Give a function:
type myFunc func(...interface{}) (interface{})
I'd like to get the type of myFunc, something like:
t := reflect.TypeOf(myFunc)
Or
t := reflect.TypeOf((*myFunc)(nil))
The only way I have found to do this is by first creating a temporary variable of myFunc and then getting the TypeOf from that.
var v myFunc
t := reflect.TypeOf(v)
Is there a better way to do this?
The simplest way to get the function type would be reflect.TypeOf(myFunc(nil)).
package main
import (
"fmt"
"reflect"
)
type myFunc func(...interface{}) interface{}
func main() {
t := reflect.TypeOf(myFunc(nil))
// Print whether function type is variadic
fmt.Println("Variadic =", t.IsVariadic())
// Print out every input type
for i := 0; i < t.NumIn(); i++ {
fmt.Println("In", i, "=", t.In(i))
}
// Print out every output type
for i := 0; i < t.NumOut(); i++ {
fmt.Println("Out", i, "=", t.Out(i))
}
}
Related
I can't explain why the following is working.
package main
import (
"fmt"
"reflect"
"strings"
)
type MyInterface interface {
someFunc()
}
type Dock struct {
}
func (d *Dock) someFunc() {
}
type Group struct {
Docks []Dock `better:"sometag"`
}
func foo(model interface{}) {
v1 := reflect.Indirect(reflect.ValueOf(model))
for i := 0; i < v1.NumField(); i++ {
tag := v1.Type().Field(i).Tag.Get("better")
if strings.HasPrefix(tag, "sometag") {
inter := v1.Field(i).Interface()
typ := reflect.TypeOf(inter).Elem()
fmt.Println("Type:", typ.String())
// Want to instantiate type like &Dock{} then assign it to some interface,
// but using reflect
n := reflect.New(typ)
_, ok := n.Interface().(MyInterface)
fmt.Println("Why is it OK?", ok)
}
}
}
func main() {
g := &Group{}
foo(g)
/*var v1, v2 interface{}
d1 := &Dock{}
v1 = d1
_, ok1 := v1.(MyInterface)
d2 := Dock{}
v2 = d2
_, ok2 := v2.(MyInterface)
fmt.Println(ok1, ok2)*/
}
It prints
Type: main.Dock
OK? true
If it's a Dock type, then it's not a pointer to Dock. Why does it conforms to MyInterface?
https://play.golang.org/p/Z9mR8amYOM7
Where as the d2 example in the comment does not.
In go doc for reflect.New
New returns a Value representing a pointer to a new zero value for the
specified type. That is, the returned Value's Type is PtrTo(typ).
n := reflect.New(typ)
fmt.Println("Type:", n.String())
It will print Type: <*main.Dock Value> means n is a pointer of Dock.You miss the part using reflect.New return the pointer.
I am trying to copy the fields from one struct value to another, where they have the same field definitions. I have this program:
package main
import (
"log"
"reflect"
)
func setExistingFields(src interface{}, dst interface{}) {
fields := reflect.TypeOf(src)
values := reflect.ValueOf(src)
num := fields.NumField()
s := reflect.ValueOf(src).Elem()
d := reflect.ValueOf(dst).Elem()
for i := 0; i < num; i++ {
field := fields.Field(i)
value := values.Field(i)
fsrc := s.FieldByName(field.Name)
fdest := d.FieldByName(field.Name)
if fdest.IsValid() && fsrc.IsValid() {
if fdest.CanSet() && fsrc.CanSet() {
fdest.Set(value)
}
}
}
}
// and then we main:
func main() {
src := struct {
Foo string
Bar string
}{
"dog",
"pony",
}
dest := struct{ Foo string; Bar string }{}
setExistingFields(&src, &dest)
log.Println("dest.Foo", dest.Foo)
}
I run that, but I get an error:
reflect: NumField of non-struct type
I can't figure out what that's about.
Here's a playground link:
https://play.golang.org/p/TsHTfAaeKhc
Try this out:
func setExistingFields(src interface{}, dst interface{}) {
srcFields := reflect.TypeOf(src).Elem()
srcValues := reflect.ValueOf(src).Elem()
dstValues := reflect.ValueOf(dst).Elem()
for i := 0; i < srcFields.NumField(); i++ {
srcField := srcFields.Field(i)
srcValue := srcValues.Field(i)
dstValue := dstValues.FieldByName(srcField.Name)
if dstValue.IsValid() {
if dstValue.CanSet() {
dstValue.Set(srcValue)
}
}
}
}
Note that you need to do additional checking if src field value is assignable to dst field type.
Edit: The reason why you are getting that error is because fields at that point is a pointer to a struct. You need to get the actual struct value by using Elem().
This won't work: A struct always gets its "schema" (eg. its fields) during compile time... You cannot add more fields during runtime.
I don't see what your exact use case is, but consider something like map[string]string or even map[string]interface{} to be able to "extend" the content/fields of the thing you are passing around...
