On Wed, Oct 10, 2018 at 10:04 AM Chris Hopkins <cbehopk...@gmail.com> wrote:
>
> Hi,
> I appreciate this is not possible, but realised I never understood why (so
> have just been working around it).
> Example code at:
> https://play.golang.org/p/BgFL9T0KiC7
>
> I can create a struct with other structs as a member. I then get all the
> fields and methods associated with that member.
Technically that's not true. If you call a method on an embedded
struct field, the method has the embedded field as the receiver, not
the containing struct.
type S struct{}
func (s S) A() {}
type X struct {
S
}
x:=X{}
Here, x.A() works on x.S, not x. The crucial difference is that this
is composition, not inheritance. The enclosing struct is not a subtype
of the enclosed struct.
One way to look at this is purely syntactical. If you have:
type Y struct {
s S
}
y:=Y()
To call B(), you have to use the member name: y.s.B(). When the member
has no name, you simply drop the indirection, thus: x.B(), which is,
in fact, the same thing as x.S.B()
> But you can't cast from that master struct back to the member struct.
> i.e. I can create a type T with subtype S and operate on the members and data
> within S, but can't cast T to S.
> I guessed that when you define type S, it exists as effectively a pointer to
> a struct, and then members within that struct are an indexed offset from that
> pointer. The act of creating S within T is simply extending that definition
> so that T's new fields start where S's finished. Casting back from T should
> therefore be trivial, but clearly it isn't :-)
>
>
> I'm sure there's a good reason, but it escapes me at the moment. Any ideas?
>
> Thanks
>
> Chris
>
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