英文:
Can someone explain this interface example in Go?
问题
从http://jordanorelli.com/post/32665860244/how-to-use-interfaces-in-go中有一个示例，说明了在Go中使用接口的可能用法。代码如下：
package mainimport ("encoding/json""fmt""reflect""time")// 从一个字符串表示的JSON数据开始var input = `{"created_at": "Thu May 31 00:00:01 +0000 2012"}`type Timestamp time.Timefunc (t *Timestamp) UnmarshalJSON(b []byte) error {v, err := time.Parse(time.RubyDate, string(b[1:len(b)-1]))if err != nil {return err}*t = Timestamp(v)return nil}func main() {// 我们的目标类型将是map[string]interface{}，这是一种非常通用的类型// 它将给我们一个哈希表，其中键是字符串，值是interface{}类型var val map[string]Timestampif err := json.Unmarshal([]byte(input), &val); err != nil {panic(err)}fmt.Println(val)for k, v := range val {fmt.Println(k, reflect.TypeOf(v))}fmt.Println(time.Time(val["created_at"]))}
运行结果如下：
map[created_at:{63474019201 0 0x59f680}]created_at main.Timestamp2012-05-31 00:00:01 +0000 UTC
我很难理解函数调用json.Unmarshal([]byte(input), &val)与之前定义的方法func (t *Timestamp) UnmarshalJSON(b []byte) error之间的关系。阅读http://golang.org/pkg/encoding/json/#Unmarshal上的文档更加令我困惑。
显然我在这里漏掉了一些东西，但我无法弄清楚。
英文:
From http://jordanorelli.com/post/32665860244/how-to-use-interfaces-in-go there an example illustrating a possible use of interfaces in Go. Code as below:
package mainimport ("encoding/json""fmt""reflect""time")// start with a string representation of our JSON datavar input = `{"created_at": "Thu May 31 00:00:01 +0000 2012"}`type Timestamp time.Timefunc (t *Timestamp) UnmarshalJSON(b []byte) error {v, err := time.Parse(time.RubyDate, string(b[1:len(b)-1]))if err != nil {return err}*t = Timestamp(v)return nil}func main() {// our target will be of type map[string]interface{}, which is a pretty generic type// that will give us a hashtable whose keys are strings, and whose values are of// type interface{}var val map[string]Timestampif err := json.Unmarshal([]byte(input), &val); err != nil {panic(err)}fmt.Println(val)for k, v := range val {fmt.Println(k, reflect.TypeOf(v))}fmt.Println(time.Time(val["created_at"]))}
with a result like this:
map[created_at:{63474019201 0 0x59f680}]created_at main.Timestamp2012-05-31 00:00:01 +0000 UTC
I am struggling to understand how the function call
json.Unmarshal([]byte(input), &val){...}
relates to the method defined earlier
func (t *Timestamp) UnmarshalJSON(b []byte) error{...}
Reading the doc at http://golang.org/pkg/encoding/json/#Unmarshal is confusing me even more.
I am obviously missing something here, but I can't figure it out.
答案1
得分: 3
在Go语言中，接口的实现只需要实现其方法。这与大多数其他流行的语言（如Java、C#、C++）非常不同，在这些语言中，类接口必须在类声明中明确指出。
关于这个概念的详细解释可以在Go文档中找到：https://golang.org/doc/effective_go.html#interfaces
因此，func (t *Timestamp) UnmarshalJSON(...)定义了一个方法，并同时实现了接口。然后，json.Unmarshal将val的元素类型断言为Unmarshaler接口（http://golang.org/pkg/encoding/json/#Unmarshaler），并调用UnmarshalJSON方法从字节切片构造它们。
英文:
In Go an interface is implemented just by implementing its methods. It is so much different from the most other popular languages (Java, C#, C++) in which the class interfaces should be explicitly mentioned in the class declaration.
The detailed explanation of this concept you can find in the Go documentation: https://golang.org/doc/effective_go.html#interfaces
So the func (t *Timestamp) UnmarshalJSON(...) defines a method and in a same time implements the interface. The json.Unmarshal then type asserts the elements of val to the Unmarshaler interface (http://golang.org/pkg/encoding/json/#Unmarshaler) and call the UnmarshalJSON method to construct them from the byte slice.
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