That would work Kortschak.
But this relies on a string's representation being the same as, but a bit
smaller thabn, a []byte. I would prefer to use the Slice/StringHeader.
It's worth noting that _most_ of the problems I described in my initial
post are hypothetical at this stage. The issue with strings being garbage
collected mid-conversion are (I think) certain not to happen in Go 1.12 and
the problems with storing a uinptr in a variable are mostly related to
moving garbage collectors (so long as the location your uintpr points to
doesn't get garbage collected). But both of these facts have a reasonable
likely hood of changing in the future.
The thing I would like the most is an pair of unsafe string <-> []byte
conversion implementations which satisfy the rules of unsafe as they are
written now which will _not_ compile if the representation of []byte/string
changes in the future.
This was what I thought I would get using the reflect.Slice/StringHeader
structs and some unsafe. It's surprising that this doesn't work in a
straight-forward way.
On Saturday, September 21, 2019 at 8:11:08 AM UTC+2, kortschak wrote:
>
> func bytesToString(b []byte) string {
> return *(*string)(unsafe.Pointer(&b))
> }
>
> https://play.golang.org/p/azJPbl946zj
>
> On Fri, 2019-09-20 at 13:30 -0700, Francis wrote:
> > Thanks Ian, that's a very interesting solution.
> >
> > Is there a solution for going in the other direction? Although I
> > excluded
> > it from the initial post, it was only to reduce the size of the
> > discussion.
> > I would also like to implement
> >
> > func BytesToString(b []byte) string {
> >
> > I don't clearly see how to avoid using the StringHeader in this case.
> >
> > F
> >
> > On Wednesday, 18 September 2019 22:46:44 UTC+2, Ian Lance Taylor
> > wrote:
> > >
> > > On Wed, Sep 18, 2019 at 2:42 AM Francis <francis...@gmail.com
> > > <javascript:>> wrote:
> > > >
> > > > I am looking at the correct way to convert from a byte slice to a
> > > > string
> > >
> > > and back with no allocations. All very unsafe.
> > > >
> > > > I think these two cases are fairly symmetrical. So to simplify
> > > > the
> > >
> > > discussion below I will only talk about converting from a string to
> > > []byte.
> > > >
> > > > func StringToBytes(s string) (b []byte)
> > >
> > > No reason to use SliceHeader, and avoiding SliceHeader avoids the
> > > problems you discuss.
> > >
> > > func StringToBytes(s string) []byte {
> > > const max = 0x7fff0000
> > > if len(s) > max {
> > > panic("string too long")
> > > }
> > > return
> > > (*[max]byte)(unsafe.Pointer((*reflect.StringHeader)(unsafe.Pointer(
> > > &s)).Data))[:len(s):len(s)]
> > >
> > > }
> > >
> > > Of course, as you say, you must not mutate the returned []byte.
> > >
> > > Ian
> > >
> >
> >
>
>
--
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to golang-nuts+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/golang-nuts/8bff6f8e-487a-4c69-a55e-513855d846da%40googlegroups.com.
