I want to use some external code that requires a pointer to a struct. At the point that the code is called, I have an interface variable.
When creating a pointer off of that variable, the pointer's type is interface{}*
when I need it to be the pointer type of the struct's type.
Image the code in TestCanGetStructPointer
does not know about the Cat class , and that it exists in some external package.
How can I cast it to this?
Here is a code sample:
import (
"reflect"
"testing"
)
type Cat struct {
name string
}
type SomethingGeneric struct {
getSomething func() interface{}
}
func getSomeCat() interface{} {
return Cat{}
}
var somethingForCats = SomethingGeneric{getSomething: getSomeCat}
func TestCanGetStructPointer(t *testing.T) {
interfaceVariable := somethingForCats.getSomething()
pointer := &interfaceVariable
interfaceVarType := reflect.TypeOf(interfaceVariable)
structPointerType := reflect.PtrTo(interfaceVarType)
pointerType := reflect.TypeOf(pointer)
if pointerType != structPointerType {
t.Errorf("Pointer type was %v but expected %v", pointerType, structPointerType)
}
}
The test fails with:
Pointer type was *interface {} but expected *parameterized.Cat
@dyoo's example does work, but it relies on you to manually cast Dog
and Cat
.
Here's a bit of a convoluted/verbose example which avoids that constraint somewhat:
package main
import (
"fmt"
"reflect"
)
type Cat struct {
name string
}
type SomethingGeneric struct {
getSomething func() interface{}
}
func getSomeCat() interface{} {
return Cat{name: "Fuzzy Wuzzy"}
}
var somethingForCats = SomethingGeneric{getSomething: getSomeCat}
func main() {
interfaceVariable := somethingForCats.getSomething()
castVar := reflect.ValueOf(interfaceVariable)
castVar.Convert(castVar.Type())
// If you want a pointer, do this:
fmt.Println(reflect.PtrTo(castVar.Type()))
// The deref'd val
if castVar.Type() != reflect.TypeOf(Cat{}) {
fmt.Printf("Type was %v but expected %v\n", castVar, reflect.TypeOf(&Cat{}))
} else {
fmt.Println(castVar.Field(0))
}
}
The following may help: http://play.golang.org/p/XkdzeizPpP
package main
import (
"fmt"
)
type Cat struct {
name string
}
type Dog struct {
name string
}
type SomethingGeneric struct {
getSomething func() interface{}
}
func getSomeCat() interface{} {
return Cat{name: "garfield"}
}
func getSomeDog() interface{} {
return Dog{name: "fido"}
}
var somethings = []SomethingGeneric{
SomethingGeneric{getSomething: getSomeCat},
SomethingGeneric{getSomething: getSomeDog},
}
func main() {
for _, something := range somethings {
interfaceVariable := something.getSomething()
cat, isCat := interfaceVariable.(Cat)
dog, isDog := interfaceVariable.(Dog)
fmt.Printf("cat %v %v\n", cat, isCat)
fmt.Printf("dog %v %v\n", dog, isDog)
}
}
I found this thread: https://groups.google.com/forum/#!topic/golang-nuts/KB3_Yj3Ny4c
package main
import (
"fmt"
"reflect"
)
type Cat struct {
name string
}
//
// Return a pointer to the supplied struct via interface{}
//
func to_struct_ptr(obj interface{}) interface{} {
fmt.Println("obj is a", reflect.TypeOf(obj).Name())
// Create a new instance of the underlying type
vp := reflect.New(reflect.TypeOf(obj))
// Should be a *Cat and Cat respectively
fmt.Println("vp is", vp.Type(), " to a ", vp.Elem().Type())
vp.Elem().Set(reflect.ValueOf(obj))
// NOTE: `vp.Elem().Set(reflect.ValueOf(&obj).Elem())` does not work
// Return a `Cat` pointer to obj -- i.e. &obj.(*Cat)
return vp.Interface()
}
//
// Dump out a pointer ...
//
func test_ptr(ptr interface{}) {
v := reflect.ValueOf(ptr)
fmt.Println("ptr is a", v.Type(), "to a", reflect.Indirect(v).Type())
}
func main() {
cat := Cat{name: "Fuzzy Wuzzy"}
// Reports "*main.Cat"
test_ptr(&cat)
// Get a "*Cat" generically via interface{}
sp := to_struct_ptr(cat)
// *should* report "*main.Cat" also
test_ptr(sp)
fmt.Println("sp is",sp)
}
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