Add support for pointer fields.

README.md

@@ -384,6 +384,20 @@ var CLI struct {
 
 For flags, multiple key+value pairs should be separated by `mapsep:"rune"` tag (defaults to `;`) eg. `--set="key1=value1;key2=value2"`.
 
+## Pointers
+
+Pointers work like the underlying type, except that you can differentiate between the presence of the zero value and no value being supplied.
+
+For example:
+
+```go
+var CLI struct {
+	Foo *int
+}
+```
+
+Would produce a nil value for `Foo` if no `--foo` argument is supplied, but would have a pointer to the value 0 if the argument `--foo=0` was supplied.
+
 ## Nested data structure
 
 Kong support a nested data structure as well with `embed:""`. You can combine `embed:""` with `prefix:""`:

kong_test.go

@@ -1542,3 +1542,106 @@ func TestPassthroughCmdOnlyStringArgs(t *testing.T) {
 	_, err := kong.New(&cli)
 	require.EqualError(t, err, "<anonymous struct>.Command: passthrough command command [<args> ...] must contain exactly one positional argument of []string type")
 }
+
+func TestStringPointer(t *testing.T) {
+	var cli struct {
+		Foo *string
+	}
+	k, err := kong.New(&cli)
+	require.NoError(t, err)
+	require.NotNil(t, k)
+	ctx, err := k.Parse([]string{"--foo", "wtf"})
+	require.NoError(t, err)
+	require.NotNil(t, ctx)
+	require.NotNil(t, cli.Foo)
+	require.Equal(t, "wtf", *cli.Foo)
+}
+
+func TestStringPointerNoValue(t *testing.T) {
+	var cli struct {
+		Foo *string
+	}
+	k, err := kong.New(&cli)
+	require.NoError(t, err)
+	require.NotNil(t, k)
+	ctx, err := k.Parse([]string{})
+	require.NoError(t, err)
+	require.NotNil(t, ctx)
+	require.Nil(t, cli.Foo)
+}
+
+func TestStringPointerDefault(t *testing.T) {
+	var cli struct {
+		Foo *string `default:"stuff"`
+	}
+	k, err := kong.New(&cli)
+	require.NoError(t, err)
+	require.NotNil(t, k)
+	ctx, err := k.Parse([]string{})
+	require.NoError(t, err)
+	require.NotNil(t, ctx)
+	require.NotNil(t, cli.Foo)
+	require.Equal(t, "stuff", *cli.Foo)
+}
+
+func TestStringPointerAliasNoValue(t *testing.T) {
+	type Foo string
+	var cli struct {
+		F *Foo
+	}
+	k, err := kong.New(&cli)
+	require.NoError(t, err)
+	require.NotNil(t, k)
+	ctx, err := k.Parse([]string{})
+	require.NoError(t, err)
+	require.NotNil(t, ctx)
+	require.Nil(t, cli.F)
+}
+
+func TestStringPointerAlias(t *testing.T) {
+	type Foo string
+	var cli struct {
+		F *Foo
+	}
+	k, err := kong.New(&cli)
+	require.NoError(t, err)
+	require.NotNil(t, k)
+	ctx, err := k.Parse([]string{"--f=value"})
+	require.NoError(t, err)
+	require.NotNil(t, ctx)
+	require.NotNil(t, cli.F)
+	require.Equal(t, Foo("value"), *cli.F)
+}
+
+func TestStringPointerEmptyValue(t *testing.T) {
+	var cli struct {
+		F *string
+		G *string
+	}
+	k, err := kong.New(&cli)
+	require.NoError(t, err)
+	require.NotNil(t, k)
+	ctx, err := k.Parse([]string{"--f", "", "--g="})
+	require.NoError(t, err)
+	require.NotNil(t, ctx)
+	require.NotNil(t, cli.F)
+	require.NotNil(t, cli.G)
+	require.Equal(t, "", *cli.F)
+	require.Equal(t, "", *cli.G)
+}
+
+func TestIntPtr(t *testing.T) {
+	var cli struct {
+		F *int
+		G *int
+	}
+	k, err := kong.New(&cli)
+	require.NoError(t, err)
+	require.NotNil(t, k)
+	ctx, err := k.Parse([]string{"--f=6"})
+	require.NoError(t, err)
+	require.NotNil(t, ctx)
+	require.NotNil(t, cli.F)
+	require.Nil(t, cli.G)
+	require.Equal(t, 6, *cli.F)
+}

mapper.go

@@ -275,7 +275,8 @@ func (r *Registry) RegisterDefaults() *Registry {
 		RegisterName("path", pathMapper(r)).
 		RegisterName("existingfile", existingFileMapper(r)).
 		RegisterName("existingdir", existingDirMapper(r)).
-		RegisterName("counter", counterMapper())
+		RegisterName("counter", counterMapper()).
+		RegisterKind(reflect.Ptr, ptrMapper(r))
 }
 
 type boolMapper struct{}
@@ -654,6 +655,22 @@ func existingDirMapper(r *Registry) MapperFunc {
 	}
 }
 
+func ptrMapper(r *Registry) MapperFunc {
+	return func(ctx *DecodeContext, target reflect.Value) error {
+		elem := reflect.New(target.Type().Elem()).Elem()
+		nestedMapper := r.ForValue(elem)
+		if nestedMapper == nil {
+			return errors.New("cannot find member for pointer element type")
+		}
+		err := nestedMapper.Decode(ctx, elem)
+		if err != nil {
+			return err
+		}
+		target.Set(elem.Addr())
+		return nil
+	}
+}
+
 func counterMapper() MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
 		if ctx.Scan.Peek().Type == FlagValueToken {