Hopefully these points help to clarify things: - The types of the parameters to compare of the shout interface type - The comparison operator == therefore operates on two interface values. With reference to the spec ( https://golang.org/ref/spec#Comparison_operators):
Interface values are comparable. Two interface values are equal if they > have identical dynamic types and equal dynamic values or if both have value > nil The dynamic types in all instances in your example are *a; the dynamic values (i.e. the *a values) are all non-nil, but only in the third case are the values (i.e. the address values) equal - For both &a and &b, you are using the address operator ( https://golang.org/ref/spec#Address_operators). The type of the variables (parameters) a and b is the shout interface type. Hence the expressions &a and &b will both be of type *shout. The value returned by &a is the address of the variable (parameter) a. Similarly for b. These values (of type *shout) do not have to be equal for the comparison a == b to return true; the equality operator on interface values is defined in a specific way, different to the comparison of pointers (which is in effect what you're doing by visually comparing &a and &b) Paul On 6 January 2017 at 10:24, Ignazio Di Napoli <neclep...@gmail.com> wrote: > I'm very curious about the answer to this unanswered StackOverflow > question: http://stackoverflow.com/questions/41357948/what- > golang-compiler-will-do-when-fmt-println > > > import ( > "fmt") > > type shout interface { > echo()} > > type a struct {} > func (*a) echo () { > fmt.Println("a")} > > type b struct {} > func (*b) echo () { > fmt.Println("b")} > > func compare(a, b shout) { > //fmt.Println(&a, &b) > if a == b { > fmt.Println("same") > } else { > fmt.Println("not same") > }} > > func main() { > a1 := &a{} > b1 := &b{} > a2 := &a{} > a1.echo() > b1.echo() > compare(a1, b1) > compare(a1, a2) > compare(a1, a1) } > > > The output is different if a fmt.Println is commented or not. > > Commented: > > not samenot same > same > > > Not commented: > > 0x1040a120 0x1040a128not same0x1040a140 0x1040a148 > same0x1040a158 0x1040a160 > same > > > > To me, it seems a compiler bug, or some undefined behaviour. > Can someone help me understand? > > > Thank you. > > > > -- > 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. > For more options, visit https://groups.google.com/d/optout. > -- 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. For more options, visit https://groups.google.com/d/optout.
