Thanks! I hadn't realized that one could constraint T to be a pointer type
by using a second type paramater Q, this in nice.
But alas, it doesn't work. When I copy&pasted your code to mine, and used
an undefined C type ... it complained of the same thing:
```
cannot use incomplete (or unallocatable) type as a type argument:
gomlx/xla._Ctype_struct_StableHLOHolder
```
I'm using it in the following snipped of code:
```
func (comp *Computation) ToStableHLO() (*StableHLO, error) {
if comp.IsNil() || comp.firstError != nil {
return nil, errors.Errorf("Computation graph is nil!?")
}
statusOr := C.ConvertComputationToStableHLO(comp.cCompPtr)
cPtr, err := PointerOrError[*C.StableHLOHolder](statusOr)
if err != nil {
return nil, errors.Wrapf(err, "failed conversion in Computation.ToStableHLO"
)
}
return NewStableHLO(cPtr), nil
}
```
I suspect it doesn't allow matching Q to an incomplete type
(`C.StableHLOHolder` in this example), same way as my original version :(
I think your example in playground doesn't capture that -- the playground
doesn't seem to allow CGO code (i tried this
<https://go.dev/play/p/ZM14sQuK8iN?v=gotip.go?download=true>, but it didn't
even try to compile).
I mean it's not the end of the world, I can always cast it in the next line
... it's just one of those little things that would be "ergonomically" very
nice if it worked :)
On Saturday, April 15, 2023 at 3:02:14 PM UTC+2 jake...@gmail.com wrote:
> What About:
>
> func PointerOrError[T *Q, Q any](s C.StatusOr ) (t T, err error)
>
> Seems to compile: https://go.dev/play/p/n4I-XkONj-O?v=gotip
>
> On Saturday, April 15, 2023 at 6:28:39 AM UTC-4 Jan wrote:
>
>> hi,
>>
>> This is a variation for a previous topic
>> <https://groups.google.com/g/golang-nuts/c/h75BwBsz4YA/m/FLBIjgFBBQAJ>,
>> but since there isn't a clear solution, I thought I would ask if anyone can
>> think of a work around.
>>
>> I've been interacting a lot with C++ libraries from Go, and one of the
>> commonly returned types is an abls::StatusOr
>> <https://abseil.io/docs/cpp/guides/status>, for which I created a simple
>> C wrapper that casts the error and value to a `char *` and `void *`
>> respectively (dropping the type information in between C++ and Go).
>>
>> In Go I want to return the type information, so I defined a small generic
>> function:
>>
>> // PointerOrError converts a StatusOr structure to either a pointer to T
>> with the data
>> // or the Status converted to an error message and then freed.
>> func PointerOrError[T any](s C.StatusOr) (*T, error) {
>> ptr, err := UnsafePointerOrError(s) // returns unsafe.Pointer, error
>> if err != nil {
>> return nil, err
>> }
>> return (*T)(ptr), nil
>> }
>>
>> Now this doesn't work for my forward declared C++ types (most of them are
>> just aliases to C++ objects) -- Go complaints with: `cannot use
>> incomplete (or unallocatable) type as a type argument`, because `T` is
>> incomplete indeed.
>>
>> But ... I will never instantiate `T`, I only care about `*T`, which is
>> not incomplete.
>>
>> But there isn't a way to say a generics attribute is a pointer. So if I
>> use the following:
>>
>> func PointerOrError2[T any](s C.StatusOr) (t T, err error) {
>> var ptr unsafe.Pointer
>> ptr, err = UnsafePointerOrError(s) // <-- unsafe.Pointer, error
>> if err != nil {
>> return
>> }
>> t = (T)(ptr)
>> return
>> }
>>
>> And instantiate it with a `PointerOrError2[*MyType](statusOr)` for
>> instance, I get, as expected:
>>
>> cannot convert ptr (variable of type unsafe.Pointer) to type T
>>
>> Any suggestions to make this work ?
>>
>> I could probably craft something using the `reflect` package, but I was
>> hoping for a smart (and likely faster?) generics solution.
>>
>> cheers
>> Jan
>>
>>
>>
>>
>>
>>
>>
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