Thanks Dave & TL.
Regards,
Peng
在 2017年7月7日星期五 UTC+8上午9:44:55，Dave Cheney写道：
>
> This is a really interesting corner case of the syntactic sugar of the
> language.
>
> The first piece of sugar is that methods are just functions with a
> predefined first parameter, or to say that another way, a method is just a
> function with the receiver as it's first parameter. So
>
> func (t *T) Foo() becomes Foo(t *T)
>
> The second piece of sugar is automatically taking the address of, or
> dereferencing, the receiver depending on the type required. For example
>
> type T struct { }
>
> func (t *T) Foo() {}
>
> func main() {
> var t T
> t.Foo()
> }
>
> Foo is defined on *T, but inside main that method is invoked on a value of
> type T, so behind the scenes the compiler does this
>
> (&t).Foo()
>
> It takes the address of t, and passes that, as the first argument to Foo,
> as the receiver.
>
> In your example, even if you could define a method on a type that you
> didn't define, int, everyone knows that you cannot take the address of a
> constant literal;
>
> &42.F00() // doesn't work
>
> So, the question is, by converting the untyped integer literal 42 to a
> duration, why doesn't that work; or to be specific why does it only work
> when `pretty` is defined on a duration, not a *duration? The easiest way to
> explain it would be this
>
> func main() {
> const d = duration(42)
> d.pretty()
> }
>
> Hopefully you agree that this is the same as your example on line 17, and
> that this makes it clear that to dispatch to d's pretty method the compiler
> would have to take the address of d. And as d is a constant, not a
> variable, this is not permitted as constants are not addressable.
>
> Thanks
>
> Dave
>
> On Thursday, 6 July 2017 22:09:45 UTC+10, Peng wrote:
>>
>> Hi,
>> In the chapter of "Method set" from "Go in Action", there is a
>> description as below "This shows that it’s not always possible to get the
>> address of a value.", the question is in which case does compiler not able
>> to get the address of a value and why, thanks.
>>
>> 01 // Sample program to show how you can't always get the
>> 02 // address of a value.
>> 03 package main
>> 04
>> 05 import "fmt"
>> 06
>> 07 // duration is a type with a base type of int.
>> 08 type duration int
>> 09
>> 10 // format pretty-prints the duration value.
>> 11 func (d *duration) pretty() string {
>> 12 return fmt.Sprintf("Duration: %d", *d)
>> 13 }
>> 14
>> 15 // main is the entry point for the application.
>> 16 func main() {
>> 17 duration(42).pretty()
>> 18
>> 19 // ./listing46.go:17: cannot call pointer method on duration(42)
>> 20 // ./listing46.go:17: cannot take the address of duration(42)
>> 21 }
>>
>
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