types: Add EncodePtrCast and DecodePtrCast

types/encoding.go

@@ -117,6 +117,18 @@ func EncodePtr[T any, P interface {
 	}
 }
 
+// EncodePtrCast encodes a pointer to an object by casting it to V.
+func EncodePtrCast[V interface {
+	Cast() T
+	EncoderTo
+}, T any](e *Encoder, p *T) {
+	e.WriteBool(p != nil)
+	if p != nil {
+		vp := *(*V)(unsafe.Pointer(p))
+		vp.EncodeTo(e)
+	}
+}
+
 // EncodeSlice encodes a slice of objects that implement EncoderTo.
 func EncodeSlice[T EncoderTo](e *Encoder, s []T) {
 	e.WriteUint64(uint64(len(s)))
@@ -254,6 +266,22 @@ func DecodePtr[T any, TP interface {
 	}
 }
 
+// DecodePtrCast decodes a pointer to an object by casting it to V.
+func DecodePtrCast[T interface {
+	Cast() V
+}, TP interface {
+	*T
+	DecoderFrom
+}, V any](d *Decoder, p **V) {
+	tp := (**T)(unsafe.Pointer(p))
+	if d.ReadBool() {
+		*tp = new(T)
+		TP(*tp).DecodeFrom(d)
+	} else {
+		*tp = nil
+	}
+}
+
 // DecodeSlice decodes a length-prefixed slice of type T, containing values read
 // from the decoder.
 func DecodeSlice[T any, DF interface {

types/encoding_test.go

@@ -11,6 +11,26 @@ import (
 	"lukechampine.com/frand"
 )
 
+func TestEncodePtrCast(t *testing.T) {
+	var buf bytes.Buffer
+	e := types.NewEncoder(&buf)
+	c := types.Siacoins(1)
+	types.EncodePtrCast[types.V1Currency](e, &c)
+	types.EncodePtrCast[types.V2Currency](e, &c)
+	types.EncodePtrCast[types.V2Currency](e, nil)
+	e.Flush()
+	var c1, c2, c3 *types.Currency
+	d := types.NewBufDecoder(buf.Bytes())
+	types.DecodePtrCast[types.V1Currency](d, &c1)
+	types.DecodePtrCast[types.V2Currency](d, &c2)
+	types.DecodePtrCast[types.V2Currency](d, &c3)
+	if err := d.Err(); err != nil {
+		t.Fatal(err)
+	} else if *c1 != c || *c2 != c || c3 != nil {
+		t.Fatal("mismatch:", c1, c2, c3)
+	}
+}
+
 func TestEncodeSlice(t *testing.T) {
 	txns := multiproofTxns(10, 10)
 	var buf bytes.Buffer