Golang中值类型/指针类型的差异

Question

type T struct {
    Name string
}
func (t T) M1 () {
    t.Name = "name1"
}
func (t *T) M2 () {
    t.Name = "name2"
}
type intf interface {
    M1()
    M2()
}
func main() {
    var t1 T = T{"t1"}
    t1.M1()
    t1.M2()
    var t2 intf = t1
    t2.M1()
    t2.M2()
}

Why when I usevar tf intf = &t1, It's correct But when I usevar tf intf = &t1, It's not correct Is there any difference in the two representations?

Answer 1

when I usevar tf intf = &t1, It's correct But when I usevar tf intf = &t1, it's not

Since in your example you are using tf intf = t1 (and it does not compile), I think you probably meant:

when I usevar tf intf = &t1, It's correct But when I usevar tf intf = t1, it's not

So the question here is why &T{"t1"} (pointer) fulfils the interface intf while T{"t1"} (value) does not.

In your example, the interface has two methods M1 and M2.

Type T implements both methods BUT:

• M1 has T value as the receiver
• M2 has T pointer as the receiver

So why does the compiler consider a T pointer to have both methods implemented but considers that a T value does not?

Answer can be found here in the spec:

https://golang.org/ref/spec#Method_sets

The method set of any other type T consists of all methods declared with receiver type T. The method set of the corresponding pointer type *T is the set of all methods declared with receiver *T or T (that is, it also contains the method set of T).

The spec says that a T pointer has all methods defined with receivers T and *T (so both M1 and M2 in this case).

But a value of T only has the methods defined with receiver T (so only M1 in this case).

Since the interface has both methods, only the pointer can fulfil it.