Go is a static language with well reflection support.
The remaining of this article will explain the reflection functionalities
provided in the reflect standard package.
It is very helpful to read the overview of Go type system and interfaces in Go articles before reading the remaining of the current article.
Go reflection brings many dynamic functionalities to Go programming.
Many standard code packages, such as the fmt and encoding
packages, heavily rely on the reflection functionalities.
We can inspect Go values through the values of the Type and
Value types defined in the reflect standard package.
The remaining of this article will show some examples on how to use
values of the two types.
One of the Go reflection design goals is any non-reflection operation should be also possible to be applied through the reflection ways. For all kinds of reasons, this goal is not 100 percent achieved. However, most non-reflection operations can be applied through the reflection ways now. On the other hand, through the reflection ways, we can do some operations which are impossible to be achieved through non-reflection ways. The operations which can't and can only be achieved through the reflection ways will be mentioned in the following sections.
reflect.Type Type and Values
In Go, we can create a reflect.Type value from an arbitrary
non-interface value by calling the reflect.TypeOf function.
The result reflect.Type value represents the type of the non-interface value.
Surely, we can also pass an interface value to a reflect.TypeOf function call,
but the call will return a reflect.Type value which represents the dynamic type of the interface value.
In fact, the reflect.TypeOf function has only one parameter of type interface{}
and always returns a reflect.Type value which represents the dynamic type of the only interface parameter.
To get a reflect.Type value which represents an interface type
by using the reflect.TypeOf function,
we must use indirect ways which will be introduced below to achieve this goal.
Since Go 1.22, we can also use
the reflect.TypeFor function
to get a reflect.Type value which represents a type which is known
at compile time. The compile-time known type may be either non-interface or interface.
The reflect.Type type is an interface type.
It specifies several methods.
We can call these methods to inspect the information of the type represented
by a reflect.Type receiver value.
Some of these methods apply for all
kinds of types,
some of them are one kind or several kinds specific.
Please read the documentation of each method for details.
Calling one of the methods through an improper reflect.Type receiver value will produce a panic.
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
}
There are 26 kinds of types in Go.
In the above example, we use the methodElem to get
the element types of some container types (a channel type, a map type,
a slice type and an array type).
In fact, we can also use this method to get the base type of a pointer type.
For example,
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
}
The above example also shows how to (indirectly) get a reflect.Type value which represents an interface type.
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
}
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.
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.
reflect.StructTag, which has two methods,
Get and Lookup, to inspect the key-value pairs specified in field tags.
An example of inspecting struct field tags:
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
}
`optional: "yes"` doesn't form key-value pairs.
`json:"author, omitempty"`,`json:" author,omitempty"` and`json:"author,omitempty"` are different from each other.
reflect.TypeOf function,
we can also use some other functions in the reflect
standard package to create reflect.Type values
which represent some unnamed composite types.
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 more reflect.Type methods which are not used
in above examples, please read the reflect package
documentation for their usages.
Note, up to now (Go 1.22), there are no ways to create interface types through reflection. This is a known limitation of Go reflection.
Another limitation is, although we can create a struct type embedding other types as anonymous fields through reflection, the struct type may or may not obtain the methods of the embedded types, and creating a struct type with anonymous fields even might panic at run time. In other words, the behavior of creating struct types with anonymous fields is partially compiler dependent.
The third limitation is we can't declare new types through reflection.
reflect.Value Type and Values
Similarly, we can create a reflect.Value value from an arbitrary
non-interface value by calling the reflect.ValueOf function.
The result reflect.Value value represents the non-interface value.
Same as the reflect.TypeOf function, the reflect.ValueOf function
also has only one parameter of type interface{}.
When an interface argument is passed to a reflect.ValueOf function call,
the call will return a reflect.Value value which represents the dynamic value of the interface argument.
To get a reflect.Value value which represents an interface value,
we must use indirect ways which will be introduced below to achieve this goal.
The value represented by a reflect.Value value v
is often called the underlying value of v.
There are plenty of methods
declared for the reflect.Value type.
We can call these methods to inspect the information of (and manipulate)
the underlying value of a reflect.Value receiver value.
Some of these methods apply for all kinds of values, some of them are one kind or several kinds specific.
Please read the reflect standard package documentation for details.
Calling a kind-specific method with an improper reflect.Value receiver value will produce a panic.
The CanSet method of a reflect.Value value
returns whether or not the underlying value of the
reflect.Value value is modifiable (can be assigned to).
If the Go value is modifiable, we can call the Set method
of the corresponding reflect.Value value to modify the Go value.
Note, the reflect.Value values returned directly by
reflect.ValueOf function calls are always read-only.
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
}
Non-exported fields of struct values can't be modified through reflections.
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
}
From the above two examples, we can learn that there are two ways to get a
reflect.Value value
whose underlying value is referenced by the underlying value (a pointer value) of another reflect.Value value.
Elem method of a reflect.Value value
which represents the pointer value.
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.)
reflect.Value.Elem method can be also used to get a reflect.Value value
which represents the dynamic value of an interface value. For example,
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
}
The
reflect standard package also declares
some reflect.Value related functions.
Each of these functions corresponds to a built-in function
or a non-reflection functionality,
The following example demonstrates how to bind a (kind of)
custom generic function to different function values.
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]
}
If the underlying value of a
reflect.Value is a function value,
then we can call the Call method of the reflect.Value
to call the underlying function.
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
}
Please note that, non-exported fields shouldn't be used as arguments of reflection calls.
If the line vt.FieldByName("A") in the above example is replaced
with vt.FieldByName("b"), a panic will occur.
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())
}
}
Please note that, the MapRange method is supported since Go 1.12.
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
}
The TrySend and TryRecv methods correspond to
one-case-one-default select
control flow code blocks.
reflect.Select function to simulate a
select code block with dynamic number of case branches
at run time.
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
}
The respective underlying values of some
reflect.Value values may be nothing.
For example, zero reflect.Value values.
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>
}
For a Go value, we can use the
reflect.ValueOf function to create a reflect.Value value
representing the Go value, through the help of interface{}.
The inverse process in similar, we can call the Interface method of a reflect.Value value
to get an interface{} value, then type assert on the interface{} value
to get the Go value represented by (a.k.a., the underlying value of ) the reflect.Value value.
But please note that, calling the Interface method of a reflect.Value value
which represents a non-exported field causes a panic.
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
}
The method reflect.Value.IsZero was introduced in Go 1.13.
It is used to check whether or not the underlying value of a reflect.Value value is a zero value.
Since Go 1.17, a slice may be converted to an array pointer.
However, such a conversion might panic if the length of the pointer base array type is too large.
The method reflect.Value.CanConvert(T reflect.Type) introduced in Go 1.17 is used to check whether or not a conversion will success.
CanConvert method:
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
}
There are more reflect.Value related functions and methods
which are not used in above examples,
please read the reflect package documentation for their usages.
In addition, please note that there are
some reflection related
details
mentioned in Go details 101.
The Go 101 project is hosted on Github. Welcome to improve Go 101 articles by submitting corrections for all kinds of mistakes, such as typos, grammar errors, wording inaccuracies, description flaws, code bugs and broken links.
If you would like to learn some Go details and facts every serveral days, please follow Go 101's official Twitter account @go100and1.
reflect standard package.sync standard package.sync/atomic standard package.