相反的reflect.TypeOf

Question

I want to retrieve back the type of value I saved once. I used reflect.Typeof() and saved the type. then try to use switch type. the type will always be "*reflect.rtype ". I couldn't retrieve either by type assertion.

package main

import (
    "fmt"
    "reflect"
)

func main() {
    var alltypes []interface{}

    alltypes = append(alltypes, reflect.TypeOf(true))
    alltypes = append(alltypes, reflect.TypeOf(0.0))
    alltypes = append(alltypes, reflect.TypeOf(0))
    fmt.Printf("%T\t%q
", alltypes, alltypes)

    for _, v := range alltypes {
        fmt.Printf("%T\t%q
", v, v)
        res, ok := v.(bool)
        fmt.Println("res: ", res, " ok: ", ok)
        switch v.(type) {
        default:
            fmt.Printf("unexpected type %T
", v)
        case bool:
            fmt.Println("bool type!")
        case int:
            fmt.Println("int type!")
        case float64:
            fmt.Println("float64 type!")
        }
    }

}

Playground: https://play.golang.org/p/kqDo4DPYjra

Answer 1

A reflect.Type holds no value you could type assert (actually you could, but that could only be reflect.Type, not what you want). A reflect.Type is just a type descriptor (which you obtained from a value).

However, you could create a value of the type represented by reflect.Type, and you can type-assert the values from that you originally wanted.

To create a new pointer value, use reflect.New(). To obtain the pointed value, use Value.Elem(). These are all wrapped in a reflect.Value. To unwrap it, use Value.Interface().

For example:

for _, v := range alltypes {
    fmt.Printf("%T\t%q
", v, v)
    value := reflect.New(v.(reflect.Type)).Elem().Interface()
    switch value.(type) {
    default:
        fmt.Printf("unexpected type %T
", v)
    case bool:
        fmt.Println("bool type!")
    case int:
        fmt.Println("int type!")
    case float64:
        fmt.Println("float64 type!")
    }
}

This will output (try it on the Go Playground):

[]interface {}  ["bool" "float64" "int"]
*reflect.rtype  "bool"
bool type!
*reflect.rtype  "float64"
float64 type!
*reflect.rtype  "int"
int type!

Also if you don't want to create new values just test the type, "save" the reflect.Type descriptors of the types you're interested in, and use a normal switch on the type:

var (
    TypeBool    = reflect.TypeOf(true)
    TypeFloat64 = reflect.TypeOf(0.0)
    TypeInt     = reflect.TypeOf(0)
)

func main() {
    var alltypes []interface{}

    alltypes = append(alltypes, reflect.TypeOf(true))
    alltypes = append(alltypes, reflect.TypeOf(0.0))
    alltypes = append(alltypes, reflect.TypeOf(0))
    fmt.Printf("%T\t%q
", alltypes, alltypes)

    for _, v := range alltypes {
        fmt.Printf("%T\t%q
", v, v)
        switch v {
        default:
            fmt.Printf("unexpected type %T
", v)
        case TypeBool:
            fmt.Println("bool type!")
        case TypeInt:
            fmt.Println("int type!")
        case TypeFloat64:
            fmt.Println("float64 type!")
        }
    }
}

This will output (try it on the Go Playground):

[]interface {}  ["bool" "float64" "int"]
*reflect.rtype  "bool"
bool type!
*reflect.rtype  "float64"
float64 type!
*reflect.rtype  "int"
int type!

Recommended reading: The Go Blog: The Laws of Reflection