feature: IntDecMode float64 and json.Number modes (encoding/json compatibility)  #439

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1. theory commented on May 18, 2025

   

   theory

   on May 18, 2025

   Contributor

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   For those looking for solutions, this comment shows how to preserve json.Number values using the recently-assigned CBOR tag 284, the JSON Number String Tag.
2. elv-gilles commented on Jan 6, 2026

   

   elv-gilles

   on Jan 6, 2026

   Last edited by elv-gilles

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   Even when activating support for JSON Number with tag 284, decoding into a struct that has a numeric field, e.g. int64, currently fails cannot unmarshal UTF-8 text string into Go struct field cbor_test.TimeSerial.update_time_epoch_sec of type int64

   This would probably need to be handled in decoder.parseToValue.

   Unit-test (click to expand)

   package cbor_test
   
   import (
   	"bytes"
   	"encoding/json"
   	"fmt"
   	"io"
   	"log"
   	"reflect"
   	"testing"
   	"time"
   
   	"github.com/fxamacker/cbor/v2"
   )
   
   type TimeSerial struct {
   	//UpdateTime         time.Time `json:"update_time"`
   	UpdateTimeEpochSec int64 `json:"update_time_epoch_sec"`
   }
   
   func makeCborCodec() (func(writer io.Writer) *cbor.Encoder, func(writer io.Reader) *cbor.Decoder) {
   	var err error
   	tagSet := cbor.NewTagSet()
   	options := cbor.TagOptions{
   		DecTag: cbor.DecTagOptional, 
   		EncTag: cbor.EncTagRequired,
   	}
   
   	err = tagSet.Add(options, reflect.TypeOf(json.Number("")), 284)
   	if err != nil {
   		log.Fatal("invalid cbor tag", err, "tag", 284)
   	}
   
   	encOptions := cbor.CoreDetEncOptions()
   	encOptions.Time = cbor.TimeUnixMicro
   	encOptions.TimeTag = cbor.EncTagRequired
   	enc, err := encOptions.EncModeWithTags(tagSet)
   	if err != nil {
   		log.Fatal("failed to create cbor encoder mode", err)
   	}
   
   	dec, err := cbor.DecOptions{
   		DefaultMapType: reflect.TypeOf((map[string]interface{})(nil)),
   	}.DecModeWithTags(tagSet)
   	if err != nil {
   		log.Fatal("failed to create cbor decoder mode", err)
   	}
   
   	return func(w io.Writer) *cbor.Encoder {
   			return enc.NewEncoder(w)
   		}, func(r io.Reader) *cbor.Decoder {
   			return dec.NewDecoder(r)
   		}
   }
   
   func TestSerialWithJsonNumber(t *testing.T) {
   	now := time.Now()
   
   	ts := &TimeSerial{
   		//UpdateTime:         now,
   		UpdateTimeEpochSec: now.Unix(),
   	}
   	var tsiFromJs interface{}
   	{
   		bb, err := json.Marshal(ts)
   		if err != nil {
   			t.Errorf("unexpected error %v", err)
   		}
   		dec := json.NewDecoder(bytes.NewReader(bb))
   		dec.UseNumber() // preserve number formats
   		err = dec.Decode(&tsiFromJs)
   		if err != nil {
   			t.Errorf("unexpected error %v", err)
   		}
   	}
   
   	encFn, decFn := makeCborCodec()
   	{
   		buf := bytes.NewBuffer(make([]byte, 0))
   		enc := encFn(buf)
   		err := enc.Encode(&tsiFromJs)
   		if err != nil {
   			t.Errorf("unexpected error %v", err)
   		}
   
   		diag, err := cbor.Diagnose(buf.Bytes())
   		if err != nil {
   			t.Errorf("unexpected error %v", err)
   		}
   		fmt.Println(diag)
   
   		dec := decFn(bytes.NewReader(buf.Bytes()))
   		ts2 := &TimeSerial{}
   		err = dec.Decode(ts2)
   		if err != nil {
   			t.Errorf("unexpected error %v", err)
   		}
   	}
   
   }
3. elv-gilles commented on Jan 15, 2026

   

   elv-gilles

   on Jan 15, 2026

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   @fxamacker - a branch is ready here to further improves interop with json.

   This covers cases where some json is received (e.g. on an API server), decoded via encoding/json before being cbor encoded. The json decoder can be configured with or without the UseNumber option of the json decoder.

   The changes in the proposed branch:

   • provide an 'opt-in' option to allow float to int conversion. This covers the case where json is decoded without the UseNumber option (and thus results in float values).
   • adding support for JSON number (tag 284). This covers the case where the UseNumber option is used.

   These changes do not modify the existing behavior since float-to-int is opt-in and support for JSON numbers has to be configured on the cbor encoder.

   Please let me know if you are interested in a PR.

   👍React with 👍1Reacted by David E. Wheeler

   👀React with 👀1Reacted by Faye Amacker
4. benluddy commented on May 19, 2026

   

   benluddy

   on May 19, 2026

   Contributor

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   I coincidentally implemented a FloatToInt option yesterday (without a lenient mode) in #777 and saw your backlink this morning (#439 (comment)). Is there any interest in spinning off the FloatToInt decode option from the proposed IntDecMode changes and json.Number support (@fxamacker @elv-gilles)?

   It's not uncommon for programs in the Kubernetes ecosystem to using encoding/json to unmarshal a JSON representation of an API object from a file into interface{}, then serialize it into an HTTP request body. Since the stdlib unmarshals even integral-valued JSON numbers to float64, and CBOR preserves the distinction between integers and floating point numbers on the wire, the API server needs to be able to accept integral-valued CBOR floats when decoding them into int/uint fields.
5. elv-gilles commented on May 20, 2026

   

   elv-gilles

   on May 20, 2026

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   @benluddy - In my opinion, these changes go together but I feel a bit conflicted:

   • on one hand, a FloatToInt option would already be a good first step (with lenient mode to help preventing random failures)
   • on the other hand, configuring the API server with useNumber would look more like the real fix
6. fxamacker commented on May 21, 2026

   

   fxamacker

   on May 21, 2026

   Owner

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   Is there any interest in spinning off the FloatToInt decode option from the proposed IntDecMode changes and json.Number support (@fxamacker @elv-gilles)?

   @benluddy Yes, I think FloatToInt would be useful to improve JSON and CBOR interoperability.

   @elv-gilles Apologies for the late reply. I took a quick look at your branch and will need to take a closer look.