Reflections in Go

Overview of Go Reflection

The reflect.Type Type and Values

package main

import "fmt"
import "reflect"

func main() {
	type A = [16]int16
	var c <-chan map[A][]byte
	tc := reflect.TypeOf(c)
	fmt.Println(tc.Kind())    // chan
	fmt.Println(tc.ChanDir()) // <-chan
	tm := tc.Elem()
	ta, tb := tm.Key(), tm.Elem()
	// The next line prints: map array slice
	fmt.Println(tm.Kind(), ta.Kind(), tb.Kind())
	tx, ty := ta.Elem(), tb.Elem()

	// byte is an alias of uint8
	fmt.Println(tx.Kind(), ty.Kind()) // int16 uint8
	fmt.Println(tx.Bits(), ty.Bits()) // 16 8
	fmt.Println(tx.ConvertibleTo(ty)) // true
	fmt.Println(tb.ConvertibleTo(ta)) // false

	// Slice and map types are incomparable.
	fmt.Println(tb.Comparable()) // false
	fmt.Println(tm.Comparable()) // false
	fmt.Println(ta.Comparable()) // true
	fmt.Println(tc.Comparable()) // true
}

package main

import "fmt"
import "reflect"

type T []interface{m()}
func (T) m() {}

func main() {
	tp := reflect.TypeOf(new(interface{}))
	tt := reflect.TypeOf(T{})
	fmt.Println(tp.Kind(), tt.Kind()) // ptr slice

	// Get two interface Types indirectly.
	ti, tim := tp.Elem(), tt.Elem()
	// The next line prints: interface interface
	fmt.Println(ti.Kind(), tim.Kind())

	fmt.Println(tt.Implements(tim))  // true
	fmt.Println(tp.Implements(tim))  // false
	fmt.Println(tim.Implements(tim)) // true

	// All types implement any blank interface type.
	fmt.Println(tp.Implements(ti))  // true
	fmt.Println(tt.Implements(ti))  // true
	fmt.Println(tim.Implements(ti)) // true
	fmt.Println(ti.Implements(ti))  // true
}

package main

import "fmt"
import "reflect"

type F func(string, int) bool
func (f F) m(s string) bool {
	return f(s, 32)
}
func (f F) M() {}

type I interface{m(s string) bool; M()}

func main() {
	var x struct {
		F F
		i I
	}
	tx := reflect.TypeOf(x)
	fmt.Println(tx.Kind())        // struct
	fmt.Println(tx.NumField())    // 2
	fmt.Println(tx.Field(1).Name) // i
	// Package path is an intrinsic property of
	// non-exported selectors (fields or methods).
	fmt.Println(tx.Field(0).PkgPath) // 
	fmt.Println(tx.Field(1).PkgPath) // main

	tf, ti := tx.Field(0).Type, tx.Field(1).Type
	fmt.Println(tf.Kind())               // func
	fmt.Println(tf.IsVariadic())         // false
	fmt.Println(tf.NumIn(), tf.NumOut()) // 2 1
	t0, t1, t2 := tf.In(0), tf.In(1), tf.Out(0)
	// The next line prints: string int bool
	fmt.Println(t0.Kind(), t1.Kind(), t2.Kind())

	fmt.Println(tf.NumMethod(), ti.NumMethod()) // 1 2
	fmt.Println(tf.Method(0).Name)              // M
	fmt.Println(ti.Method(1).Name)              // m
	_, ok1 := tf.MethodByName("m")
	_, ok2 := ti.MethodByName("m")
	fmt.Println(ok1, ok2) // false true
}

1. for non-interface types, the reflect.Type.NumMethod only returns the number of exported methods (including implicitly declared ones) of a type. We are unable to get the information of a non-exported method by using the reflect.Type.MethodByName method. For interface types, the limits don't exist (the fact was not mentioned in the docs of the two methods before Go 1.16). Such situation also applies to the corresponding methods of the reflect.Value type introduced in the next section.
2. although a reflect.Type.NumField method call returns the number of all fields (including non-exported ones) of a struct type, it is not a good idea to use the reflect.Type.FieldByName method to get the information of a non-exported field.

package main

import "fmt"
import "reflect"

type T struct {
	X    int  `max:"99" min:"0" default:"0"`
	Y, Z bool `optional:"yes"`
}

func main() {
	t := reflect.TypeOf(T{})
	x := t.Field(0).Tag
	y := t.Field(1).Tag
	z := t.Field(2).Tag
	fmt.Println(reflect.TypeOf(x)) // reflect.StructTag
	// v is a string
	v, present := x.Lookup("max")     
	fmt.Println(len(v), present)      // 2 true
	fmt.Println(x.Get("max"))         // 99
	fmt.Println(x.Lookup("optional")) //  false
	fmt.Println(y.Lookup("optional")) // yes true
	fmt.Println(z.Lookup("optional")) // yes true
}

