如何获得指向被掩盖为接口的变量的指针? [英] How to get a pointer to a variable that's masked as an interface?
问题描述
我宁愿不深入探讨以下情况的理由.它与拆封序列化对象有关,该序列化对象可以是一组固定的类型,但是您不知道哪种类型.
I prefer not to dive into the rationale of the situation below. It has to do with unmarshaling an serialized object that can be any of a fixed set of types, but you don't know which type.
我有以下几种类型:
type I interface {
Do()
}
type someI struct {}
func (i *someI) Do() {}
type otherI struct {}
func (i *otherI) Do() {}
因此,两个结构的指针实现了接口I
.
So, two structs of which the pointers implement interface I
.
现在,我有此方法想要返回类型为I
的值:
Now I have this method that wants to return a value of type I
:
func GetSomeI(marshalled []byte) (I, error) {
var obj interface{}
// The following method magically puts an instance
// of either someI or otherI into obj.
magicUnmarshall(marshalled, obj)
// The problem now is that we cannot return obj,
// because the raw structs don't implement I.
// One solution would be to do a type switch like this:
switch obj.(type) {
case someI:
i := obj.(someI)
return &i, nil
case otherI:
i := obj.(otherI)
return &i, nil
default:
return nil, errors.New("marschalled object was not of type I")
}
// But now consider the case that there are quite some
// different implementations of I.
// We would prefer to have a general way of getting
// a reference to obj.
}
推荐答案
要判断包装在interface{}
中的值是否实现了其他接口(I
),您可以简单地使用
To tell if a value wrapped in an interface{}
implements some other interface (I
), you may simply use a type assertion.
请注意,您必须将要将结果解组的变量的地址传递给它.
Note that you must pass the address of the variable you want results unmarshaled to.
出于演示目的,让我们使用以下magicUnmarshal()
函数:
For demonstration purposes, let's use the following magicUnmarshal()
function:
func magicUnmarshal(what int, obj interface{}) {
v := reflect.ValueOf(obj).Elem()
switch what {
case 0:
v.Set(reflect.ValueOf(&someI{}))
case 1:
v.Set(reflect.ValueOf(&otherI{}))
case 2:
v.Set(reflect.ValueOf("just a string"))
case 3:
v.Set(reflect.ValueOf(someI{}))
case 4:
v.Set(reflect.ValueOf(otherI{}))
}
}
请注意,case 3
和case 4
返回非指针.
Note that case 3
and case 4
are returning non-pointers.
您的GetSomeI()
实现可以是:
func GetSomeI(what int) (I, error) {
var obj interface{}
magicUnmarshal(what, &obj)
// Try the value as-is:
if i, ok := obj.(I); ok {
return i, nil
}
// No success. Try a pointer to the value:
v := reflect.Indirect(reflect.New(reflect.TypeOf(obj)))
v.Set(reflect.ValueOf(obj))
pobj := v.Addr().Interface()
if i, ok := pobj.(I); ok {
return i, nil
}
return nil, fmt.Errorf("%T does not implement I!", obj)
}
第一个GeSomeI()
测试值是否由magicUnmarshal()
实现了I
,如果是,则按原样使用它.如果不是,我们使用反射构造一个新的,并获取其地址(指向值的指针),然后进行测试.如果该指针实现了I
,我们将其返回.
First GeSomeI()
tests if the value got form magicUnmarshal()
implements I
, and if so, it is used as-is. If not, we construct a new using reflection, and get its address (a pointer to a value), and we test that. If that pointer implements I
, we return it.
测试:
func main() {
for what := 0; what < 5; what++ {
i, err := GetSomeI(what)
fmt.Printf("%T %v\n", i, err)
}
}
输出是(在进入游乐场上尝试):
*main.someI <nil>
*main.otherI <nil>
<nil> string does not implement I!
*main.someI <nil>
*main.otherI <nil>
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