Create a new struct with reflect from type defined by a nil pointer

Question

I would like know if it is possible to allocate a struct from a type specified by a nil pointer by using reflect.New()

type SomeType struct{
   A int
}

sometype := (*SomeType)(nil)

v := reflect.valueOf(sometype)
// I would like to allocate a new struct based on the type defined by the pointer
// newA := reflect.New(...)
//
newA.A = 3

How should I do this ?

Ainar-G · Accepted Answer · 2014-12-05 13:56:14Z

12

Use reflect.Type.Elem():

s := (*SomeType)(nil)
t := reflect.TypeOf(s).Elem()

v := reflect.New(t)
sp := (*SomeType)(unsafe.Pointer(v.Pointer()))
sp.A = 3

Playground: http://play.golang.org/p/Qq8eo-W2yq

EDIT: Elwinar in comments below pointed out that you can get the struct without unsafe.Pointer by using reflect.Indirect():

s := (*SomeType)(nil)
t := reflect.TypeOf(s).Elem()

ss := reflect.Indirect(reflect.New(t)).Interface().(SomeType)
ss.A = 3

Playground: http://play.golang.org/p/z5xgEMR_Vx

edited Dec 5, 2014 at 13:56

answered Dec 5, 2014 at 13:45

Ainar-G

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3 Comments

yageek Over a year ago

Is it the only possibility ? Is there a way without using the unsafe package ?

Elwinar Over a year ago

To prevent having to use the unsafe package, you should use the reflect.Indirect method. One-liner : reflect.Indirect(reflect.New(reflect.TypeOf(sometype).Elem())).Interface().(SomeType)

yageek Over a year ago

Is it the same operation to proceed but starting with an interface ? I mean you have an object A implementing the I interface. I start from the interface I and I would like to create the same underlying object that implement the interface I using reflect ?

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