• tag keys may not contain space (Unicode value 32), quote (Unicode value 34) and colon (Unicode value 58) characters.
• to form a valid key-value pair, no space characters are allowed to follow the semicolon in the supposed key-value pair. So
  `optional: "yes"` doesn't form key-value pairs.
• space characters in tag values are important (not be ignored). So
  `json:"author, omitempty"`,
  `json:" author,omitempty"` and
  `json:"author,omitempty"` are different from each other.
• each struct field tag should present as a single line to be wholly meaningful.

package main

import "fmt"
import "reflect"

func main() {
	ta := reflect.ArrayOf(5, reflect.TypeOf(123))
	fmt.Println(ta) // [5]int
	tc := reflect.ChanOf(reflect.SendDir, ta)
	fmt.Println(tc) // chan<- [5]int
	tp := reflect.PtrTo(ta)
	fmt.Println(tp) // *[5]int
	ts := reflect.SliceOf(tp)
	fmt.Println(ts) // []*[5]int
	tm := reflect.MapOf(ta, tc)
	fmt.Println(tm) // map[[5]int]chan<- [5]int
	tf := reflect.FuncOf([]reflect.Type{ta},
				[]reflect.Type{tp, tc}, false)
	fmt.Println(tf) // func([5]int) (*[5]int, chan<- [5]int)
	tt := reflect.StructOf([]reflect.StructField{
		{Name: "Age", Type: reflect.TypeOf("abc")},
	})
	fmt.Println(tt)            // struct { Age string }
	fmt.Println(tt.NumField()) // 1
}

• There are no ways to create interface types through reflection.
• Although we can create a struct type with embedding fields through reflection, the way has many pitfalls.

The reflect.Value Type and Values

package main

import "fmt"
import "reflect"

func main() {
	n := 123
	p := &n
	vp := reflect.ValueOf(p)
	fmt.Println(vp.CanSet(), vp.CanAddr()) // false false
	vn := vp.Elem() // get the value referenced by vp
	fmt.Println(vn.CanSet(), vn.CanAddr()) // true true
	vn.Set(reflect.ValueOf(789)) // <=> vn.SetInt(789)
	fmt.Println(n)               // 789
}

package main

import "fmt"
import "reflect"

func main() {
	var s struct {
		X interface{} // an exported field
		y interface{} // a non-exported field
	}
	vp := reflect.ValueOf(&s)
	// If vp represents a pointer. the following
	// line is equivalent to "vs := vp.Elem()".
	vs := reflect.Indirect(vp)
	// vx and vy both represent interface values.
	vx, vy := vs.Field(0), vs.Field(1)
	fmt.Println(vx.CanSet(), vx.CanAddr()) // true true
	// vy is addressable but not modifiable.
	fmt.Println(vy.CanSet(), vy.CanAddr()) // false true
	vb := reflect.ValueOf(123)
	vx.Set(vb)     // okay, for vx is modifiable
	// vy.Set(vb)  // will panic, for vy is unmodifiable
	fmt.Println(s) // {123 <nil>}
	fmt.Println(vx.IsNil(), vy.IsNil()) // false true
}

1. One way is by calling the Elem method of a reflect.Value value which represents the pointer value.
2. The other way is to pass a reflect.Value value which represents the pointer value to a reflect.Indirect function call. (If the argument passed to a reflect.Indirect function call doesn't represent a pointer value, then the call returns a copy of the argument.)

package main

import "fmt"
import "reflect"

func main() {
	var z = 123
	var y = &z
	var x interface{} = y
	v := reflect.ValueOf(&x)
	vx := v.Elem()
	vy := vx.Elem()
	vz := vy.Elem()
	vz.Set(reflect.ValueOf(789))
	fmt.Println(z) // 789
}

package main

import "fmt"
import "reflect"

func InvertSlice(args []reflect.Value) []reflect.Value {
	inSlice, n := args[0], args[0].Len()
	outSlice := reflect.MakeSlice(inSlice.Type(), 0, n)
	for i := n-1; i >= 0; i-- {
		element := inSlice.Index(i)
		outSlice = reflect.Append(outSlice, element)
	}
	return []reflect.Value{outSlice}
}

func Bind(p interface{},
		f func ([]reflect.Value) []reflect.Value) {
	// invert represents a function value.
	invert := reflect.ValueOf(p).Elem()
	invert.Set(reflect.MakeFunc(invert.Type(), f))
}

func main() {
	var invertInts func([]int) []int
	Bind(&invertInts, InvertSlice)
	fmt.Println(invertInts([]int{2, 3, 5})) // [5 3 2]

	var invertStrs func([]string) []string
	Bind(&invertStrs, InvertSlice)
	fmt.Println(invertStrs([]string{"Go", "C"})) // [C Go]
}

package main

import "fmt"
import "reflect"

type T struct {
	A, b int
}

func (t T) AddSubThenScale(n int) (int, int) {
	return n * (t.A + t.b), n * (t.A - t.b)
}

