Support default for flag.Value implementations registered via AddStruct

flag_set.go

@@ -458,16 +458,17 @@ func (cfg FlagConfig) getPlaceholder() string {
 	}
 
 	// Otherwise, use a transformation of the flag value type name.
-	var typeName string
-	{
-		typeName = strings.ToUpper(fmt.Sprintf("%T", cfg.Value))
-		typeName = genericTypeNameRegexp.ReplaceAllString(typeName, "$1")
-		typeName = strings.TrimSuffix(typeName, "VALUE")
-		if lastDot := strings.LastIndex(typeName, "."); lastDot > 0 {
-			typeName = typeName[lastDot+1:]
-		}
+	return reflectPlaceholder(cfg.Value)
+}
+
+func reflectPlaceholder(v flag.Value) (typeName string) {
+	typeName = strings.ToUpper(fmt.Sprintf("%T", v))
+	typeName = genericTypeNameRegexp.ReplaceAllString(typeName, "$1")
+	typeName = strings.TrimSuffix(typeName, "VALUE")
+	if lastDot := strings.LastIndex(typeName, "."); lastDot > 0 {
+		typeName = typeName[lastDot+1:]
 	}
-	return typeName
+	return
 }
 
 func (cfg FlagConfig) getHelpDefault() string {
@@ -721,7 +722,7 @@ func (fs *FlagSet) AddStruct(val any) error {
 			)
 			if fieldValAddrTyp.Implements(flagValueElemTyp) {
 				// The field implements flag.Value, we can use it directly.
-				cfg.Value = fieldValAddrIface.(flag.Value)
+				cfg.Value = &defaultValue{Value: fieldValAddrIface.(flag.Value), def: def}
 			} else {
 				// Try to construct a new flag value.
 				v, err := ffval.NewValueReflect(fieldValAddrIface, def)
@@ -746,6 +747,34 @@ func (fs *FlagSet) AddStruct(val any) error {
 	return nil
 }
 
+type defaultValue struct {
+	flag.Value
+
+	isSet bool
+	def   string
+}
+
+func (d *defaultValue) GetPlaceholder() string {
+	return reflectPlaceholder(d.Value)
+}
+
+func (d *defaultValue) String() string {
+	if d.isSet {
+		return d.Value.String()
+	}
+
+	return d.def
+}
+
+func (d *defaultValue) Set(s string) error {
+	if err := d.Value.Set(s); err != nil {
+		return err
+	}
+
+	d.isSet = true
+	return nil
+}
+
 // Value defines a new flag in the flag set, and panics on any error.
 func (fs *FlagSet) Value(short rune, long string, value flag.Value, usage string) Flag {
 	f, err := fs.AddFlag(FlagConfig{

flag_set_test.go

@@ -317,9 +317,10 @@ func TestFlagSet_structs(t *testing.T) {
 		Beta  int    `ff:"          long: beta,  placeholder: β,         usage: beta int"`
 		Delta bool   `ff:"short: d,              nodefault,              usage: delta bool"`
 
-		Epsilon bool    `ff:"| short=e | long=epsilon | nodefault    | usage: epsilon bool          |"`
-		Gamma   string  `ff:"| short=g | long=gamma   |              | usage: 'usage, with a comma' |"`
-		Iota    float64 `ff:"|         | long=iota    | default=0.43 | usage: iota float            |"`
+		Epsilon bool        `ff:"| short=e | long=epsilon | nodefault    | usage: epsilon bool              |"`
+		Gamma   string      `ff:"| short=g | long=gamma   |              | usage: 'usage, with a comma'     |"`
+		Iota    float64     `ff:"|         | long=iota    | default=0.43 | usage: iota float                |"`
+		Value   customValue `ff:"|         | long=value   | default=abcd | usage: some value implementation |"`
 	}
 
 	var flags myFlags
@@ -336,6 +337,7 @@ func TestFlagSet_structs(t *testing.T) {
 		  -e, --epsilon        epsilon bool
 		  -g, --gamma STRING   usage, with a comma
 		      --iota FLOAT64   iota float (default: 0.43)
+		      --value CUSTOM   some value implementation (default: abcd)
 	`), fftest.UnindentString(ffhelp.Flags(fs).String()); want != have {
 		t.Error(fftest.DiffString(want, have))
 	}
@@ -346,15 +348,19 @@ func TestFlagSet_structs(t *testing.T) {
 	}{
 		{
 			args: "--alpha=x",
-			want: myFlags{Alpha: "x", Iota: 0.43},
+			want: myFlags{Alpha: "x", Iota: 0.43, Value: "abcd"},
 		},
 		{
 			args: "-e --iota=1.23",
-			want: myFlags{Alpha: "alpha-default", Epsilon: true, Iota: 1.23},
+			want: myFlags{Alpha: "alpha-default", Epsilon: true, Iota: 1.23, Value: "abcd"},
 		},
 		{
 			args: "-gabc -d",
-			want: myFlags{Alpha: "alpha-default", Delta: true, Gamma: "abc", Iota: 0.43},
+			want: myFlags{Alpha: "alpha-default", Delta: true, Gamma: "abc", Iota: 0.43, Value: "abcd"},
+		},
+		{
+			args: "--value=bcde",
+			want: myFlags{Alpha: "alpha-default", Iota: 0.43, Value: "bcde"},
 		},
 	} {
 		t.Run(testcase.args, func(t *testing.T) {
@@ -593,3 +599,14 @@ func TestFlagSet_Std(t *testing.T) {
 		t.Errorf("flag names: want %v, have %v", want, have)
 	}
 }
+
+type customValue string
+
+func (v *customValue) String() string {
+	return (string)(*v)
+}
+
+func (v *customValue) Set(s string) error {
+	*v = customValue(s)
+	return nil
+}