Feature allow unset pointer

[rostislaved]

Description

This pull request adds a config flag AllowUnsetPointer to allow pointer fields in structs to be treated as optional during decoding. When enabled, missing pointer fields will not be reported as unset even if ErrorUnset is true.

This is useful when decoding into structs with optional pointer fields, while still maintaining strict handling (ErrorUnset) for required non-pointer fields.

---

Current behavior example

type Config struct {
	URL  *string
	Port int
}

input := map[string]any{
	"Port": 8080,
}

var cfg Config

dec, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
	ErrorUnset: true,
	Result:     &cfg,
})
if err != nil {
	panic(err)
}

err = dec.Decode(input)
// err is not nil: "has unset fields: URL"

New behavior with AllowUnsetPointer: true

type Config struct {
	URL  *string
	Port int
}

input := map[string]any{
	"Port": 8080,
}

var cfg Config

dec, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
	ErrorUnset:        true,
	AllowUnsetPointer: true,
	Result:            &cfg,
})
if err != nil {
	panic(err)
}

err = dec.Decode(input)
// err is nil
// cfg.URL == nil
// cfg.Port == 8080

[sagikazarmark]

I think it would be more elegant to continue here and leave setting the map after the condition.

WDYT?

[rostislaved]

Hi! Did you mean like this?

		if !rawMapVal.IsValid() {
				// There was no matching key in the map for the value in
				// the struct. Remember it for potential errors and metadata.
				if !(d.config.AllowUnsetPointer && fieldValue.Kind() == reflect.Ptr) {
					targetValKeysUnused[fieldName] = struct{}{}

					continue
				}

				continue
			}

If yes, I am fine with this

[sagikazarmark]

I think the correct implementation would be:

		if !rawMapVal.IsValid() {
				if d.config.AllowUnsetPointer && fieldValue.Kind() == reflect.Ptr {
					continue
				}

				// There was no matching key in the map for the value in
				// the struct. Remember it for potential errors and metadata.
				targetValKeysUnused[fieldName] = struct{}{}


				continue
		}

Or did I misread what it does?

[rostislaved]

Your implementation did the thing. However, if we set AllowUnsetPointer == true and use "continue" we won't add the fieldName to the set and it seems to be formally incorrect, because there is still no "matching key in the map for the value in the struct".

[rostislaved]

@sagikazarmark What do you think?

[rostislaved]

@sagikazarmark could you please have a look?