Re: [go-nuts] Re: Equality of interface of an empty struct - why?

[brien colwell]

I learned a lot from this thread, thank you.  Intuitively the spec seems to conclude a pointer to an empty struct is a different type of pointer? Normally a pointer wouldn't be able to change values during execution, so we can do things like key maps by pointers. But if every evaluation of the empty struct pointer can lead to a different outcome, isn't that the same as the address of the pointer arbitrarily changing? Otherwise if two pointers are different, then the interface comparison of two pointers must be different also since the pointer address does not change? On Feb 25, 2024, at 1:02 AM, tapi...@gmail.com <tapir....@gmail.com> wrote:  The behavior of Go 1.9 or 1.10 is even more weird. They make the following code print false. ;D package main type T struct {} func main() {   var a, b = &T{}, &T{}   println(a == b || a != b) } On Sunday, February 25, 2024 at 4:30:22 PM UTC+8 tapi...@gmail.com wrote: Absolutely a bug. On Thursday, February 22, 2024 at 6:55:49 PM UTC+8 Brien Colwell wrote: I'm confused by this output. It appears that the interface of two different pointers to an empty struct are equal. In all other cases, interface equality seems to be the pointer equality. What's going on in the empty struct case? ``` package main import "fmt" type Foo struct { } func (self *Foo) Hello() { } type FooWithValue struct { A int } func (self *FooWithValue) Hello() { } type Bar interface { Hello() } func main() { a := &Foo{} b := &Foo{} fmt.Printf("%t\n", *a == *b) fmt.Printf("%t\n", a == b) fmt.Printf("%t\n", Bar(a) == Bar(b)) c := &FooWithValue{A: 1} d := &FooWithValue{A: 1} fmt.Printf("%t\n", *c == *d) fmt.Printf("%t\n", c == d) fmt.Printf("%t\n", Bar(c) == Bar(d)) } ``` Prints (emphasis added on the strange case): ``` true false **true** true false false ``` -- You received this message because you are subscribed to a topic in the Google Groups "golang-nuts" group. To unsubscribe from this topic, visit https://groups.google.com/d/topic/golang-nuts/JBVqWYFdtC4/unsubscribe. To unsubscribe from this group and all its topics, 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/e4429119-5c91-44d1-93c4-dc877efdd7b9n%40googlegroups.com. -- 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/96D5EEE6-5E5B-4064-BAA3-AD8B985FE1F0%40gmail.com.