How to get function arguments in Go at runtime, all I know is only how to get function name:
pc, file, line, ok := runtime.Caller(2)
rt := runtime.FuncForPC(pc)
return rt.Name() // Foo
What I need is something like this:
Foo(1,2,3)
// Foo_1_2_3
Not a full answer, but maybe this can help :
package main
import (
"fmt"
"reflect"
)
func main() {
fmt.Println(reflect.TypeOf(f1))
for index := 0; index < reflect.TypeOf(f1).NumIn(); index++ {
fmt.Println(reflect.TypeOf(f1).In(index))
}
}
func f1(a int, b string) {}
prints :
func(int, string)
int
string
package main
import (
"fmt"
)
type alias int
type aliases []*alias
func main() {
a1 := alias(1)
t := aliases{&a1}
fmt.Println([]*int([]*alias(t)))
}
The type type aliases []*alias is essentially []*int
I want to be able to type convert aliases back to []*int
You can with unsafe.Pointer, a little bit unsafe so not recommended
PointerToSliceOfPointersToInt := (*([]*int))(unsafe.Pointer(&t))
try it works https://play.golang.org/p/6AWd1W_it3
Try this, you could do that by doing right casting.
type alias int
type aliases []*alias
func main() {
a1 := alias(1)
t := aliases{&a1}
orig := int(*([]*alias(t)[0]))
fmt.Println(orig)
}
Example on http://play.golang.org/p/1WosCIUZSa
If you want to get all values (not just the first index) you have to loop and cast each element.
func main() {
a1 := alias(1)
t := aliases{&a1}
orig := []*int{}
for _, each := range t {
temp := int(*each)
orig = append(orig, &temp)
}
fmt.Printf("%#v\n", orig) // []*int{(*int)(0x10434114)}
}
Example: http://play.golang.org/p/Sx4JK3kA45
I found this question with this great answers:
How to find a type of a object in Golang?
I played around with the answer and tried to get the name of a struct in the same way:
package main
import (
"fmt"
"reflect"
)
type Ab struct {
}
func getType(myvar interface{}) string {
return reflect.TypeOf(myvar).Name()
}
func main() {
fmt.Println("Hello, playground")
tst := "string"
tst2 := 10
tst3 := 1.2
tst4 := new(Ab)
fmt.Println(getType(tst))
fmt.Println(getType(tst2))
fmt.Println(getType(tst3))
fmt.Println(getType(tst4))
}
Go playground: http://play.golang.org/p/tD8mygvETH
But the output is:
Hello, playground
string
int
float64
Program exited.
Expected output would be:
Hello, playground
string
int
float64
Ab
Program exited.
I tried to figure out by reading the documentation but didn't find the issue about that. So, sorry for the very general question, but:
What's the reason, reflect.TypeOf().Name() does not work with (this) struct(s)?
In your example you pass a value of pointer type (*Ab), not a struct type.
Sticking to Type.Name()
If it is not a pointer, Type.Name() will properly return Ab. In case of pointer if you still want the struct's name, you can use Type.Elem() to get the element's type:
func getType(myvar interface{}) string {
if t := reflect.TypeOf(myvar); t.Kind() == reflect.Ptr {
return "*" + t.Elem().Name()
} else {
return t.Name()
}
}
Testing it:
tst4 := Ab{}
tst5 := new(Ab)
fmt.Println(getType(tst4))
fmt.Println(getType(tst5))
Output (try your modified example on the Go Playground):
Ab
*Ab
Note:
Note that as Type.Name() does not resolve pointers, it would not work if the value passed is a pointer to pointer, e.g. **Ab, while as Type.String() automatically resolves pointers, would work in this case too.
We can easily make our getType() function to work with **Ab too (or with any depth of pointers):
func getType(myvar interface{}) (res string) {
t := reflect.TypeOf(myvar)
for t.Kind() == reflect.Ptr {
t = t.Elem()
res += "*"
}
return res + t.Name()
}
Calling it with values:
tst4 := Ab{}
tst5 := new(Ab)
tst6 := &tst5 // type of **Ab
tst7 := &tst6 // type of ***Ab
Output (try it on the Go Playground):
Ab
*Ab
**Ab
***Ab
Using Type.String()
A simpler and better approach would be to use Type.String() instead of Type.Name() which automatically handles pointers and also includes package name. E.g.:
func getType(myvar interface{}) string {
return reflect.TypeOf(myvar).String()
}
For the modified example it outputs:
string
int
float64
main.Ab
*main.Ab
Try this variant on the Go Playground.
fmt has a cool %T tag as well
package main
import (
"fmt"
"net/http"
)
type Potato struct {
}
func main() {
fmt.Printf("I have a %T, an %T and a %T\n", Potato{}, http.StatusMultipleChoices, &http.Response{})
}
outputs I have a main.Potato, an int and a *http.Response
https://play.golang.org/p/6z7_0BSitm
The problem is new returns pointer, following should get the desired result.
package main
import (
"fmt"
"reflect"
)
type Ab struct {
}
func getType(myvar interface{}) {
valueOf := reflect.ValueOf(myvar)
if valueOf.Type().Kind() == reflect.Ptr {
fmt.Println(reflect.Indirect(valueOf).Type().Name())
} else {
fmt.Println(valueOf.Type().Name())
}
}
func main() {
fmt.Println("Hello, playground")
tst := "string"
tst2 := 10
tst3 := 1.2
tst4 := new(Ab)
getType(tst)
getType(tst2)
getType(tst3)
getType(tst4)
}
Output is
Hello, playground
string
int
float64
Ab