Convert interface to struct

Question

type SipField interface {
    Info() (id, name, defaultValue string, length int)
}

type Field string

func (f *Field) Get() string {
    return string(*f)
}

func (f *Field) Set(s string) {
    *f = Field(s)
}

type CommandID Field

func (cid *CommandID) Info() (id, name, defaultValue string, length int) {
    return "", "command ID", "", 2
}

type Language Field

func (l *Language) Info() (id, name, defaultValue string, length int) 
{
    return "", "language", "019", 3
}

func InitField(f interface{}, val string) error {
    sipField, ok := f.(SipField)
    if !ok {
        return errors.New("InitField: require a SipField")
    }
    _, _, defaultValue, length := sipField.Info()
    field, ok := f.(*Field)
    if !ok {
     return errors.New("InitField: require a *Field")
    }
    return nil
}

How should I do for converting interface{} to Field(CommandID, Language...) in InitField() function? I try to directly type assert by

field, ok := f.(*Field)

but it not working.I have tried to use unsafe.Pointer but failed also.

Answer 1

Have a look at Type assertions chapter in Go reference. It states:

x.(T)

More precisely, if T is not an interface type, x.(T) asserts that the dynamic type of x is identical to the type T.

Types CommandID and Field are not identical as described in Type identity.

A defined type is always different from any other type.

Both types CommandId and Fields are defined as described in Type definitions.

A type definition creates a new, distinct type with the same underlying type and operations as the given type, and binds an identifier to it.

TypeDef = identifier Type .

You can only do

field, ok := f.(*CommandID)

or

field, ok := f.(*Language)

As @mkopriva mentioned in the comment, you can do type conversion later to *Field but this it does not seem to be your goal.

Other solution is to introduce a Field interface with Set and Get methods. Then you will need to provide an implementation for every implementing type.