英文:
Go methods sets — Calling method for pointer type *T with receiver T
问题
根据Go语言规范中的说明，类型T的方法集由所有接收者类型为T的方法组成。相应指针类型T的方法集由所有接收者类型为T或T的方法组成（也就是说，它还包含了类型T的方法集）。
你的理解是正确的：类型T有自己的方法集，而类型T有自己的方法集以及类型T的方法集，因为它可以将接收者 T解引用为T并调用方法。因此，我们可以使用接收者类型为*T的方法来调用变量类型为T的方法。
你决定验证自己的逻辑，提供了一个示例代码。你有点困惑。看起来我可以在T上调用“*T的方法”？你还提供了一个更复杂的示例，也让你感到困惑。是否存在某种相反的类型推断？
你是否漏掉了什么，还是我的逻辑有误？
谢谢！
英文:
Go spec says:
> The method set of any other type T consists of all methods with receiver type T. The method set of the corresponding pointer type *T is the set of all methods with receiver *T or T (that is, it also contains the method set of T).
I understand this as: T has its own method set, while *T has it own method set plus the method set of T, because it can dereference receiver *T to T and call the method. Therefore, we can call some method with receiver *T of variable type T.
So I decided to verify my logic:
package main
import (
"fmt"
"reflect"
)
type User struct{}
func (self *User) SayWat() {
fmt.Println(self)
fmt.Println(reflect.TypeOf(self))
fmt.Println("WAT\n")
}
func main() {
var user User = User{}
fmt.Println(reflect.TypeOf(user), "\n")
user.SayWat()
}
http://play.golang.org/p/xMKuLzUbIf
I am a bit confused. It looks like I can call methods "of *T" on T? I have a bit wider example http://play.golang.org/p/RROPMj534A, which confuses me too. Is there some vice versa type inference?
Am I missing something, or my logic is incorrect?
Thanks!
答案1
得分: 10
你不能在T上调用*T的方法，但编译器足够智能，会为你获取变量的引用，从而有效地调用
(&user).SayWat()
这在这里有解释：
> 调用：如果方法集（类型的）x包含m，并且参数列表可以分配给m的参数列表，则方法调用x.m()是有效的。如果x是可寻址的，并且&x的方法集包含m，则x.m()是(&x).m()的简写。
为了理解区别，你可以考虑一个返回值（不可寻址）：
func aUser() User {
return User{}
}
...
aUser().SayWat()
会出现错误：
prog.go:40: cannot call pointer method on aUser()
prog.go:40: cannot take the address of aUser()
http://play.golang.org/p/HOTKiiOK7S
英文:
You cannot call a method of *T on T, but the compiler is smart enough to take the reference of the variable for you, effectively calling
(&user).SayWat()
This is explained here:
> Calls: A method call x.m() is valid if the method set of (the type of) x
contains m and the argument list can be assigned to the parameter list of
m. If x is addressable and &x's method set contains m, x.m() is shorthand
for (&x).m().
To understand the difference, you can for instance take a return value (non-addressable):
func aUser() User {
return User{}
}
...
aUser().SayWat()
Fails with error:
prog.go:40: cannot call pointer method on aUser()
prog.go:40: cannot take the address of aUser()
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