Add encoder options to serialize 'nil' maps/slices as empty object/array.

gen/encoder.go

@@ -121,7 +121,7 @@ func (g *Generator) genTypeEncoderNoCheck(t reflect.Type, in string, tags fieldT
 		if t.Elem().Kind() == reflect.Uint8 {
 			fmt.Fprintln(g.out, ws+"out.Base64Bytes("+in+")")
 		} else {
-			fmt.Fprintln(g.out, ws+"if "+in+" == nil {")
+			fmt.Fprintln(g.out, ws+"if "+in+" == nil && (out.Flags & jwriter.NilSliceAsEmpty) == 0 {")
 			fmt.Fprintln(g.out, ws+`  out.RawString("null")`)
 			fmt.Fprintln(g.out, ws+"} else {")
 			fmt.Fprintln(g.out, ws+"  out.RawByte('[')")
@@ -178,7 +178,7 @@ func (g *Generator) genTypeEncoderNoCheck(t reflect.Type, in string, tags fieldT
 		}
 		tmpVar := g.uniqueVarName()
 
-		fmt.Fprintln(g.out, ws+"if "+in+" == nil {")
+		fmt.Fprintln(g.out, ws+"if "+in+" == nil && (out.Flags & jwriter.NilMapAsEmpty) == 0 {")
 		fmt.Fprintln(g.out, ws+"  out.RawString(`null`)")
 		fmt.Fprintln(g.out, ws+"} else {")
 		fmt.Fprintln(g.out, ws+"  out.RawByte('{')")

jwriter/writer.go

@@ -10,8 +10,19 @@ import (
 	"github.com/mailru/easyjson/buffer"
 )
 
+// Flags describe various encoding options. The behavior may be actually implemented in the encoder, but
+// Flags field in Writer is used to set and pass them around.
+type Flags int
+
+const (
+	NilMapAsEmpty   Flags = 1 << iota // Encode nil map as '{}' rather than 'null'.
+	NilSliceAsEmpty                   // Encode nil slice as '[]' rather than 'null'.
+)
+
 // Writer is a JSON writer.
 type Writer struct {
+	Flags Flags
+
 	Error  error
 	Buffer buffer.Buffer
 }

tests/basic_test.go

@@ -156,3 +156,30 @@ func TestUnderflowArray(t *testing.T) {
 		t.Errorf("Unmarshal(%v) = %+v; want %+v", arrayUnderflowString, a, arrayUnderflowValue)
 	}
 }
+
+func TestEncodingFlags(t *testing.T) {
+	for i, test := range []struct {
+		Flags jwriter.Flags
+		In    easyjson.Marshaler
+		Want  string
+	}{
+		{0, EncodingFlagsTestMap{}, `{"F":null}`},
+		{0, EncodingFlagsTestSlice{}, `{"F":null}`},
+		{jwriter.NilMapAsEmpty, EncodingFlagsTestMap{}, `{"F":{}}`},
+		{jwriter.NilSliceAsEmpty, EncodingFlagsTestSlice{}, `{"F":[]}`},
+	} {
+		w := &jwriter.Writer{Flags: test.Flags}
+		test.In.MarshalEasyJSON(w)
+
+		data, err := w.BuildBytes()
+		if err != nil {
+			t.Errorf("[%v] easyjson.Marshal(%+v) error: %v", i, test.In, err)
+		}
+
+		v := string(data)
+		if v != test.Want {
+			t.Errorf("[%v] easyjson.Marshal(%+v) = %v; want %v", i, test.In, v, test.Want)
+		}
+	}
+
+}

tests/data.go

@@ -624,3 +624,13 @@ type RequiredOptionalStruct struct {
 	FirstName string `json:"first_name,required"`
 	Lastname  string `json:"last_name"`
 }
+
+//easyjson:json
+type EncodingFlagsTestMap struct {
+	F map[string]string
+}
+
+//easyjson:json
+type EncodingFlagsTestSlice struct {
+	F []string
+}