英文:
Go: Pointer to interface{} loses underyling type
问题
我正在使用Go语言中的一些“通用”函数，这些函数在interface{}上操作并在通道中发送数据等。简化一下，假设我有以下代码：
type MyType struct {
// 字段
}
func (m *MyType) MarshalJSON() ([]byte, error) {
// 自定义的MarshalJSON方法
log.Print("custom JSON marshal")
return []byte("hello"), nil
}
func GenericFunc(v interface{}) {
// 做一些操作...
log.Print(reflect.TypeOf(v))
log.Print(reflect.TypeOf(&v))
b, _ := json.Marshal(&v)
fmt.Println(string(b))
}
func main() {
m := MyType{}
GenericFunc(m)
}
这将输出：
2014/11/16 12:41:44 MyType
2014/11/16 12:41:44 *interface {}
而不是自定义的输出。据我所知，这是因为调用Marshal时，它看到的是指向接口的指针类型，而不是指向MyType的指针类型。
为什么当我使用&v时会丢失类型信息？我期望输出的第二行是*MyType而不是*interface{}。
有没有办法在不显式转换的情况下调用自定义的JSON编组方法？
英文:
I'm working with some "generic" functions in Go that operate on interface{} and send things around channels, etc. Slimmed down, let's say I have something like:
type MyType struct {
// Fields
}
func (m *MyType) MarshalJSON() ([]byte, error) {
// MarshalJSON
log.Print("custom JSON marshal")
return []byte("hello"), nil
}
func GenericFunc(v interface{}) {
// Do things...
log.Print(reflect.TypeOf(v))
log.Print(reflect.TypeOf(&v))
b, _ = json.Marshal(&v)
fmt.Println(string(b))
}
func main() {
m := MyType{}
GenericFunc(m)
}
This outputs:
2014/11/16 12:41:44 MyType
2014/11/16 12:41:44 *interface {}
Followed by the default json.Marshal output, rather than the custom one. As far as I can tell, that's because the call to Marshal sees a value of type pointer-to-interface rather than pointer-to-MyType.
Why do I lose type information when I take &v? I would expect the second line of the output to be *MyType and not *interface {}.
Is there any way for me have the custom JSON Marshaller called without explicitly casting?
答案1
得分: 2
只需将指向您的结构体的指针传递给函数，而不是传递其值。指针仍然是interface{}类型，但指向接口的指针是没有意义的。
英文:
Just pass a pointer to your struct and not its value to the function. The pointer is still interface{} but a pointer to the interface is meaningless.
答案2
得分: 0
看起来你想要在chan interface{}上发送非指针值，并且希望自定义的MarshalJSON方法能够正常工作。在这种情况下，只需不在指针类型上定义该方法。
请参考这里
package main
import (
"encoding/json"
"fmt"
"log"
"time"
)
func printer(in chan interface{}) {
for val := range in {
buf, err := json.Marshal(val)
if err != nil {
log.Println(err.Error())
}
log.Println(string(buf))
}
}
type MyType struct {
name string
}
func (m MyType) MarshalJSON() ([]byte, error) {
return []byte(fmt.Sprintf(`"%s"`, m.name)), nil
}
func main() {
ch := make(chan interface{})
go printer(ch)
ch <- "string value"
ch <- 25
ch <- MyType{
name: "foo",
}
time.Sleep(time.Second)
}
唯一的真正区别是方法的接收者。使用func (m MyType) MarshalJSON ([]byte, error)而不是func (m *MyType) MarshalJSON ([]byte, error)。
英文:
It sounds like you want to send non-pointer values over a chan interface{} and have a custom MarshalJSON method work as expected. In that case, just don't define the method on the pointer type.
See here
package main
import (
"encoding/json"
"fmt"
"log"
"time"
)
func printer(in chan interface{}) {
for val := range in {
buf, err := json.Marshal(val)
if err != nil {
log.Println(err.Error())
}
log.Println(string(buf))
}
}
type MyType struct {
name string
}
func (m MyType) MarshalJSON() ([]byte, error) {
return []byte(fmt.Sprintf(`"%s"`, m.name)), nil
}
func main() {
ch := make(chan interface{})
go printer(ch)
ch <- "string value"
ch <- 25
ch <- MyType{
name: "foo",
}
time.Sleep(time.Second)
}
The only real difference is the method receiver. func (m MyType) MarshalJSON ([]byte, error) instead of func (m *MyType) MarshalJSON ([]byte, error)
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