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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>
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@ -80,6 +80,7 @@ pkg net/url, method (Values) Has(string) bool
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pkg reflect, func VisibleFields(Type) []StructField
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pkg reflect, method (Method) IsExported() bool
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pkg reflect, method (StructField) IsExported() bool
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pkg reflect, method (Value) CanConvert(Type) bool
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pkg runtime/cgo (darwin-amd64-cgo), func NewHandle(interface{}) Handle
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pkg runtime/cgo (darwin-amd64-cgo), method (Handle) Delete()
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pkg runtime/cgo (darwin-amd64-cgo), method (Handle) Value() interface{}
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@ -989,6 +989,18 @@ func Foo() bool {
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<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
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<dd>
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<p><!-- CL 334669 -->
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The new
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<a href="/pkg/reflect/#Value.CanConvert"><code>Value.CanConvert</code></a>
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method reports whether a value can be converted to a type.
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This may be used to avoid a panic when converting a slice to an
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array pointer type if the slice is too short.
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Previously it was sufficient to use
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<a href="/pkg/reflect/#Type.ConvertibleTo"><code>Type.ConvertibleTo</code></a>
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for this, but the newly permitted conversion from slice to array
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pointer type can panic even if the types are convertible.
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</p>
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<p><!-- CL 266197 -->
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The new
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<a href="/pkg/reflect/#StructField.IsExported"><code>StructField.IsExported</code></a>
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@ -4304,6 +4304,9 @@ func TestConvert(t *testing.T) {
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// vout1 represents the in value converted to the in type.
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v1 := tt.in
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if !v1.CanConvert(t1) {
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t.Errorf("ValueOf(%T(%[1]v)).CanConvert(%s) = false, want true", tt.in.Interface(), t1)
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}
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vout1 := v1.Convert(t1)
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out1 := vout1.Interface()
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if vout1.Type() != tt.in.Type() || !DeepEqual(out1, tt.in.Interface()) {
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@ -4311,6 +4314,9 @@ func TestConvert(t *testing.T) {
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}
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// vout2 represents the in value converted to the out type.
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if !v1.CanConvert(t2) {
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t.Errorf("ValueOf(%T(%[1]v)).CanConvert(%s) = false, want true", tt.in.Interface(), t2)
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}
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vout2 := v1.Convert(t2)
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out2 := vout2.Interface()
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if vout2.Type() != tt.out.Type() || !DeepEqual(out2, tt.out.Interface()) {
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@ -4371,6 +4377,9 @@ func TestConvertPanic(t *testing.T) {
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if !v.Type().ConvertibleTo(pt) {
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t.Errorf("[]byte should be convertible to *[8]byte")
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}
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if v.CanConvert(pt) {
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t.Errorf("slice with length 4 should not be convertible to *[8]byte")
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}
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shouldPanic("reflect: cannot convert slice with length 4 to pointer to array with length 8", func() {
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_ = v.Convert(pt)
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})
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@ -2811,6 +2811,26 @@ func (v Value) Convert(t Type) Value {
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return op(v, t)
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}
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// CanConvert reports whether the value v can be converted to type t.
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// If v.CanConvert(t) returns true then v.Convert(t) will not panic.
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func (v Value) CanConvert(t Type) bool {
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vt := v.Type()
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if !vt.ConvertibleTo(t) {
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return false
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}
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// Currently the only conversion that is OK in terms of type
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// but that can panic depending on the value is converting
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// from slice to pointer-to-array.
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if vt.Kind() == Slice && t.Kind() == Ptr && t.Elem().Kind() == Array {
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n := t.Elem().Len()
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h := (*unsafeheader.Slice)(v.ptr)
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if n > h.Len {
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return false
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}
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}
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return true
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}
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// convertOp returns the function to convert a value of type src
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// to a value of type dst. If the conversion is illegal, convertOp returns nil.
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func convertOp(dst, src *rtype) func(Value, Type) Value {
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