On Fri, Jan 11, 2019 at 5:59 PM Xinhu Liu <xinhu.li...@gmail.com> wrote:
> Hi Ian,
>
> thanks for your reply.
>
> After reading and experimenting a lot I think I understand the slight
> differences between nil and interface{} with nil value.
>
> The only thing I find confusing is that interface{}(nil) == nil returns
> true.
>
In that context, they are the same thing -- the nil interface. In fact
SomeInterface(nil)
== interface{}(nil) is also true -- the values are both the nil interface.
(The nil identifier is quite overloaded in Go since it can also indicate a
nil pointer, a nil map, a nil slice, a nil function, or a nil channel,
depending on context).
The issue at hand is that reflect.ValueOf(nil) does *not* give you an
instance with type interface{} and value nil -- it gives you a zero-value
(e.g. invalid) instance. This is because reflect.ValueOf returns the
*concrete* type and value of the input value, but nil interface has no
concrete type or value. So, in order to get a reflect.Value of type
interface{} and nil value, you have to go through some hoops.
>
> (The reason why I use reflect on interface is that I want to operate slice
> with different element types, like []interface{}, []int and so on.)
>
> Best regards,
> Xinhu
>
> Am Fr., 11. Jan. 2019 um 01:39 Uhr schrieb Ian Lance Taylor <
> i...@golang.org>:
>
>> On Thu, Jan 10, 2019 at 3:40 PM <xinhu.li...@gmail.com> wrote:
>> >
>> > I want to append a nil value using reflect.Append(), but I got a panic:
>> reflect: call of reflect.Value.Set on zero Value.
>> >
>> > This is my code.
>> >
>> > s := make([]interface{}, 10)
>> > v := reflect.ValueOf(nil)
>> > reflect.Append(reflect.ValueOf(s), v)
>> >
>> > So is there a way i can append nil? Currently I do this as workaround:
>> >
>> > s := make([]interface{}, 10)
>> > s0 := make([]interface{}, 1)
>> > r := reflect.AppendSlice(reflect.ValueOf(s), reflect.ValueOf(s0))
>> >
>> > It works but I think there must be a better way.
>>
>> First you have to clearly distinguish between nil, an uninitialized
>> value with no type, and the value interface{}(nil), which is an
>> uninitialized value of type interface{}. You want the latter. Then,
>> it's always a bit awkward to work with interface types with the
>> reflect package, because the reflect package naturally tries to look
>> for the value stored in the interface type, whereas you actually want
>> the interface type.
>>
>> Here is one way to do it:
>>
>> https://play.golang.org/p/Qt5-AmYa9Mk
>>
>> Ian
>>
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