reflect: add Value.CanConvert

For #395
For #46746

Change-Id: I4bfc094cf1cecd27ce48e31f92384cf470f371a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/334669
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
This commit is contained in:
Ian Lance Taylor 2021-07-14 14:46:09 -07:00
parent 9e26569293
commit 48c88f1b1b
4 changed files with 42 additions and 0 deletions

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@ -80,6 +80,7 @@ pkg net/url, method (Values) Has(string) bool
pkg reflect, func VisibleFields(Type) []StructField
pkg reflect, method (Method) IsExported() bool
pkg reflect, method (StructField) IsExported() bool
pkg reflect, method (Value) CanConvert(Type) bool
pkg runtime/cgo (darwin-amd64-cgo), func NewHandle(interface{}) Handle
pkg runtime/cgo (darwin-amd64-cgo), method (Handle) Delete()
pkg runtime/cgo (darwin-amd64-cgo), method (Handle) Value() interface{}

View file

@ -989,6 +989,18 @@ func Foo() bool {
<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
<dd>
<p><!-- CL 334669 -->
The new
<a href="/pkg/reflect/#Value.CanConvert"><code>Value.CanConvert</code></a>
method reports whether a value can be converted to a type.
This may be used to avoid a panic when converting a slice to an
array pointer type if the slice is too short.
Previously it was sufficient to use
<a href="/pkg/reflect/#Type.ConvertibleTo"><code>Type.ConvertibleTo</code></a>
for this, but the newly permitted conversion from slice to array
pointer type can panic even if the types are convertible.
</p>
<p><!-- CL 266197 -->
The new
<a href="/pkg/reflect/#StructField.IsExported"><code>StructField.IsExported</code></a>

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@ -4304,6 +4304,9 @@ func TestConvert(t *testing.T) {
// vout1 represents the in value converted to the in type.
v1 := tt.in
if !v1.CanConvert(t1) {
t.Errorf("ValueOf(%T(%[1]v)).CanConvert(%s) = false, want true", tt.in.Interface(), t1)
}
vout1 := v1.Convert(t1)
out1 := vout1.Interface()
if vout1.Type() != tt.in.Type() || !DeepEqual(out1, tt.in.Interface()) {
@ -4311,6 +4314,9 @@ func TestConvert(t *testing.T) {
}
// vout2 represents the in value converted to the out type.
if !v1.CanConvert(t2) {
t.Errorf("ValueOf(%T(%[1]v)).CanConvert(%s) = false, want true", tt.in.Interface(), t2)
}
vout2 := v1.Convert(t2)
out2 := vout2.Interface()
if vout2.Type() != tt.out.Type() || !DeepEqual(out2, tt.out.Interface()) {
@ -4371,6 +4377,9 @@ func TestConvertPanic(t *testing.T) {
if !v.Type().ConvertibleTo(pt) {
t.Errorf("[]byte should be convertible to *[8]byte")
}
if v.CanConvert(pt) {
t.Errorf("slice with length 4 should not be convertible to *[8]byte")
}
shouldPanic("reflect: cannot convert slice with length 4 to pointer to array with length 8", func() {
_ = v.Convert(pt)
})

View file

@ -2811,6 +2811,26 @@ func (v Value) Convert(t Type) Value {
return op(v, t)
}
// CanConvert reports whether the value v can be converted to type t.
// If v.CanConvert(t) returns true then v.Convert(t) will not panic.
func (v Value) CanConvert(t Type) bool {
vt := v.Type()
if !vt.ConvertibleTo(t) {
return false
}
// Currently the only conversion that is OK in terms of type
// but that can panic depending on the value is converting
// from slice to pointer-to-array.
if vt.Kind() == Slice && t.Kind() == Ptr && t.Elem().Kind() == Array {
n := t.Elem().Len()
h := (*unsafeheader.Slice)(v.ptr)
if n > h.Len {
return false
}
}
return true
}
// 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 {