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draft!! dot parenthesis

 2 years ago
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draft!! dot parenthesis

# Type Switch

  • https://golang.org/doc/effective_go#type_switch
var t interface{}
t = functionOfSomeType()
switch t := t.(type) {
default:
    fmt.Printf("unexpected type %T\n", t)     // %T prints whatever type t has
case bool:
    fmt.Printf("boolean %t\n", t)             // t has type bool
case int:
    fmt.Printf("integer %d\n", t)             // t has type int
case *bool:
    fmt.Printf("pointer to boolean %t\n", *t) // t has type *bool
case *int:
    fmt.Printf("pointer to integer %d\n", *t) // t has type *int
}
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# Type Assertion

Type Switch 使用了 type 标识符，如果我们只关心一种类型的话，可以使用确定 的 type 类型来替代 type 标识符，如 value.(typeName)

如果 interface{} 中不包含类型 typeName 就会报错。

var i interface{}
i = int(42)

a, ok := i.(int)
// a == 42 and ok == true

b, ok := i.(string)
// b == "" (default value) and ok == false
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type Stringer interface {
    String() string
}

if str, ok := value.(string); ok {
	  return str
} else if str, ok := value.(Stringer); ok {
    return str.String()
}
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# 案例

由于历史原因或代码对接过程中没有做到统一数据类型

如出现下面两种类型

type Message struct {
   Name string
   Time string
}
type Message struct {
   Name string
   Time float64
}
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处理的方式就是结合使用 interface{} 和 type switch

type Message struct {
   Name string
   Time interface{}
}

b := []byte(`{"Name":"Wednesday","Time":6}`)

var f Message
json.Unmarshal(b, &f)

switch v := f.Time.(type) {
case float64:
   fmt.Printf("Name: %v; Time: %v\n", f.Name, v)
   fmt.Printf("Time type %T!\n", v)
case string:
   fmt.Printf("Name: %v; Time: %v\n", f.Name, v)
   fmt.Printf("Time type %T!\n", v)
default:
   fmt.Printf("I don't know about type %T!\n", v)
}
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参考：Golang Type switches


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