On Thu, Feb 22, 2024 at 9:00 AM Axel Wagner
<axel.wagner...@googlemail.com> wrote:
>
> Note that in the Spec section I quoted above it says "Two distinct zero-size
> variables may have the same address in memory" (emphasis mine).
> There is no guarantee, that all zero-sized values have the same address
> (otherwise, you'd get into inefficiencies when taking the address of a
> zero-sized field in a larger struct, or when converting a zero-capacity slice
> into an array-pointer). But it is allowed.
> If you require two pointers returned from different code paths to be
> different, for correctness, then you have to make them point at something
> that has non-zero size. Otherwise, all potential combinations are valid
> according to the spec.
Yes. But in that case, you'd expect either a==b and Bar(a)==Bar(b) to
be both true, or both false. In this case, one is true and the other
is not.
>
> On Thu, Feb 22, 2024 at 4:53 PM burak serdar <bser...@computer.org> wrote:
>>
>> The compiler can allocate the same address for empty structs, so I
>> actually expected a==b to be true, not false. However, there's
>> something more interesting going on here because:
>>
>> a := &Foo{}
>> b := &Foo{}
>> fmt.Printf("%t\n", *a == *b)
>> fmt.Printf("%t\n", a == b)
>> fmt.Printf("%p %p\n", a, b)
>> x := Bar(a)
>> y := Bar(b)
>> fmt.Printf("%t\n", Bar(a) == Bar(b))
>> fmt.Printf("%t\n", x == y)
>>
>> Prints:
>>
>> true
>> true
>> 0x58e360 0x58e360 // Note that a and be are pointing to the same address
>> true
>> true
>>
>>
>> But:
>> a := &Foo{}
>> b := &Foo{}
>> fmt.Printf("%t\n", *a == *b)
>> fmt.Printf("%t\n", a == b)
>> //fmt.Printf("%p %p\n", a, b) // Comment out the print
>> x := Bar(a)
>> y := Bar(b)
>> fmt.Printf("%t\n", Bar(a) == Bar(b))
>> fmt.Printf("%t\n", x == y)
>>
>>
>> Prints:
>>
>> true
>> false
>> true
>> true
>>
>>
>> On Thu, Feb 22, 2024 at 3:56 AM Brien Colwell <xcolw...@gmail.com> 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
>> > ```
>> >
>> >
>> > --
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>>
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