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 <[email protected]> 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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