Add example of Embedded JSON Tag for CBOR

For more info, see
https://github.com/toravir/CBOR-Tag-Specs/blob/master/embeddedJSON.md

encode.go

@@ -314,7 +314,7 @@ const (
 	// TimeUnixMicro causes time.Time to encode to a CBOR time (tag 1) with a floating point content
 	// representing seconds elapsed (with up to 1-microsecond precision) since UNIX Epoch UTC.
 	// NOTE: The floating point content is encoded to the shortest floating-point encoding that preserves
-	// the 64-bit floating point content. I.e., the floating point encoding can be IEEE 764:
+	// the 64-bit floating point value. I.e., the floating point encoding can be IEEE 764:
 	// binary64, binary32, or binary16 depending on the content's value.
 	TimeUnixMicro
 

example_embedded_json_tag_for_cbor_test.go

@@ -0,0 +1,130 @@
+// Copyright (c) Faye Amacker. All rights reserved.
+// Licensed under the MIT License. See LICENSE in the project root for license information.
+
+package cbor_test
+
+// fxamacker/cbor allows user apps to use almost any current or future
+// CBOR tag number by implementing cbor.Marshaler and cbor.Unmarshaler
+// interfaces.  Essentially, MarshalCBOR and UnmarshalCBOR functions that
+// are implemented by user apps will automatically be called by this
+// CBOR codec's Marshal, Unmarshal, etc.
+//
+// This example shows how to encode and decode a tagged CBOR data item with
+// tag number 262 and the tag content is a JSON object "embedded" as a
+// CBOR byte string (major type 2).
+//
+// NOTE: RFC 8949 does not mention tag number 262. IANA assigned
+// CBOR tag number 262 as "Embedded JSON Object" specified by the
+// document Embedded JSON Tag for CBOR:
+//
+//	"Tag 262 can be applied to a byte string (major type 2) to indicate
+//	that the byte string is a JSON Object. The length of the byte string
+//	indicates the content."
+//
+// For more info, see Embedded JSON Tag for CBOR at:
+// https://github.com/toravir/CBOR-Tag-Specs/blob/master/embeddedJSON.md
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+
+	"github.com/fxamacker/cbor/v2"
+)
+
+// cborTagNumForEmbeddedJSON is the CBOR tag number 262.
+const cborTagNumForEmbeddedJSON = 262
+
+// EmbeddedJSON represents a Go value to be encoded as a tagged CBOR data item
+// with tag number 262 and the tag content is a JSON object "embedded" as a
+// CBOR byte string (major type 2).
+type EmbeddedJSON struct {
+	any
+}
+
+func NewEmbeddedJSON(val any) EmbeddedJSON {
+	return EmbeddedJSON{val}
+}
+
+// MarshalCBOR encodes EmbeddedJSON to a tagged CBOR data item with the
+// tag number 262 and the tag content is a JSON object that is
+// "embedded" as a CBOR byte string.
+func (v EmbeddedJSON) MarshalCBOR() ([]byte, error) {
+	// Encode v to JSON object.
+	data, err := json.Marshal(v)
+	if err != nil {
+		return nil, err
+	}
+
+	// Create cbor.Tag representing a tagged CBOR data item.
+	tag := cbor.Tag{
+		Number:  cborTagNumForEmbeddedJSON,
+		Content: data,
+	}
+
+	// Marshal to a tagged CBOR data item.
+	return cbor.Marshal(tag)
+}
+
+// UnmarshalCBOR decodes a tagged CBOR data item to EmbeddedJSON.
+// The byte slice provided to this function must contain a single
+// tagged CBOR data item with the tag number 262 and tag content
+// must be a JSON object "embedded" as a CBOR byte string.
+func (v *EmbeddedJSON) UnmarshalCBOR(b []byte) error {
+	// Unmarshal tagged CBOR data item.
+	var tag cbor.Tag
+	if err := cbor.Unmarshal(b, &tag); err != nil {
+		return err
+	}
+
+	// Check tag number.
+	if tag.Number != cborTagNumForEmbeddedJSON {
+		return fmt.Errorf("got tag number %d, expect tag number %d", tag.Number, cborTagNumForEmbeddedJSON)
+	}
+
+	// Check tag content.
+	jsonData, isByteString := tag.Content.([]byte)
+	if !isByteString {
+		return fmt.Errorf("got tag content type %T, expect tag content []byte", tag.Content)
+	}
+
+	// Unmarshal JSON object.
+	return json.Unmarshal(jsonData, v)
+}
+
+// MarshalJSON encodes EmbeddedJSON to a JSON object.
+func (v EmbeddedJSON) MarshalJSON() ([]byte, error) {
+	return json.Marshal(v.any)
+}
+
+// UnmarshalJSON decodes a JSON object.
+func (v *EmbeddedJSON) UnmarshalJSON(b []byte) error {
+	dec := json.NewDecoder(bytes.NewReader(b))
+	dec.UseNumber()
+	return dec.Decode(&v.any)
+}
+
+func Example_embeddedJSONTagForCBOR() {
+	value := NewEmbeddedJSON(map[string]any{
+		"name": "gopher",
+		"id":   json.Number("42"),
+	})
+
+	data, err := cbor.Marshal(value)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("cbor: %x\n", data)
+
+	var v EmbeddedJSON
+	err = cbor.Unmarshal(data, &v)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("%+v\n", v.any)
+	for k, v := range v.any.(map[string]any) {
+		fmt.Printf("  %s: %v (%T)\n", k, v, v)
+	}
+}