
+// interface value, so it panics.
+func (fv *FuncValue) Interface() interface{} {
+ if fv.first != nil {
+ panic("FuncValue: cannot create interface value for method with bound receiver")
+ }
+ return fv.value.Interface()
+}
+
// Call calls the function fv with input parameters in.
// It returns the function's output parameters as Values.
func (fv *FuncValue) Call(in []Value) []Value {
@@ -902,7 +962,7 @@
if x != nil {
i = x.Interface()
}
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
// Two different representations; see comment in Get.
@@ -933,11 +993,11 @@
// Interface is two words: itable, data.
tab := *(**runtime.Itable)(v.addr)
- data := &value{Typeof((*byte)(nil)), addr(uintptr(v.addr) + ptrSize), true}
+ data := &value{Typeof((*byte)(nil)), addr(uintptr(v.addr) + ptrSize), 0}
// 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}
+ fv := &FuncValue{value: value{toType(*p.typ), addr(&fn), 0}, first: data, isInterface: true}
return fv
}
@@ -956,7 +1016,7 @@
// 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 {
+ if !v.CanSet() {
panic(cannotSet)
}
if x == nil {
@@ -1075,15 +1135,18 @@
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.
+// The new value x must have the same type as v, and x.Elem().CanSet() must be true.
func (v *PtrValue) Set(x *PtrValue) {
if x == nil {
*(**uintptr)(v.addr) = nil
return
}
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
+ if x.flag&canStore == 0 {
+ panic("cannot copy pointer obtained from unexported struct field")
+ }
typesMustMatch(v.typ, x.typ)
// TODO: This will have to move into the runtime
// once the new gc goes in
@@ -1112,7 +1175,7 @@
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()
+ *(*uintptr)(v.addr) = x.UnsafeAddr()
}
// Elem returns the value that v points to.
@@ -1121,7 +1184,11 @@
if v.IsNil() {
return nil
}
- return newValue(v.typ.(*PtrType).Elem(), *(*addr)(v.addr), v.canSet)
+ flag := canAddr
+ if v.flag&canStore != 0 {
+ flag |= canSet | canStore
+ }
+ return newValue(v.typ.(*PtrType).Elem(), *(*addr)(v.addr), flag)
}
// Indirect returns the value that v points to.
@@ -1148,7 +1215,7 @@
func (v *StructValue) Set(x *StructValue) {
// TODO: This will have to move into the runtime
// once the gc goes in.
- if !v.canSet {
+ if !v.CanSet() {
panic(cannotSet)
}
typesMustMatch(v.typ, x.typ)
@@ -1165,7 +1232,12 @@
return nil
}
f := t.Field(i)
- return newValue(f.Type, addr(uintptr(v.addr)+f.Offset), v.canSet && f.PkgPath == "")
+ flag := v.flag
+ if f.PkgPath != "" {
+ // unexported field
+ flag &^= canSet | canStore
+ }
+ return newValue(f.Type, addr(uintptr(v.addr)+f.Offset), flag)
}
// FieldByIndex returns the nested field corresponding to index.
@@ -1221,11 +1293,11 @@
return nil
}
t, a := unsafe.Reflect(i)
- return newValue(toType(t), addr(a), true)
+ return newValue(toType(t), addr(a), canSet|canAddr|canStore)
}
-func newValue(typ Type, addr addr, canSet bool) Value {
- v := value{typ, addr, canSet}
+func newValue(typ Type, addr addr, flag uint32) Value {
+ v := value{typ, addr, flag}
switch typ.(type) {
case *ArrayType:
return &ArrayValue{v}
@@ -1266,5 +1338,5 @@
if typ == nil {
return nil
}
- return newValue(typ, addr(unsafe.New(typ)), true)
+ return newValue(typ, addr(unsafe.New(typ)), canSet|canAddr|canStore)
}
