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README.md

Golang reflect package examples

This repository contains a bunch of examples for dealing with the reflect package. Mainly, for decoding/encoding stuff, and calling functions dynamically.
Most of the examples were taken from projects I worked on in the past, and some from projects I am currently working on.

You will also find informative comments in the examples, that will help you to understand the code. some of them are mine, and some of them were taken from the godoc website.

If you want to contribute to this repository, don't hesitate to create a PR.

The awesome gopher in the logo was taken from @egonelbre/gophers.

Table Of Content

Read struct tags

package main

import (
	"fmt"
	"reflect"
)

type User struct {
	Email  string `mcl:"email"`
	Name   string `mcl:"name"`
	Age    int    `mcl:"age"`
	Github string `mcl:"github" default:"a8m"`
}

func main() {
	var u interface{} = User{}
	// TypeOf returns the reflection Type that represents the dynamic type of u.
	t := reflect.TypeOf(u)
	// Kind returns the specific kind of this type. 
	if t.Kind() != reflect.Struct {
		return
	}
	for i := 0; i < t.NumField(); i++ {
		f := t.Field(i)
		fmt.Println(f.Tag.Get("mcl"), f.Tag.Get("default"))
	}
}

Calling to Kind() can returns one of this list.

Get and set struct fields

package main

import (
	"fmt"
	"reflect"
)

type User struct {
	Email  string `mcl:"email"`
	Name   string `mcl:"name"`
	Age    int    `mcl:"age"`
	Github string `mcl:"github" default:"a8m"`
}

func main() {
	u := &User{Name: "Ariel Mashraki"}
	// Elem returns the value that the pointer u points to.
	v := reflect.ValueOf(u).Elem()
	f := v.FieldByName("Github")
	// make sure that this field is defined, and can be changed.
	if !f.IsValid() || !f.CanSet() {
		return
	}
	if f.Kind() != reflect.String || f.String() != "" {
		return
	}
	f.SetString("a8m")
	fmt.Printf("Github username was changed to: %q\n", u.Github)
}

Fill slice with strings, without knowing its type. Use case: decoder.

package main

import (
	"fmt"
	"io"
	"reflect"
)

func main() {
	var (
		a []string
		b []interface{}
		c []io.Writer
	)
	fmt.Println(fill(&a), a) // pass
	fmt.Println(fill(&b), b) // pass
	fmt.Println(fill(&c), c) // fail
}

func fill(i interface{}) error {
	v := reflect.ValueOf(i)
	if v.Kind() != reflect.Ptr {
		return fmt.Errorf("non-pointer %v", v.Type())
	}
	// get the value that the pointer v points to.
	v = v.Elem()
	if v.Kind() != reflect.Slice {
		return fmt.Errorf("can't fill non-slice value")
	}
	v.Set(reflect.MakeSlice(v.Type(), 3, 3))
	// validate the type of the slice. see below.
	if !canAssign(v.Index(0)) {
		return fmt.Errorf("can't assign string to slice elements")
	}
	for i, w := range []string{"foo", "bar", "baz"} {
		v.Index(i).Set(reflect.ValueOf(w))
	}
	return nil
}

// we accept strings, or empty interfaces.
func canAssign(v reflect.Value) bool {
	return v.Kind() == reflect.String || (v.Kind() == reflect.Interface && v.NumMethod() == 0)
}

Set a value of a number. Use case: decoder.

package main

import (
	"fmt"
	"reflect"
)

const n = 255

func main() {
	var (
		a int8
		b int16
		c uint
		d float32
		e string
	)
	fmt.Println(fill(&a), a)
	fmt.Println(fill(&b), b)
	fmt.Println(fill(&c), c)
	fmt.Println(fill(&d), c)
	fmt.Println(fill(&e), e)
}

func fill(i interface{}) error {
	v := reflect.ValueOf(i)
	if v.Kind() != reflect.Ptr {
		return fmt.Errorf("non-pointer %v", v.Type())
	}
	// get the value that the pointer v points to.
	v = v.Elem()
	switch v.Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		if v.OverflowInt(n) {
			return fmt.Errorf("can't assign value due to %s-overflow", v.Kind())
		}
		v.SetInt(n)
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		if v.OverflowUint(n) {
			return fmt.Errorf("can't assign value due to %s-overflow", v.Kind())
		}
		v.SetUint(n)
	case reflect.Float32, reflect.Float64:
		if v.OverflowFloat(n) {
			return fmt.Errorf("can't assign value due to %s-overflow", v.Kind())
		}
		v.SetFloat(n)
	default:
		return fmt.Errorf("can't assign value to a non-number type")
	}
	return nil
}

