英文:
Replace value of interface
问题
我想要替换一个接口的值，像这样：
package main
import "fmt"
type Fooer interface {Foo(string)}
type Foo struct {foo string}
func (f *Foo) Foo(bar string) {f.foo = bar}
var z = &Foo{foo : "new"}
func swap(fooer Fooer) {fooer = z}
func main() {
f := &Foo{foo: "old"}
fmt.Printf("%s (want &{old})\n", f)
swap(f)
fmt.Printf("%s (want &{new})", f)
}
但是我得到的结果是：
&{old}
&{old}
我尝试了各种调用（fooer *= z，*fooer = *z，..），但是我似乎无法弄对。
你可以在 play.golang 上尝试这个例子：http://play.golang.org/p/EZEh3X8yHC
好的，我认为可以这样实现：
func swap(fooer Fooer) {
foo, _ := fooer.(*Foo)
*foo = *z
}
英文:
I want to replace an interface's value like this:
package main
import "fmt"
type Fooer interface {Foo(string)}
type Foo struct {foo string}
func (f *Foo) Foo(bar string) {f.foo = bar}
var z = &Foo{foo : "new"}
func swap(fooer Fooer) {fooer = z}
func main() {
f := &Foo{foo: "old"}
fmt.Printf("%s (want &{old})\n", f)
swap(f)
fmt.Printf("%s (want &{new})", f)
}
But i get:
&{old}
&{old}
I tried around with various calls (fooer *= z, *fooer = *z, ..) but I cant seem to get it right.
You can try this example at play.golang: http://play.golang.org/p/EZEh3X8yHC
Okay, I think it works like this:
func swap(fooer Fooer) {
foo, _ := fooer.(*Foo)
*foo = *z
}
答案1
得分: 10
与Go语言中的大多数内容一样，接口(interface)只是值。在函数内部分配一个新值并不会改变复制到函数参数中的值。
因为你想要替换接口值，所以你需要一个指向该值的指针，就像你对其他值一样。使用指向接口的指针是非常罕见的情况：http://play.golang.org/p/EZEh3X8yHC
func swap(fooer *Fooer) {
z := Fooer(&Foo{foo: "new"})
*fooer = z
}
func main() {
var f Fooer = &Foo{foo: "old"}
fmt.Printf("%s (want &{old})\n", f)
swap(&f)
fmt.Printf("%s (want &{new})", f)
}
但是，由于指向接口的指针几乎总是一个错误（你可以看到我们必须非常明确地使其工作），你应该真的有一个很好的理由以这种方式实现，并且要对其进行良好的文档记录。
你最有可能想要的是从接口中提取指针，并在其中分配一个新值（这是你在问题末尾添加的内容）。这是一个更好的构造，但类型必须匹配，所以接口是不需要的。
英文:
Like most things in Go, interfaces are simply values. Assigning a new value within a function isn't going to change the value copied into the function arguments.
Because you want to replace the interface value, you need a pointer to the value just like you would any other. It's a very rare case where you could use a pointer to an interface: http://play.golang.org/p/EZEh3X8yHC
func swap(fooer *Fooer) {
z := Fooer(&Foo{foo: "new"})
*fooer = z
}
func main() {
var f Fooer = &Foo{foo: "old"}
fmt.Printf("%s (want &{old})\n", f)
swap(&f)
fmt.Printf("%s (want &{new})", f)
}
But, since a pointer to an interface is almost always a mistake (you can see we have to be very explicit to make this work), you should really have a good reason for implementing something this way, and document it well.
What you most likely want is to extract the pointer from the interface, and assign a new value in there (which is what you added to end of your question). This is a much better construct, but the type must match, so the interface isn't needed.
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