> newItem = new(KdElem)
> if itemfunc(arg, &newItem.item, newItem.size) != 0
"newItem.item" is a "interface{}" with a nil value.
Doing "&newItem.item" then turns it into a "*interface{}"
That is why you cannot do "var realval *int64 = val.(*int64)"
To make it work as what I think you intend to do you would have to do:
https://play.golang.org/p/kpErE88hEyz
> newItem = new(KdElem)
> newItem.item = new(int64)
> if itemfunc(arg, newItem.item, newItem.size) != 0
or
https://play.golang.org/p/rV39qFOnZpW
> newItem = new(KdElem)
> var item int64
> if itemfunc(arg, &item, newItem.size) != 0
> newItem.item = item
On Tuesday, 12 January 2021 at 11:16:21 pm UTC+8 Steve Murphy wrote:
> Ian--
>
> You missed the part about the arguments being "shifted left" above, which
> kinda looks like a compiler issue. Maybe something I wrote contributed to
> the problem, but I have no idea what.
> If this is a known problem, perhaps I can hand-install aa newer version,
> or something...?
>
> murf
>
> On Tuesday, January 12, 2021 at 7:52:40 AM UTC-7 Ian Davis wrote:
>
>> Note the error says that val was nil, so your type assertion will panic.
>> You can read more about how to make type assertions that report success or
>> failure in the Go Tour: https://tour.golang.org/methods/15
>>
>> Have you tried printing the type of what you receive before attempting
>> the type assertion? I suspect that the system you are working with is
>> passing you a variety of different types depending on the context, so
>> you'll need to be flexible in what you handle.
>>
>>
>>
>>
>>
>> On Tue, 12 Jan 2021, at 5:55 AM, Steve Murphy wrote:
>>
>> Many thanks, Ian, I appreciate your suggestions!
>>
>> I added your code snippet to my program, and got:
>>
>> panic: interface conversion: interface {} is nil, not *interface {}
>> [recovered]
>> panic: interface conversion: interface {} is nil, not *interface {}
>>
>> According to the trace, it looks like it's crashing while trying to
>> evaluate the valDeref := line.
>>
>> Looking at the trace provided:
>>
>> genBox: arg type=*int64 argval=0xc00001a150, val type=<nil>, val
>> val=<nil> offset=0
>> genBox: box=<nil>
>> --- FAIL: TestKd1 (0.09s)
>> panic: interface conversion: interface {} is nil, not *interface {}
>> [recovered]
>> panic: interface conversion: interface {} is nil, not *interface {}
>>
>> goroutine 6 [running]:
>> testing.tRunner.func1(0xc000020200)
>> /usr/lib/go-1.13/src/testing/testing.go:874 +0x3a3
>> panic(0x523360, 0xc0000bc330)
>> /usr/lib/go-1.13/src/runtime/panic.go:679 +0x1b2
>> parsetree.com/kdtree.genBox(0x50eec0, 0xc00001a150, 0x0, 0x0, 0x0, 0x2)
>> /home/murf/go/kdtree/kdtree1_test.go:43 +0x43e
>>
>> It kinda looks like the %T and %v aren't working for val; but the stack
>> trace shows 0xc00001a150 in the second argument to genBox.
>>
>> Uh, wait a minute! According to the stack, the first arg (called arg) is
>> 0x50eec0; but the debug output from genbox shows arg's val is 0xc00001a150,
>> which is what the second arg should be. So it looks like the args have
>> shifted to the left one position! And no errors on compilation?
>>
>> I don't know what to think.
>>
>> BTW, I'm on ubuntu 20.04, and the version of go is 1.13.
>>
>> murf
>>
>> On Monday, January 11, 2021 at 10:45:29 AM UTC-7 Ian Davis wrote:
>>
>>
>> In genBox your code is saying that val contains a pointer to
>> interface{}. In other words its an interface{} that contains a
>> *interface{}. That is a weird but valid construct.
>>
>> I suggest you dereference it and see what the contained interface holds.
>> Something like:
>>
>> valDeref := val.(*interface{})
>>
>> fmt.Printf("val contains=%T", valDeref)
>>
>> You may find that valDeref contains the *int64 you are looking for.
>>
>> As an aside, *interface{} is usually an indication that someone somewhere
>> is passing the wrong value to the function since it almost never makes
>> sense to pass a pointer to interface{}.
>>
>>
>>
>>
>>
>> On Mon, 11 Jan 2021, at 4:03 PM, Steve Murphy wrote:
>>
>> Hello!