Decode key-value pairs into map

package main

import (
	"fmt"
	"reflect"
	"strconv"
	"strings"
)

func main() {
	var (
		m0 = make(map[int]string)
		m1 = make(map[int64]string)
		m2 map[interface{}]string
		m3 map[interface{}]interface{}
		m4 map[bool]string
	)
	s := "1=foo,2=bar,3=baz"
	fmt.Println(decodeMap(s, &m0), m0) // pass
	fmt.Println(decodeMap(s, &m1), m1) // pass
	fmt.Println(decodeMap(s, &m2), m2) // pass
	fmt.Println(decodeMap(s, &m3), m3) // pass
	fmt.Println(decodeMap(s, &m4), m4) // fail
}

func decodeMap(s string, i interface{}) error {
	v := reflect.ValueOf(i)
	if v.Kind() != reflect.Ptr {
		return fmt.Errorf("non-pointer %v", v.Type())
	}
	// get the value that the pointer v points to.
	v = v.Elem()
	t := v.Type()
	// allocate a new map, if v is nil. see: m2, m3, m4.
	if v.IsNil() {
		v.Set(reflect.MakeMap(t))
	}
	// assume that the input is valid.
	for _, kv := range strings.Split(s, ",") {
		s := strings.Split(kv, "=")
		n, err := strconv.Atoi(s[0])
		if err != nil {
			return fmt.Errorf("failed to parse number: %v", err)
		}
		k, e := reflect.ValueOf(n), reflect.ValueOf(s[1])
		// get the type of the key.
		kt := t.Key()
		if !k.Type().ConvertibleTo(kt) {
			return fmt.Errorf("can't convert key to type %v", kt.Kind())
		}
		k = k.Convert(kt)
		// get the element type.
		et := t.Elem()
		if et.Kind() != v.Kind() && !e.Type().ConvertibleTo(et) {
			return fmt.Errorf("can't assign value to type %v", kt.Kind())
		}
		v.SetMapIndex(k, e.Convert(et))
	}
	return nil
}

Decode key-value pairs into struct

package main

import (
	"fmt"
	"reflect"
	"strings"
)

type User struct {
	Name    string
	Github  string
	private string
}

func main() {
	var (
		v0 User
		v1 *User
		v2 = new(User)
		v3 struct{ Name string }
		s  = "Name=Ariel,Github=a8m"
	)
	fmt.Println(decode(s, &v0), v0) // pass
	fmt.Println(decode(s, v1), v1)  // fail
	fmt.Println(decode(s, v2), v2)  // pass
	fmt.Println(decode(s, v3), v3)  // fail
	fmt.Println(decode(s, &v3), v3) // pass
}

func decode(s string, i interface{}) error {
	v := reflect.ValueOf(i)
	if v.Kind() != reflect.Ptr || v.IsNil() {
		return fmt.Errorf("decode requires non-nil pointer")
	}
	// get the value that the pointer v points to.
	v = v.Elem()
	// assume that the input is valid.
	for _, kv := range strings.Split(s, ",") {
		s := strings.Split(kv, "=")
		f := v.FieldByName(s[0])
		// make sure that this field is defined, and can be changed.
		if !f.IsValid() || !f.CanSet() {
			continue
		}
		// assume all the fields are type string.
		f.SetString(s[1])
	}
	return nil
}

Encode struct into key-value pairs

package main

import (
	"fmt"
	"reflect"
	"strings"
)

type User struct {
	Email   string `kv:"email,omitempty"`
	Name    string `kv:"name,omitempty"`
	Github  string `kv:"github,omitempty"`
	private string
}

func main() {
	var (
		u = User{Name: "Ariel", Github: "a8m"}
		v = struct {
			A, B, C string
		}{
			"foo",
			"bar",
			"baz",
		}
		w = &User{}
	)
	fmt.Println(encode(u))
	fmt.Println(encode(v))
	fmt.Println(encode(w))
}

