Playground link for above code: https://play.golang.org/p/rTrkkzEjRl
On Tuesday, 9 August 2016 17:48:22 UTC+1, Sam Salisbury wrote:
>
> Thanks for the explanation,
>
> The update I suggest is to add the following sentence to the end of the
> reflect.Value.IsNil documentation:
>
> Likewise, if v was created by calling ValueOf on an initialised
>> interface{} value with a nil value pointer j, v.IsNil will return true,
>> whereas j == nil will return false.
>
>
> Based on the following observation:
>
> var m map[string]interface{}
> var i interface{} = m
> fmt.Printf("reflect.ValueOf(i).IsNil() == %t\n",
> reflect.ValueOf(i).IsNil())
> fmt.Printf("i == nil == %t\n", i == nil)
> // output:
> // reflect.ValueOf(i).IsNil() == true
> // i == nil == false
>
>
>
> On Tuesday, 9 August 2016 14:43:41 UTC+1, Ian Lance Taylor wrote:
>>
>> On Tue, Aug 9, 2016 at 6:23 AM, Sam Salisbury <[email protected]>
>> wrote:
>> >
>> > All this gets me thinking, is there any use case where this fact is
>> useful?
>> > (I.e. a nil-valued interface not being equal to nil via the ==
>> operator.)
>>
>> This has been discussed several times on the mailing list. An
>> interface value == nil if it is the zero value of the interface type.
>> It does not == nil if it holds the zero value of some other type, even
>> if the zero value of that other type is nil. Many people would be
>> surprised if
>> var v interface{} = 0
>> fmt.Printf("%t\n", v == nil)
>> printed true. It should be equally surprising if
>> var v interface{} = (*byte)(nil)
>> fmt.Printf("%t\n", v == nil)
>> printed true.
>>
>> The confusion results because Go, perhaps unfortunately, uses the name
>> nil to designate both the zero value of an interface and the zero
>> value of a pointer (and a slice or map or channel too, for that
>> matter). If the names were different, this would be less confusing.
>>
>> > Also, should the reflect.Value.IsNil documentation be updated? It
>> doesn't
>> > mention this case where it differs*, only the one about it panicking on
>> a
>> > zero reflect.Value.
>> >
>> > * IsNil returns true for an interface{} with a nil value pointer, even
>> > though '== nil' return false.
>>
>> What update do you suggest? You say there is a "case where it
>> differs", but by my reading there is not. It may help to read
>> https://blog.golang.org/laws-of-reflection .
>>
>> Ian
>>
>
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