即使同一接口也不能将X类型用作Y类型

Question

I am trying to write a function which takes anything that implements a certain interface as an argument.

I have defined an interface KeyProvider which specifies a GetKey() method. I have defined a function that uses this interface ToKeys().

type KeyProvider interface {
    GetKey() *datastore.Key 
}

func ToKeys(l []*KeyProvider) []*datastore.Key {
    keys := make([]*datastore.Key, len(l))
    for i, vp := range l {
        v := *vp
        keys[i] = v.GetKey()
    }
    return keys
}

I have implemented this interface on a struct User.

type User struct {
    Key *datastore.Key
}

func (u *User) GetKey() *datastore.Key {
    return u.Key
}

I try to use the ToKeys() method.

func foo(users []*User) {
    keys := ToKeys(users)
}

But I get this error:

cannot use users (type []*User) as type []*KeyProvider in argument to ToKeys

Why? And how can I get something like this working?

Answer 1

always keep in mind, Go requires explicit type conversions.

1.func ToKeys(l []*KeyProvider) []*datastore.Key should be func ToKeys(l []KeyProvider) []*datastore.Key.
should almost never use pointer to an interface, use interface.

2.not allow to convert []*User to []KeyProvider directly, because of different representation in memory (if you are interesting about slice's internal).

you need to convert the slice manually, for example:

func foo(t []*User) {
    s := make([]KeyProvider, len(t))
    for i, v := range t {
        s[i] = v
    }

    keys := ToKeys(s)
}

check this two urls:
https://golang.org/doc/faq#convert_slice_of_interface https://golang.org/doc/faq#convert_slice_with_same_underlying_type