// this example supports only structs, and assume their
// fields are type string.
func encode(i interface{}) (string, error) {
	v := reflect.ValueOf(i)
	t := v.Type()
	if t.Kind() != reflect.Struct {
		return "", fmt.Errorf("type %s is not supported", t.Kind())
	}
	var s []string
	for i := 0; i < t.NumField(); i++ {
		f := t.Field(i)
		// skip unexported fields. from godoc:
		// PkgPath is the package path that qualifies a lower case (unexported)
		// field name. It is empty for upper case (exported) field names.
		if f.PkgPath != "" {
			continue
		}
		fv := v.Field(i)
		key, omit := readTag(f)
		// skip empty values when "omitempty" set.
		if omit && fv.String() == "" {
			continue
		}
		s = append(s, fmt.Sprintf("%s=%s", key, fv.String()))
	}
	return strings.Join(s, ","), nil
}

func readTag(f reflect.StructField) (string, bool) {
	val, ok := f.Tag.Lookup("kv")
	if !ok {
		return f.Name, false
	}
	opts := strings.Split(val, ",")
	omit := false
	if len(opts) == 2 {
		omit = opts[1] == "omitempty"
	}
	return opts[0], omit
}

Check if the underlying type implements an interface

package main

import (
	"fmt"
	"reflect"
)

type Marshaler interface {
	MarshalKV() (string, error)
}

type User struct {
	Email   string `kv:"email,omitempty"`
	Name    string `kv:"name,omitempty"`
	Github  string `kv:"github,omitempty"`
	private string
}

func (u User) MarshalKV() (string, error) {
	return fmt.Sprintf("name=%s,email=%s,github=%s", u.Name, u.Email, u.Github), nil
}

func main() {
	fmt.Println(encode(User{"boring", "Ariel", "a8m", ""}))
	fmt.Println(encode(&User{Github: "posener", Name: "Eyal", Email: "boring"}))
}

var marshalerType = reflect.TypeOf(new(Marshaler)).Elem()

func encode(i interface{}) (string, error) {
	t := reflect.TypeOf(i)
	if !t.Implements(marshalerType) {
		return "", fmt.Errorf("encode only supports structs that implement the Marshaler interface")
	}
	m, _ := reflect.ValueOf(i).Interface().(Marshaler)
	return m.MarshalKV()
}

Wrap a reflect.Value with pointer (T => *T)

package main

import (
	"fmt"
	"reflect"
)

type User struct {
	Name string
}

func main() {
	// T => *T
	u1 := User{"a8m"}
	p1 := ptr(reflect.ValueOf(u1))
	fmt.Println(u1 == p1.Elem().Interface())

	// *T => **T
	u2 := &User{"a8m"}
	p2 := ptr(reflect.ValueOf(u2))
	fmt.Println(*u2 == p2.Elem().Elem().Interface())
}

// ptr wraps the given value with pointer: V => *V, *V => **V, etc.
func ptr(v reflect.Value) reflect.Value {
	pt := reflect.PtrTo(v.Type()) // create a *T type.
	pv := reflect.New(pt.Elem())  // create a reflect.Value of type *T.
	pv.Elem().Set(v)              // sets pv to point to underlying value of v.
	return pv
}

Playground

Function calls

Call method without prameters, and without return value

package main

import (
	"fmt"
	"reflect"
)

type A struct{}

func (A) Hello() { fmt.Println("World") }

func main() {
	// ValueOf returns a new Value, which is the reflection interface to a Go value.
	v := reflect.ValueOf(A{})
	m := v.MethodByName("Hello")
	if m.Kind() != reflect.Func {
		return
	}
	m.Call(nil)
}

Call function with list of arguments, and validate return values

package main

import (
	"fmt"
	"reflect"
)

func Add(a, b int) int { return a + b }

func main() {
	v := reflect.ValueOf(Add)
	if v.Kind() != reflect.Func {
		return
	}
	t := v.Type()
	argv := make([]reflect.Value, t.NumIn())
	for i := range argv {
		// validate the type of parameter "i".
		if t.In(i).Kind() != reflect.Int {
			return
		}
		argv[i] = reflect.ValueOf(i)
	}
	// note that, len(result) == t.NumOut()
	result := v.Call(argv)
	if len(result) != 1 || result[0].Kind() != reflect.Int {
		return
	}
	fmt.Println(result[0].Int())
}

