In https://blog.golang.org/error-handling-and-go, it shows the well-known
error interface:
type error interface {
Error() string
}
and it gives an example of a user-defined error:
// errorString is a trivial implementation of error.
type errorString struct {
s string
}
func (e *errorString) Error() string {
return e.s
}
// New returns an error that formats as the given text.
func New(text string) error {
return &errorString{text}
}
I notice that it's returning a *pointer* to an error object. My question
is: is there a particular reason for returning a pointer here, rather than
a direct instance of an error?
The word "pointer" doesn't appear anywhere in that blog posting. Clearly a
pointer allows the possibility of returning "nil" - but an interface
variable can have a nil value <https://tour.golang.org/methods/13> too.
Without the pointers, it becomes:
func (e errorString) Error() string {
return e.s
}
func New(text string) error {
return errorString{text}
}
and this appears to work the same - see a longer example on play
<https://play.golang.org/p/joVzZ-PPs8E> (based on someone else's example,
but I just removed the pointers).
I'm aware that the method set of *T includes the methods of T, so a pointer
functions perfectly well in place of a concrete value to implement an
interface. Also, passing a pointer might be slightly more efficient, in
that it reduces a bit of copying. Other than that, is there a reason why
code should return pointers to error objects rather than error objects?
I did try looking through the FAQ, which asks "Should I define methods on
values or pointers?
<https://golang.org/doc/faq#methods_on_values_or_pointers>" (although not
"should I return values or pointers?"), and ends up by saying:
*For types such as basic types, slices, and small structs, a value receiver
is very cheap so unless the semantics of the method requires a pointer, a
value receiver is efficient and clear.*
Which makes me wonder if I'm missing some other reason for using pointers
in this context.
Many thanks,
Brian.
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