除了将方法临时分配给变量之外，是否可以将方法绑定到Go映射？

Question

this code works fine but the temp var used to call the function feels clunky

package main

import "fmt"

type Foo struct {
    name  string
    value int
}

// SetName receives a pointer to Foo so it can modify it.
func (f *Foo) SetName(name string) {
    f.name = name
}

var users = map[string]Foo{}

func main() {
    // Notice the Foo{}. The new(Foo) was just a syntactic sugar for &Foo{}
    // and we don't need a pointer to the Foo, so I replaced it.
    // Not relevant to the problem, though.
    //p := Foo{}
    users["a"] = Foo{value: 1}
    x := users["a"]
    x.SetName("Abc")
    users["a"] = x
    fmt.Println(users)
}

http://play.golang.org/p/vAXthNBfdP

Answer 1

Unfortunately no. In Go typically pointers are transparent, and values get auto-addressed when you call pointer methods on them. You managed to find one of the few cases where they aren't. That case is map storage -- values in maps are not considered addressable. That is, you can never do val := &map[key].

When you have a value val := Typ{} and methods defined on *Typ, when you try to call val.Method() Go will super secretly do (&val).Method(). Since you can't do &map[key], then this doesn't work so that temporary variable dance you do is the only way.

As for why that's the case, the internals of a map are considered a bit secret to the user, since it's a hashmap it reserves the right to reallocate itself, shuffle around data, etc, allowing you to take the address of any value undermines that. There have been proposals considered to allow this specific case to work (that is: calling a method with a pointer receiver on it), since the fix is so easy, but none have been accepted yet. It may be allowed someday, but not right now.