> That is true in current implementations, but Go, the language, does not
guarantee that pointers will never move.
That is what I thought, but it is allowed to use a pointer far map keys.
And I have seen some programs/package using this feature.
Apparently all these programs and packages would break if objects were to
be moved around ?
Le vendredi 4 juin 2021 à 07:25:11 UTC+2, Ian Lance Taylor a écrit :
> On Thu, Jun 3, 2021 at 9:13 PM Robert Engels <ren...@ix.netcom.com> wrote:
> >
> > Doesn’t that depend on what the uintptr refers to? Eg if it was
> allocated off heap to begin with then it should be stable and computing a
> hash on it is valid.
>
> That is true in current implementations, but Go, the language, does
> not guarantee that pointers will never move.
>
> Ian
>
>
> > On Jun 3, 2021, at 9:42 AM, 'Axel Wagner' via golang-nuts <
> golan...@googlegroups.com> wrote:
> >
> >
> > On Thu, Jun 3, 2021 at 4:19 PM christoph...@gmail.com <
> christoph...@gmail.com> wrote:
> >>
> >> The documentation doesn’t specify that we can cast an uintptr into a
> uint64.
> >
> >
> > Both are integer types, so they can be converted. You might be worried
> about their respective sizes and you are correct that the spec does not
> guarantee that a uintptr is at most 64 bit. However, at least for any
> implementation I know on any supported architecture, that's the case. If
> you *really* want to make sure, you can do something like this:
> >
> > func toBytes(v uintptr) []byte {
> > var out []byte
> > mask := ^uintptr(0)
> > for mask != 0 {
> > out = append(out, uint8(v))
> > v >>= 8
> > mask >>= 8
> > }
> > return out
> > }
> >
> > But really, you can just trust that a uintptr fits into a uint64. Or, if
> you want to future-proof, assume it's uint64, but guard by build tags to
> known architectures (so it at least fails to compile if that assumption
> ever changes).
> >
> >> To give some context, the application is for a cache with the hash key
> computed over a pointer to a heap allocated struct. I need to store the
> pointer in the cache entry to test for matching, but this would prevent the
> struct to be garbage collected. Storing the pointer as an uintptr would do
> the trick.
> >
> >
> > This is problematic. The address of a value can change, so the uintptr
> would change as well. So there is no guarantee that your hash stays the
> same, in subsequent calls.
> > Today, that usually doesn't happen, but there is no guarantee it stays
> that way. If you are willing to assume that it doesn't happen, you should
> definitely also be willing to assume a uintptr fits into a uint64
> >
> >>
> >>
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