1 files changed, 81 insertions, 0 deletions
diff --git a/src/encoding/gob/example_interface_test.go b/src/encoding/gob/example_interface_test.go
new file mode 100644
index 000000000..4681e6307
--- /dev/null
+++ b/src/encoding/gob/example_interface_test.go
@@ -0,0 +1,81 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gob_test
+
+import (
+	"bytes"
+	"encoding/gob"
+	"fmt"
+	"log"
+	"math"
+)
+
+type Point struct {
+	X, Y int
+}
+
+func (p Point) Hypotenuse() float64 {
+	return math.Hypot(float64(p.X), float64(p.Y))
+}
+
+type Pythagoras interface {
+	Hypotenuse() float64
+}
+
+// This example shows how to encode an interface value. The key
+// distinction from regular types is to register the concrete type that
+// implements the interface.
+func Example_interface() {
+	var network bytes.Buffer // Stand-in for the network.
+
+	// We must register the concrete type for the encoder and decoder (which would
+	// normally be on a separate machine from the encoder). On each end, this tells the
+	// engine which concrete type is being sent that implements the interface.
+	gob.Register(Point{})
+
+	// Create an encoder and send some values.
+	enc := gob.NewEncoder(&network)
+	for i := 1; i <= 3; i++ {
+		interfaceEncode(enc, Point{3 * i, 4 * i})
+	}
+
+	// Create a decoder and receive some values.
+	dec := gob.NewDecoder(&network)
+	for i := 1; i <= 3; i++ {
+		result := interfaceDecode(dec)
+		fmt.Println(result.Hypotenuse())
+	}
+
+	// Output:
+	// 5
+	// 10
+	// 15
+}
+
+// interfaceEncode encodes the interface value into the encoder.
+func interfaceEncode(enc *gob.Encoder, p Pythagoras) {
+	// The encode will fail unless the concrete type has been
+	// registered. We registered it in the calling function.
+
+	// Pass pointer to interface so Encode sees (and hence sends) a value of
+	// interface type.  If we passed p directly it would see the concrete type instead.
+	// See the blog post, "The Laws of Reflection" for background.
+	err := enc.Encode(&p)
+	if err != nil {
+		log.Fatal("encode:", err)
+	}
+}
+
+// interfaceDecode decodes the next interface value from the stream and returns it.
+func interfaceDecode(dec *gob.Decoder) Pythagoras {
+	// The decode will fail unless the concrete type on the wire has been
+	// registered. We registered it in the calling function.
+	var p Pythagoras
+	err := dec.Decode(&p)
+	if err != nil {
+		log.Fatal("decode:", err)
+	}
+	return p
+}