英文:
How to get a pointer of interface{} to type check interface implementation
问题
所以我基本上想找出实现类似以下代码的最佳方法:
package main
import "fmt"
type SomeStruct struct {
}
type SomeInterface interface {
SomeMethodWhichNeedsAPointerReceiver() string
}
func (s *SomeStruct) SomeMethodWhichNeedsAPointerReceiver() string {
return "Well, believe me, I wrote something"
}
func Run(s interface{}) {
// 在这里如何将 s 转换为指针?
casted, ok := s.(SomeInterface)
if ok {
fmt.Println("Awesome: " + casted.SomeMethodWhichNeedsAPointerReceiver())
return
}
fmt.Println("Fail :(")
}
func SomeThirdPartyMethod() interface{} {
return SomeStruct{}
}
func main() {
x := SomeThirdPartyMethod()
Run(x)
}
我的问题在于 Run 方法中的类型转换。
我只知道它的类型是 interface{},现在我需要调用一个具有指针接收器的接口方法。
我目前唯一的解决方案是使用反射动态构建一个切片,将元素设置到切片中,然后使其可寻址。
这真的是找到解决方案的唯一可能性吗?
英文:
So I'm basically trying to find out the best way to get to something like this:
package main
import "fmt"
type SomeStruct struct {
}
type SomeInterface interface {
SomeMethodWhichNeedsAPointerReceiver() string
}
func (s *SomeStruct) SomeMethodWhichNeedsAPointerReceiver() string {
return "Well, believe me, I wrote something"
}
func Run(s interface{}) {
// how can I cast s to a pointer here?
casted, ok := (s).(SomeInterface)
if ok {
fmt.Println("Awesome: " + casted.SomeMethodWhichNeedsAPointerReceiver())
return
}
fmt.Println("Fail :(")
}
func SomeThirdPartyMethod() interface{} {
return SomeStruct{}
}
func main() {
x := SomeThirdPartyMethod()
Run(x)
}
My problem here is, at the typecast in the Run method.
I basically only know that it is of type interface{} and now I need
to call an interface method, which has a pointer receiver.
My only solution at the moment is to dynamically construct a slice with reflection, set the element into the slice, and than make it adressable.
Is this really the only possibility to find a solution?
答案1
得分: 6
在Go语言中,接口只是一组方法的集合(规范:接口类型)。
接口不指定接收者是指针还是非指针。在Go语言中,实现接口是隐式的:没有声明意图。
你需要阅读并理解方法集。引用其中的一个重要部分:
> 任何其他类型 T
的方法集由所有声明了接收者类型为 T
的方法组成。相应的指针类型 *T
的方法集是由所有声明了接收者为 *T
或 T
的方法组成的(也就是说,它还包含了 T
的方法集)。
一个值(可能是指针,也可能不是指针)实现了一个接口,如果它的类型的方法集是接口的超集(即接口类型的方法)。
在你的例子中:
func (s *SomeStruct) SomeMethodWhichNeedsAPointerReceiver() string{}
SomeMethodWhichNeedsAPointerReceiver()
有一个指针接收者,所以类型为 SomeStruct
的值将没有这个方法,但是类型为 *SomeStruct
的值将有。
因此,类型为 SomeStruct
的值不实现你的 SomeInterface
接口(因为它没有 SomeMethodWhichNeedsAPointerReceiver()
方法),但是 *SomeStruct
实现你的 SomeInterface
,因为它的方法集包含了 SomeMethodWhichNeedsAPointerReceiver()
方法。
由于你创建并使用的是一个简单的 SomeStruct
值(而不是指向它的指针),类型断言将失败。
如果你使用了 *SomeStruct
,类型断言将会成功。
修改你的 SomeThirdPartyMethod()
函数来创建并返回一个 *SomeStruct
(一个指针),它将按照你的期望工作:
func SomeThirdPartyMethod() interface{} {
return &SomeStruct{}
}
或者作为另一种选择:
func SomeThirdPartyMethod() interface{} {
return new(SomeStruct)
}
在Go Playground上试一试。
如果你不能修改 SomeThirdPartyMethod()
如果你不能修改 SomeThirdPartyMethod()
函数:首先,它是否有意义/需要返回一个实现 SomeStruct
的值?如果是这样,那么它当前的实现是错误的,你可以期望它返回一个实现 SomeStruct
的值,而不需要你对其进行解引用(为了获得一个实现 SomeStruct
的值)。
在这种特殊情况下,你还可以尝试对 SomeStruct
本身进行类型断言:
if ss, ok := s.(SomeStruct); ok {
fmt.Println(ss.SomeMethodWhichNeedsAPointerReceiver())
}
调用 ss.SomeMethodWhichNeedsAPointerReceiver()
将自动对 ss
进行解引用,以获取其地址(这将作为调用 SomeMethodWhichNeedsAPointerReceiver()
方法的指针接收者值)。
英文:
In Go, an interface is just a set of methods (spec: Interface types).
