To give a real-world example:
https://pkg.go.dev/github.com/prometheus/client_golang/prometheus
type metrics struct {
cpuTemp prometheus.Gauge
hdFailures *prometheus.CounterVec
}
Question: why is prometheus.Gauge not a pointer, but *prometheus.CounterVec
is a pointer?
Answer: because prometheus.Gauge is an interface, whereas
prometheus.CounterVec is a struct.
An interface essentially already *contains* a pointer to a data value - in
fact, a tuple of (type, pointer). So you should never take a pointer to an
interface. Just pass the interface value, and it will copy the (type,
pointer) pair.
However, when you extract a value from an interface, it *will* always copy
the internal value. This avoids aliasing issues: someone who passes an
interface value to someone else, won't expect the recipient to be able to
change the sender's copy.
e.g.
https://go.dev/play/p/u5y3Kwm9Ydz
Of course, if your interface *contains* a pointer, then the recipient of
the interface can modify the thing being pointed to.
https://go.dev/play/p/o_XAJtNuyGF
But they can't modify the pointer itself held within the interface, to make
it point to something else.
The short version is: avoid pointers to interfaces, as the FAQ says.
Instead, let a concrete pointer value satisfy an interface.
On Friday, 25 November 2022 at 07:40:59 UTC Nigel Tao wrote:
> Possibly relevant:
> https://go.dev/doc/faq#pointer_to_interface
>
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