fmt: add example for GoStringer interface

Updates golang/go#27376.

Change-Id: Ia8608561eb6a268aa7eae8c39c7098df100b643a
Reviewed-on: https://go-review.googlesource.com/133075
Reviewed-by: Kevin Burke <kev@inburke.com>
Run-TryBot: Kevin Burke <kev@inburke.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/fmt/gostringer_example_test.go b/src/fmt/gostringer_example_test.go
new file mode 100644 (file)
index 0000000..ab19ee3
--- /dev/null
+++ b/src/fmt/gostringer_example_test.go
@@ -0,0 +1,59 @@
+// Copyright 2018 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 fmt_test
+
+import (
+       "fmt"
+)
+
+// Address has a City, State and a Country.
+type Address struct {
+       City    string
+       State   string
+       Country string
+}
+
+// Person has a Name, Age and Address.
+type Person struct {
+       Name string
+       Age  uint
+       Addr *Address
+}
+
+// GoString makes Person satisfy the GoStringer interface.
+// The return value is valid Go code that can be used to reproduce the Person struct.
+func (p Person) GoString() string {
+       if p.Addr != nil {
+               return fmt.Sprintf("Person{Name: %q, Age: %d, Addr: &Address{City: %q, State: %q, Country: %q}}", p.Name, int(p.Age), p.Addr.City, p.Addr.State, p.Addr.Country)
+       }
+       return fmt.Sprintf("Person{Name: %q, Age: %d}", p.Name, int(p.Age))
+}
+
+func ExampleGoStringer() {
+       p1 := Person{
+               Name: "Warren",
+               Age:  31,
+               Addr: &Address{
+                       City:    "Denver",
+                       State:   "CO",
+                       Country: "U.S.A.",
+               },
+       }
+       // If GoString() wasn't implemented, the output of `fmt.Printf("%#v", p1)` would be similar to
+       // Person{Name:"Warren", Age:0x1f, Addr:(*main.Address)(0x10448240)}
+       fmt.Printf("%#v\n", p1)
+
+       p2 := Person{
+               Name: "Theia",
+               Age:  4,
+       }
+       // If GoString() wasn't implemented, the output of `fmt.Printf("%#v", p2)` would be similar to
+       // Person{Name:"Theia", Age:0x4, Addr:(*main.Address)(nil)}
+       fmt.Printf("%#v\n", p2)
+
+       // Output:
+       // Person{Name: "Warren", Age: 31, Addr: &Address{City: "Denver", State: "CO", Country: "U.S.A."}}
+       // Person{Name: "Theia", Age: 4}
+}