On Tuesday, 29 December 2020 at 16:25:41 UTC axel.wa...@googlemail.com
wrote:
> On Tue, Dec 29, 2020 at 4:37 PM Arnaud Delobelle <arn...@gmail.com> wrote:
>
>> Question 1: I *think* that the compiler has all the information necessary
>> to implement type assertion to the Cont interface as I have, i.e. it knows
>> only 3 types implement that interface, so could it not do the optimisation
>> on my behalf?
>>
>
>>
> Question 2: Or is it possible that other Go values can be made at runtime
>> that would implement this interface but not be one of the three known types
>> that implement it?
>>
>
> Yes, re 2. `reflect` can create new types at runtime. AFAIK the
> implementation for interface-type-assertions is basically to look it up in
> a global hashmap, which is pre-seeded with compile-time known types and
> then gets filled on each (successful) inteface-type-assertion with the
> correct method tables. But, I'm handwaving.
>
> Under these assumptions, it might be *possible* to first check against the
> statically known types and only fall back on the map if none of that
> matches. But it doesn't seem 100% clear to me that that's always faster.
>
> I think concrete type-assertions will always be faster than interface
> type-assertions though - for a concrete type-assertions, it's really just
> comparing the two type-pointers and copying the value (which, in your case,
> is a pointer itself), whereas for an interface type-assertion, the actual
> method table must be assembled or looked up.
>
> But, here's a question: If `v.iface` always contains a `Cont` - at least
> that's what I assume from the fact that you don't use a ,ok assertion - why
> not just declare it as such, instead of `interface{}`?
>
>>
>>
It doesn't always contain a Cont. Sometimes it should, which is why this
function panics (programming error). There is also TryCont() which returns
a boolean as well and doesn't panic.
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