Why do you need fields? If you need fields, you are likely putting too many
things into your context (note, that the lookup is O(N), with N being the
number of distinct keys - so you want the number of distinct keys to be
small). The intended use of `context.WithValue` is to only have one key per
"use-case" (normally, that means one key per package) and then store the
actual data in the value.
So, yes, you *can* have fields and if you have fields and re-use one type
for many keys that way. And if you do that, you need to specify the exact
field values on the lookup as well. But if you want "many keys" from a
single type, that's a code-smell and I would suggest to change your design.
If you think you need it, feel free to expand on your use-case and we might
come up with a better solution :)
On Wed, Jan 6, 2021 at 11:52 PM Amit Saha <amitsaha...@gmail.com> wrote:
> On Thu, Jan 7, 2021 at 9:48 AM Axel Wagner
> <axel.wagner...@googlemail.com> wrote:
> >
> >
> >
> > On Wed, Jan 6, 2021 at 11:39 PM Amit Saha <amitsaha...@gmail.com> wrote:
> >>
> >> Is it fair to say then that any user-defined type here is acceptable as
> a key?
> >
> >
> > No. You can create a user-defined type out of *any* type T, by just
> writing `type X T`. In particular, you can write `type X func()` and get an
> incomparable user-defined type.
> >
> > I highly recommend sticking to the recommendation from the docs: Use
> `type ctxKey struct{}` as a key type and `ctxKey{}` as the key - it's
> comparable and the only value equal to `ctxKey{}` is `ctxKey{}`, which
> gives exactly the desirable semantics. And as it's zero-sized, it also
> doesn't occur any allocation (at least with gc, all zero-sized objects have
> the same address). You could also use `[0]int` or similar, but `struct{}`
> is very common for use-cases like this.
>
> Thanks. If I wanted to have a struct like this
>
> type myReqContext struct {
> ID1 string
> ID2 string
> }
>
> Would the correct key and value be: myReqContext{}, myReqContext{ID1:
> "abcd", ID2: "defg"}?
>
> The key isn't zero value any more, or is it? If so, should I have a
> dedicated empty struct for the key?
>
>
>
> >
> >>
> >>
> >>
> >> >>
> >> >> Thanks,
> >> >> Amit.
> >> >>
> >> >> --
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> .
>
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