英文:
golang type assertion, what the different between interface{}(pointer) and interface{}(object)?
问题
为什么在将接口分配给指针时，对类型断言结果进行赋值是可以的，但是对分配给结构体对象的接口进行赋值时会出现“无法分配”的错误？
这是我的代码：
package main
import (
"fmt"
)
type Person interface {
SayHi()
}
type Student struct {
id int
name string
}
func (s Student) SayHi() {
fmt.Println("hi, i am", s.name, "my id is:", s.id)
}
func main() {
p1 := Person(&Student{id: 123, name: "William"})
p1.SayHi() // 正常
p1.(*Student).SayHi() // 正常
p1.(*Student).id = 456 // 正常
p2 := Person(Student{id: 123, name: "William"})
p2.SayHi() // 正常
p2.(Student).SayHi() // 正常
p2.(Student).id = 456 // 出现错误，为什么？
fmt.Println("p1:", p1, "p2:", p2)
}
链接：https://play.golang.org/p/dwkvLzng_n
英文:
Why do I assign a value to a type assertion result where the interface is assigned by a pointer, and it comes to a "cannot assign" error when I do it to a interface which is assigned by a struct object?
Here's my code:
package main
import (
"fmt"
)
type Person interface {
SayHi()
}
type Student struct {
id int
name string
}
func (s Student) SayHi() {
fmt.Println("hi, i am", s.name, " my id is:", s.id)
}
func main() {
p1 := Person(&Student{id: 123, name: "William"})
p1.SayHi() // ok
p1.(*Student).SayHi() // ok here
p1.(*Student).id = 456 // ok here
p2 := Person(Student{id: 123, name: "William"})
p2.SayHi() //ok
p2.(Student).SayHi() // ok here
p2.(Student).id = 456 // error here and why?
fmt.Println("p1:", p1, " p2:", p2)
}
答案1
得分: 5
value.(typeName) 的结果是一个具有静态类型 typeName 的新值（副本）。
p2.(Student).id=456 将创建一个临时的 Student 值，对该值的任何修改都将被丢弃。因此，语言只是禁止了这个错误。
英文:
the result of value.(typeName) is a new (copy) value with the static type typeName.
p2.(Student).id=456 will create a temporary Student value, any modifications of that value would be discarded. So the language just disallows this mistake.
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