The interface does not specify whether receivers are pointers or not. Implementing an interface in Go is implicit: there is no declaration of intent.
You need to read and understand Method sets. Quoting an important part of it:
> 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
).
A value (which may or may not be a pointer) implements an interface if the method set of its type is a superset of the interface (the methods of the interface type).
In your example:
func (s *SomeStruct) SomeMethodWhichNeedsAPointerReceiver() string{}
SomeMethodWhichNeedsAPointerReceiver()
has a pointer receiver, so a value of type SomeStruct
will not have this method, but a value of type *SomeStruct
will.
So as a consequence a value of type SomeStruct
does not implement your SomeInterface
interface (because it does not have a SomeMethodWhichNeedsAPointerReceiver()
method), but *SomeStruct
does implement your SomeInterface
because its method set contains the SomeMethodWhichNeedsAPointerReceiver()
method.
Since you create and use a simple SomeStruct
value (and not a pointer to it), the type assertion will fail.
Should you have used a *SomeStruct
, the type assertion would have succeeded.
Modify your SomeThirdPartyMethod()
function to create and return a *SomeStruct
(a pointer) and it will work as you expect it:
func SomeThirdPartyMethod() interface{} {
return &SomeStruct{}
}
Or as an alternative:
func SomeThirdPartyMethod() interface{} {
return new(SomeStruct)
}
Try it on the Go Playground.
If you can't modify SomeThirdPartyMethod()
If you can't modify the SomeThirdPartyMethod()
function: first of all, is it intended/required that it returns a value which implements SomeStruct
? If so, then its current implementation is wrong, and you can expect it to return a value which does implement SomeStruct
without you having to dereference it (in order to gain a value which implements SomeStruct
).
In this specific case you can also try type assertion for SomeStruct
itself:
if ss, ok := s.(SomeStruct); ok {
fmt.Println(ss.SomeMethodWhichNeedsAPointerReceiver())
}
Calling ss.SomeMethodWhichNeedsAPointerReceiver()
will automatically dereference ss
to take its address (which will be the pointer receiver value for calling the SomeMethodWhichNeedsAPointerReceiver()
method).
答案2
得分: 4
最好的解决方案确实是从SomeThirdPartyMethod
返回一个指针。但如果这不可能,你可以使用反射构造一个指向该值的指针:
v := reflect.Indirect(reflect.New(reflect.TypeOf(s)))
v.Set(reflect.ValueOf(s))
sp := v.Addr().Interface()
casted, ok := sp.(SomeInterface)
if ok {
fmt.Println("也可以:", casted.SomeMethodWhichNeedsAPointerReceiver())
return
}
工作示例:http://play.golang.org/p/JYJT8mRxWN
英文:
The best solution would indeed be to return a pointer from SomeThirdPartyMethod
. But if this is not possible, you can construct a pointer to that value using reflection:
v := reflect.Indirect(reflect.New(reflect.TypeOf(s)))
v.Set(reflect.ValueOf(s))
sp := v.Addr().Interface()
casted, ok = (sp).(SomeInterface)
if ok {
fmt.Println("Good as well: " + casted.SomeMethodWhichNeedsAPointerReceiver())
return
}
Working example: http://play.golang.org/p/JYJT8mRxWN
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