allow custom encoders to handle nil pointers

If a custom encoder is defined on a pointer of a type, allow that
encoder to handle nil values.  Encoders defined on non-pointer types
will continue to be given the zero value of the type to encode (the
existing behavior).

Fixes #46

query/encode.go

@@ -178,15 +178,10 @@ func reflectValue(values url.Values, val reflect.Value, scope string) error {
 			continue
 		}
 
-		for sv.Kind() == reflect.Ptr {
-			if sv.IsNil() {
-				break
-			}
-			sv = sv.Elem()
-		}
-
 		if sv.Type().Implements(encoderType) {
-			if !reflect.Indirect(sv).IsValid() {
+			// if sv is a nil pointer and the custom encoder is defined on a non-pointer
+			// method receiver, set sv to the zero value of the underlying type
+			if !reflect.Indirect(sv).IsValid() && sv.Type().Elem().Implements(encoderType) {
 				sv = reflect.New(sv.Type().Elem())
 			}
 
@@ -197,6 +192,14 @@ func reflectValue(values url.Values, val reflect.Value, scope string) error {
 			continue
 		}
 
+		// recursively dereference pointers. break on nil pointers
+		for sv.Kind() == reflect.Ptr {
+			if sv.IsNil() {
+				break
+			}
+			sv = sv.Elem()
+		}
+
 		if sv.Kind() == reflect.Slice || sv.Kind() == reflect.Array {
 			var del string
 			if opts.Contains("comma") {

query/encode_test.go

@@ -553,9 +553,10 @@ type customEncodedIntPtr int
 // values cause an error.
 func (m *customEncodedIntPtr) EncodeValues(key string, v *url.Values) error {
 	if m == nil {
-		return nil
+		v.Set(key, "undefined")
+	} else {
+		v.Set(key, fmt.Sprintf("_%d", *m))
 	}
-	v.Set(key, fmt.Sprintf("_%d", *m))
 	return nil
 }
 
@@ -594,7 +595,7 @@ func TestValues_CustomEncodingPointer(t *testing.T) {
 			struct {
 				V *customEncodedIntPtr `url:"v"`
 			}{},
-			url.Values{"v": {"_0"}},
+			url.Values{"v": {"undefined"}},
 		},
 		{
 			struct {
@@ -606,19 +607,19 @@ func TestValues_CustomEncodingPointer(t *testing.T) {
 			struct {
 				V *customEncodedIntPtr `url:"v"`
 			}{&zero},
-			url.Values{"v": {"0"}},
+			url.Values{"v": {"_0"}},
 		},
 		{
 			struct {
 				V *customEncodedIntPtr `url:"v,omitempty"`
 			}{&zero},
-			url.Values{"v": {"0"}},
+			url.Values{"v": {"_0"}},
 		},
 		{
 			struct {
 				V *customEncodedIntPtr `url:"v"`
 			}{&one},
-			url.Values{"v": {"1"}},
+			url.Values{"v": {"_1"}},
 		},
 	}