Hi,
On Mon, Mar 8, 2021 at 10:07 PM Ian Lance Taylor <i...@golang.org> wrote:
> > 2. When I need to iterate map.
> > Using standard reflect package there is no way to iterate map[string]int
> without allocation of string header and int for every key-value pair. When
> > key or values is struct, things become worse.
>
> That is how the language works also. Map keys and values are not
> addressable.
I'm not sure I fully understand the issue, but
for k, v := range m
only needs to allocate storage for k/v *once*. AIUI, `MapIter.Key` and
`MapIter.Value` need to allocate on every iteration (or at least that's how
I read the comment by Yura). However, if MapIter's methods would have
signatures
Key(v Value)
Value(v Value)
we could create *one* value of the key/value type. The methods would then
call `v.Set` to copy the non-addressable internal value into the
addressable iteration variables. This would be closer to the for loop -
we'd still (maybe) need to heap-allocate the iteration variables, but that
would only have to happen once per loop, not once per iteration.
Disclaimer: I haven't verified if `MapIter.{Key,Value}` actually do
allocate on every call, that's just how I read the comment. It's possible
escape analysis is already clever enough for this (though I don't know how
it would be).
> Keeping an address for them is very deeply unsafe, and
> could break irretrievably with future changes to the garbage
> collector. Or, to put it another way, supporting this functionality
> imposes limitations on the garbage collector that I don't think we
> want to impose. I'm not sure what we can do here. Go's map type
> intentionally does not give programs complete control over memory.
> It's possible that generics will address some of these use cases.
>
> Ian
>
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>
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