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// Copyright 2012 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 reflect_test
import (
"bytes"
"encoding/json"
"fmt"
"io"
"os"
"reflect"
)
func ExampleKind() {
for _, v := range []any{"hi", 42, func() {}} {
switch v := reflect.ValueOf(v); v.Kind() {
case reflect.String:
fmt.Println(v.String())
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
fmt.Println(v.Int())
default:
fmt.Printf("unhandled kind %s", v.Kind())
}
}
// Output:
// hi
// 42
// unhandled kind func
}
func ExampleMakeFunc() {
// swap is the implementation passed to MakeFunc.
// It must work in terms of reflect.Values so that it is possible
// to write code without knowing beforehand what the types
// will be.
swap := func(in []reflect.Value) []reflect.Value {
return []reflect.Value{in[1], in[0]}
}
// makeSwap expects fptr to be a pointer to a nil function.
// It sets that pointer to a new function created with MakeFunc.
// When the function is invoked, reflect turns the arguments
// into Values, calls swap, and then turns swap's result slice
// into the values returned by the new function.
makeSwap := func(fptr any) {
// fptr is a pointer to a function.
// Obtain the function value itself (likely nil) as a reflect.Value
// so that we can query its type and then set the value.
fn := reflect.ValueOf(fptr).Elem()
// Make a function of the right type.
v := reflect.MakeFunc(fn.Type(), swap)
// Assign it to the value fn represents.
fn.Set(v)
}
// Make and call a swap function for ints.
var intSwap func(int, int) (int, int)
makeSwap(&intSwap)
fmt.Println(intSwap(0, 1))
// Make and call a swap function for float64s.
var floatSwap func(float64, float64) (float64, float64)
makeSwap(&floatSwap)
fmt.Println(floatSwap(2.72, 3.14))
// Output:
// 1 0
// 3.14 2.72
}
func ExampleStructTag() {
type S struct {
F string `species:"gopher" color:"blue"`
}
s := S{}
st := reflect.TypeOf(s)
field := st.Field(0)
fmt.Println(field.Tag.Get("color"), field.Tag.Get("species"))
// Output:
// blue gopher
}
func ExampleStructTag_Lookup() {
type S struct {
F0 string `alias:"field_0"`
F1 string `alias:""`
F2 string
}
s := S{}
st := reflect.TypeOf(s)
for i := 0; i < st.NumField(); i++ {
field := st.Field(i)
if alias, ok := field.Tag.Lookup("alias"); ok {
if alias == "" {
fmt.Println("(blank)")
} else {
fmt.Println(alias)
}
} else {
fmt.Println("(not specified)")
}
}
// Output:
// field_0
// (blank)
// (not specified)
}
func ExampleTypeOf() {
// As interface types are only used for static typing, a
// common idiom to find the reflection Type for an interface
// type Foo is to use a *Foo value.
writerType := reflect.TypeOf((*io.Writer)(nil)).Elem()
fileType := reflect.TypeOf((*os.File)(nil))
fmt.Println(fileType.Implements(writerType))
// Output:
// true
}
func ExampleStructOf() {
typ := reflect.StructOf([]reflect.StructField{
{
Name: "Height",
Type: reflect.TypeOf(float64(0)),
Tag: `json:"height"`,
},
{
Name: "Age",
Type: reflect.TypeOf(int(0)),
Tag: `json:"age"`,
},
})
v := reflect.New(typ).Elem()
v.Field(0).SetFloat(0.4)
v.Field(1).SetInt(2)
s := v.Addr().Interface()
w := new(bytes.Buffer)
if err := json.NewEncoder(w).Encode(s); err != nil {
panic(err)
}
fmt.Printf("value: %+v\n", s)
fmt.Printf("json: %s", w.Bytes())
r := bytes.NewReader([]byte(`{"height":1.5,"age":10}`))
if err := json.NewDecoder(r).Decode(s); err != nil {
panic(err)
}
fmt.Printf("value: %+v\n", s)
// Output:
// value: &{Height:0.4 Age:2}
// json: {"height":0.4,"age":2}
// value: &{Height:1.5 Age:10}
}
func ExampleValue_FieldByIndex() {
// This example shows a case in which the name of a promoted field
// is hidden by another field: FieldByName will not work, so
// FieldByIndex must be used instead.
type user struct {
firstName string
lastName string
}
type data struct {
user
firstName string
lastName string
}
u := data{
user: user{"Embedded John", "Embedded Doe"},
firstName: "John",
lastName: "Doe",
}
s := reflect.ValueOf(u).FieldByIndex([]int{0, 1})
fmt.Println("embedded last name:", s)
// Output:
// embedded last name: Embedded Doe
}
func ExampleValue_FieldByName() {
type user struct {
firstName string
lastName string
}
u := user{firstName: "John", lastName: "Doe"}
s := reflect.ValueOf(u)
fmt.Println("Name:", s.FieldByName("firstName"))
// Output:
// Name: John
}
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