>
>
>
> In this code,
> starting at 0,
> is case 1 not an aberration,
> is case 3 is useless
> https://play.golang.org/p/VyOfZyt7rw
>
> Note case 0 is expected to fail and might be detected.
>
>>
>> As a dumb hardware engineer I too have struggled with this in the past.
2 rules I find helped:
1) Everything into a function is passed as a copy
2) If you need to modify something passed to you in a function then you
need not a copy of the thing but the address of the thing you want to modify
so taking your code for both cases 0 & 1:
func (v valueType) SetName(n string) {
v.Name = n
}
when this function runs a local bit of storage is created (probably on the
stack as escape analysis would determine that the value can't escape the
scope) and the current value of the data is copied into that storage. That
storage is then modified. The function then returns. The local storage goes
out of scope and you are left with your original data unchanged.
for cases 2&3
func (v *refType) SetName(n string) {
v.Name = n
}
This time again the function runs and there is a local piece of storage
created (again called v) this time the local storage (probably on the stack
as v cannot escape the scope of that function) and v gets the pointer to
the variable it was called on copied into it instead of the value.
So when you set the name it uses the local copy of the pointer to access
the Name field. Because this is pointing to the underlying storage for the
original structure that structure is modified.
What might be causing confusion here (because it used to confuse me with
go) is that go is doing something automatically under the hood. When you
type v.Name it knows that *refType does not have a field Name. It knows
that you're trying to access the structure addressed by the pointer and so
automatically de-references the pointer for you i.e. takes the address held
in v, works out what offset from that address it has to use to get at the
Name field and then directs the assignment to that new address.
At least that's how I think it works. I'm sure someone will correct me, as
this would otherwise be the first time I had this low level detail of
software correct ;-)
HTH
Chris
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