Call to a function dynamically. similar to the template/text package

package main

import (
	"fmt"
	"html/template"
	"reflect"
	"strconv"
	"strings"
)

func main() {
	funcs := template.FuncMap{
		"trim":    strings.Trim,
		"lower":   strings.ToLower,
		"repeat":  strings.Repeat,
		"replace": strings.Replace,
	}
	fmt.Println(eval(`{{ "hello" 4 | repeat }}`, funcs))
	fmt.Println(eval(`{{ "Hello-World" | lower }}`, funcs))
	fmt.Println(eval(`{{ "foobarfoo" "foo" "bar" -1 | replace }}`, funcs))
	fmt.Println(eval(`{{ "-^-Hello-^-" "-^" | trim }}`, funcs))
}

// evaluate an expression. note that this implemetation is assuming that the
// input is valid, and also very limited. for example, whitespaces are not allowed
// inside a quoted string.
func eval(s string, funcs template.FuncMap) (string, error) {
	args, name := parseArgs(s)
	fn, ok := funcs[name]
	if !ok {
		return "", fmt.Errorf("function %s is not defined", name)
	}
	v := reflect.ValueOf(fn)
	t := v.Type()
	if len(args) != t.NumIn() {
		return "", fmt.Errorf("invalid number of arguments. got: %v, want: %v", len(args), t.NumIn())
	}
	argv := make([]reflect.Value, len(args))
	// go over the arguments, validate and build them.
	// note that we support only int, and string in this simple example.
	for i := range argv {
		var argType reflect.Kind
		// if the argument "i" is string.
		if strings.HasPrefix(args[i], "\"") {
			argType = reflect.String
			argv[i] = reflect.ValueOf(strings.Trim(args[i], "\""))
		} else {
			argType = reflect.Int
			// assume that the input is valid.
			num, _ := strconv.Atoi(args[i])
			argv[i] = reflect.ValueOf(num)
		}
		if t.In(i).Kind() != argType {
			return "", fmt.Errorf("Invalid argument. got: %v, want: %v", argType, t.In(i).Kind())
		}
	}
	result := v.Call(argv)
	// in real-world code, we validate it before executing the function,
	// using the v.NumOut() method. similiar to the text/template package.
	if len(result) != 1 || result[0].Kind() != reflect.String {
		return "", fmt.Errorf("function %s must return a one string value", name)
	}
	return result[0].String(), nil
}

// parseArgs is an auxiliary function, that extract the function and its
// parameter from the given expression.
func parseArgs(s string) ([]string, string) {
	args := strings.Split(strings.Trim(s, "{ }"), "|")
	return strings.Split(strings.Trim(args[0], " "), " "), strings.Trim(args[len(args)-1], " ")
}

Call function with variadic parameter

package main

import (
	"fmt"
	"math/rand"
	"reflect"
)

func Sum(x1, x2 int, xs ...int) int {
	sum := x1 + x2
	for _, xi := range xs {
		sum += xi
	}
	return sum
}

func main() {
	v := reflect.ValueOf(Sum)
	if v.Kind() != reflect.Func {
		return
	}
	t := v.Type()
	argc := t.NumIn()
	if t.IsVariadic() {
		argc += rand.Intn(10)
	}
	argv := make([]reflect.Value, argc)
	for i := range argv {
		argv[i] = reflect.ValueOf(i)
	}
	result := v.Call(argv)
	fmt.Println(result[0].Int()) // assume that t.NumOut() > 0 tested above.
}

Create function at runtime

package main

import (
	"fmt"
	"reflect"
)

type Add func(int64, int64) int64

func main() {
	t := reflect.TypeOf(Add(nil))
	mul := reflect.MakeFunc(t, func(args []reflect.Value) []reflect.Value {
		a := args[0].Int()
		b := args[1].Int()
		return []reflect.Value{reflect.ValueOf(a+b)}
	})
	fn, ok := mul.Interface().(Add)
	if !ok {
		return
	}
	fmt.Println(fn(2,3))
}
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