>>
>> I keep getting:
>> panic: interface conversion: interface {} is *interface {}, not *int64
>> [recovered]
>> panic: interface conversion: interface {} is *interface {}, not *int64
>>
>> I *suspect* what is happening is that the KDElem struct has an item field
>> whose
>> type is interface{}, so that you can store a pointer to some struct (of
>> your own making),
>> or an index into an array of structs, or... *whatever*, but in my case,
>> it's an index into an array
>> of objects. The itemfunc (or genBox in my test code) is supposed to set
>> the item field in
>> the KDElem struct to the proper index, as the struct has just been
>> created, and the job
>> of the itemfunc is to make it point to the right object, and set the
>> bounds info in the new struct.
>>
>> type KdElem struct {
>> item interface{} // a ptr to a particular struct, or an index
>> into an array of objects, or....
>> ...
>> }
>>
>> And the func that calls the itemfunc (genBox) looks like this:
>>
>> func loadItems(itemfunc func(arg interface{}, val interface{}, size
>> *KdBox) int, arg interface{}, extent KdBox, length *int64, mean *float64)
>> *KdElem {
>> ...
>> newItem = new(KdElem)
>> if itemfunc(arg, &newItem.item, newItem.size) != 0 {
>> ...
>>
>>
>> And, in this case the itemfunc declaration looks like this:
>>
>> func genBox(arg interface{}, val interface{}, box *KdBox) int {
>> var offsetp = arg.(*int64) // successful
>> var offset = *offsetp // successful
>> // fmt.Printf("genBox: offset=%v offsetp=%v\n", offset, offsetp)
>> fmt.Printf("genBox: arg type=%T argval=%v, val type=%T, val
>> val=%v\n", arg, arg, val, val)
>> if offset < KDBoxes {
>> fmt.Printf("genBox: val=%v *int64=%v\n", val, val)
>> var realval *int64 = val.(*int64) // <<-- This is line
>> 43! Compiles.... but... Crash!!!
>> *realval = offset + 1
>> ...
>>
>> Now, I get this from a go test:
>>
>> KdBuild: arg type=*int64 argval=0xc0000d2078;
>> genBox: arg type=*int64 argval=0xc0000d2078, val type=*interface {},
>> val val=0xc0000ba640
>> genBox: val=0xc0000ba640 *int64=0xc0000ba640
>> --- FAIL: TestKd1 (0.08s)
>> panic: interface conversion: interface {} is *interface {}, not *int64
>> [recovered]
>> panic: interface conversion: interface {} is *interface {}, not *int64
>>
>> goroutine 19 [running]:
>> testing.tRunner.func1(0xc000108100)
>> /usr/lib/go-1.13/src/testing/testing.go:874 +0x3a3
>> panic(0x523360, 0xc0000c2360)
>> /usr/lib/go-1.13/src/runtime/panic.go:679 +0x1b2
>> parsetree.com/kdtree.genBox(0x50eec0, 0xc0000d2078, 0x50ef40,
>> 0xc0000ba640, 0x0, 0x0)
>> /home/murf/go/kdtree/kdtree1_test.go:43 +0x270
>> parsetree.com/kdtree.loadItems(0x5570f8, 0x50eec0, 0xc0000d2078,
>> 0x7fffffffffffffff, 0x7fffffffffffffff, 0x8000000000000000,
>> 0x8000000000000000, 0xc00096ab38, 0xc00096ab30, 0x0)
>> /home/murf/go/kdtree/kdtree.go:683 +0xec
>> parsetree.com/kdtree.KdBuild(0x5570f8, 0x50eec0, 0xc0000d2078, 0x0)
>> /home/murf/go/kdtree/kdtree.go:168 +0x17e
>> parsetree.com/kdtree.TestKd1(0xc000108100)
>> /home/murf/go/kdtree/kdtree1_test.go:72 +0x13a
>> testing.tRunner(0xc000108100, 0x5570f0)
>> /usr/lib/go-1.13/src/testing/testing.go:909 +0
>>
>> How do I set realval to the (*int64) value that's in val?
>>
>> murf
>>
>>
>> --
>>
>> Steve Murphy
>> ParseTree Corporation
>> 57 Lane 17
>> Cody, WY 82414
>> ✉ murf at parsetree dot com
>> ☎ 307-899-0510 <(307)%20899-0510>
>>
>>
>> --
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>>
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>> .
>>
>>
>>
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