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