Hello awesome community!
I am so honored to be a part of the great community of gophers around the
world. Thank you to everyone who makes it so inviting to everyone who wants
to participate.
I am writing today to ask about some puzzling behavior with respect to
dereferencing pointers, function evaluation and tuple assignment. I have
the following code snippet:
type T struct {
i int
}
func (t *T) sideEffectPtr() *int {
t.i += 4
return &t.i
}
func derefIntPtr(i *int) int {
return *i
}
func main() {
var d = T{i: 200}
var e = T{i: 200}
a, b := derefIntPtr(d.sideEffectPtr()), derefIntPtr(d.sideEffectPtr())
aa, bb := *e.sideEffectPtr(), *e.sideEffectPtr()
fmt.Println(a, b)
fmt.Println(aa, bb)
}
You can see the code in action on the playground here:
https://play.golang.org/p/wGsvYWXEqf
The output is surprising, to say the least:
204 208
208 208
>From what I understand, the right hand side of a tuple assignment is
completely evaluated before the assignment happens. I also think that I
understand that for evaluations not covered specifically by the language
specification, the particular order of the evaluation is not defined. In
other words, in
aa, bb := *e.sideEffectPtr(), *e.sideEffectPtr()
the calls to sideEffectPtr() may happen in any order.
However, I would imagine that, no matter what the order, the dereference
operation would happen immediately after the first function call and before
the second. That would lead me to believe that one output would be 204 and
the other would be 208.
Indeed that is what happens when you perform the dereference operation via
a function:
a, b := derefIntPtr(d.sideEffectPtr()), derefIntPtr(d.sideEffectPtr())
but that is NOT what happens when you use the * operator directly.
I have spent a significant amount of time thinking about this and
consulting (what I think) is the relevant part of the spec:
https://golang.org/ref/spec#Order_of_evaluation. I've tried to write a
relatively succinct test case and provide this brief description.
If anyone can shed light on what I am missing, I would really appreciate
it.
Again, thank you so much for creating such a great community and for any
time that you can spare to help me through this issue.
Will
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