func main() {
	t := T{5, 2}
	vt := reflect.ValueOf(t)
	vm := vt.MethodByName("AddSubThenScale")
	results := vm.Call([]reflect.Value{reflect.ValueOf(3)})
	fmt.Println(results[0].Int(), results[1].Int()) // 21 9

	neg := func(x int) int {
		return -x
	}
	vf := reflect.ValueOf(neg)
	fmt.Println(vf.Call(results[:1])[0].Int()) // -21
	fmt.Println(vf.Call([]reflect.Value{
		vt.FieldByName("A"), // panic on changing to "b"
	})[0].Int()) // -5
}

package main

import "fmt"
import "reflect"

func main() {
	valueOf := reflect.ValueOf
	m := map[string]int{"Unix": 1973, "Windows": 1985}
	v := valueOf(m)
	// A zero second Value argument means to delete an entry.
	v.SetMapIndex(valueOf("Windows"), reflect.Value{})
	v.SetMapIndex(valueOf("Linux"), valueOf(1991))
	for i := v.MapRange(); i.Next(); {
		fmt.Println(i.Key(), "\t:", i.Value())
	}
}

package main

import "fmt"
import "reflect"

func main() {
	c := make(chan string, 2)
	vc := reflect.ValueOf(c)
	vc.Send(reflect.ValueOf("C"))
	succeeded := vc.TrySend(reflect.ValueOf("Go"))
	fmt.Println(succeeded) // true
	succeeded = vc.TrySend(reflect.ValueOf("C++"))
	fmt.Println(succeeded) // false
	fmt.Println(vc.Len(), vc.Cap()) // 2 2
	vs, succeeded := vc.TryRecv()
	fmt.Println(vs.String(), succeeded) // C true
	vs, sentBeforeClosed := vc.Recv()
	fmt.Println(vs.String(), sentBeforeClosed) // Go true
	vs, succeeded = vc.TryRecv()
	fmt.Println(vs.String()) // <invalid Value>
	fmt.Println(succeeded)   // false
}

package main

import "fmt"
import "reflect"

func main() {
	c := make(chan int, 1)
	vc := reflect.ValueOf(c)
	succeeded := vc.TrySend(reflect.ValueOf(123))
	fmt.Println(succeeded, vc.Len(), vc.Cap()) // true 1 1

	vSend, vZero := reflect.ValueOf(789), reflect.Value{}
	branches := []reflect.SelectCase{
		{Dir: reflect.SelectDefault, Chan: vZero, Send: vZero},
		{Dir: reflect.SelectRecv, Chan: vc, Send: vZero},
		{Dir: reflect.SelectSend, Chan: vc, Send: vSend},
	}
	selIndex, vRecv, sentBeforeClosed := reflect.Select(branches)
	fmt.Println(selIndex)         // 1
	fmt.Println(sentBeforeClosed) // true
	fmt.Println(vRecv.Int())      // 123
	vc.Close()
	// Remove the send case branch this time,
	// for it may cause panic.
	selIndex, _, sentBeforeClosed = reflect.Select(branches[:2])
	fmt.Println(selIndex, sentBeforeClosed) // 1 false
}

package main

import "reflect"
import "fmt"

func main() {
	var z reflect.Value // a zero Value value
	fmt.Println(z)      // <invalid reflect.Value>
	v := reflect.ValueOf((*int)(nil)).Elem()
	fmt.Println(v)      // <invalid reflect.Value>
	fmt.Println(v == z) // true
	var i = reflect.ValueOf([]interface{}{nil}).Index(0)
	fmt.Println(i)             // <nil>
	fmt.Println(i.Elem() == z) // true
	fmt.Println(i.Elem())      // <invalid reflect.Value>
}

package main

import (
	"fmt"
	"reflect"
	"time"
)

func main() {
	vx := reflect.ValueOf(123)
	vy := reflect.ValueOf("abc")
	vz := reflect.ValueOf([]bool{false, true})
	vt := reflect.ValueOf(time.Time{})

	x := vx.Interface().(int)
	y := vy.Interface().(string)
	z := vz.Interface().([]bool)
	m := vt.MethodByName("IsZero").Interface().(func() bool)
	fmt.Println(x, y, z, m()) // 123 abc [false true] true

	type T struct {x int}
	t := &T{3}
	v := reflect.ValueOf(t).Elem().Field(0)
	fmt.Println(v)             // 3
	fmt.Println(v.Interface()) // panic
}

package main

import (
	"fmt"
	"reflect"
)

func main() {
	s := reflect.ValueOf([]int{1, 2, 3, 4, 5})
	ts := s.Type()
	t1 := reflect.TypeOf(&[5]int{})
	t2 := reflect.TypeOf(&[6]int{})
	fmt.Println(ts.ConvertibleTo(t1)) // true
	fmt.Println(ts.ConvertibleTo(t2)) // true
	fmt.Println(s.CanConvert(t1))     // true
	fmt.Println(s.CanConvert(t2))     // false
}