Ok, thanks for the clarification. Is there some way to reliably handle these situations? I really like the idea of custom value types as keys.
On Tue, Nov 28, 2017, 5:22 PM roger peppe <rogpe...@gmail.com> wrote: > > What tripped me up aside from reusing the map in the example was that in > case of errors a value type will still be put in the with it's default > value which my or may not for me. > > The reason for that is that your UnmarshalText method was being called > on a pointer to the zero value, > but was a value method, so the value was copied (all value methods are > implemented by pointer > values too), then your method couldn't change the original value, so > the JSON unmarshaler > just saw the zero value unchanged. > > On 28 November 2017 at 16:10, Henrik Johansson <dahankz...@gmail.com> > wrote: > > The time example I have was just an example. > > I have a trivial struct as key. > > > > What tripped me up aside from reusing the map in the example was that in > > case of errors a value type will still be put in the with it's default > value > > which my or may not for me. I avoided the whole thing by just using > strings > > as the key. A bit less typed but not unmanageable. > > > > Thanks for clarifying everyone! > > > > > > On Tue, Nov 28, 2017, 3:34 PM Marvin Renich <m...@renich.org> wrote: > >> > >> * Henrik Johansson <dahankz...@gmail.com> [171128 07:43]: > >> > But wouldn't unmarshal just overwrite in case of more trivial keys? > >> > > >> > So pointer receivers on the marshaling methods is better. > >> > I think I tried it but something else blew up. > >> > >> While MarshalText can use a value or pointer receiver, it makes no sense > >> to use a value receiver with UnmarshalText. The UnmarshalText method > >> must be able to change the caller's copy of the variable that is to hold > >> the unmarshalled data. > >> > >> As for time.Time, if you read its documentation, it says that it holds > >> both a wall clock time and a monotonic time. time.Now() returns a > >> structure that has both values, but some operations (e.g. time.Parse and > >> time.Time.UnmarshalJSON) return a time.Time that only has a wall clock > >> time. t.Round(0) is the canonical way to strip the monotonic time from > >> a time.Time value t. > >> > >> So after t := time.Now(); t2 := t.Round(0), t and t2 represent the same > >> wall clock time, but compare as different, because the t has both wall > >> clock and monotonic times, whereas t2 only has wall clock time. > >> > >> So your map with key time.Now() has a key with both times. When you > >> marshal it, the marshalled value only has wall clock time. When you > >> unmarshal it back to the same map, the unmarshalled time is different > >> (but represents the same wall clock time) from the existing map key, so > >> it creates an additional map element with the new key. > >> > >> If you create your map with keys that only have wall clock times with > >> UTC location, as in https://play.golang.org/p/BCB3TAZADB, the > >> unmarshalled key will match the existing key and overwrite it. If you > >> remove either .UTC() or .Round(0) or both from that code, you will > >> notice that the map keys are different, and you end up with two values > >> in the map after unmarshalling. > >> > >> ...Marvin > >> > >> -- > >> 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. > -- 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.
