On Sat, Feb 27, 2021 at 7:03 PM Deiter <hwaterf...@gmail.com> wrote:
>
> Go: go1.15.8 darwin/amd64
> OS: MacOS 11.2.1
>
> The program here includes a type and a method that acts on that type:
>
> type Prefix struct {
> prefix string
> }
>
> func (rec *Prefix) outputString(s string) {
> fmt.Printf("%v: %v\n", rec.prefix, s)
> }
>
> Very straightforward. I expected that this would work too:
>
> type PrefixPtr *Prefix
> func (rec PrefixPtr) outputString(s string) {
> fmt.Printf("%v: %v\n", rec.prefix, s)
> }
>
> But it led to:
> # receiver
> ./receiver.go:24:6: invalid receiver type PrefixPtr (PrefixPtr is a pointer
> type)
> ./receiver.go:30:10: prefixer.outputString undefined (type PrefixPtr has no
> field or method outputString)
>
> I understand that golfing doesn’t have implicit type conversions, and that
> PrefixPtr is not the same as *Prefix, so I guess what I’m asking is twofold:
>
> Why can’t receivers be a pointer type?
Because the language supports pointer methods, as in
func (p *S) Set(...) { ... }
That is a pointer method on the type S, and it can only be called on
values of type *S or addressable values of type S. If a receiver type
could be a type defined as a pointer type, it would be confusing
whether methods declared on that type were pointer methods or value
methods. And it would be confusing whether it were valid to declare a
method on a pointer to that pointer type. We avoid those potential
confusions by simply forbidding methods on types defined as pointer
types. This does not cause any loss of functionality.
> Why isn’t *Prefix considered a pointer type?
It is, but a method defined with a receiver type of *Prefix is a
pointer method of Prefix. That method can be called on any value of
type *Prefix and also on any addressable value of type Prefix.
Ian
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