How to cast reflect.Value to its type?

转载自 https://stackoverflow.com/questions/17262238/how-to-cast-reflect-value-to-its-type

43

 

9

 

How to cast reflect.Value to its type?

type Cat struct { 
    Age int
}

cat := reflect.ValueOf(obj)
fmt.Println(cat.Type()) // Cat

fmt.Println(Cat(cat).Age) // doesn't compile
fmt.Println((cat.(Cat)).Age) // same

Thanks!

reflectiongo

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asked Jun 23 '13 at 15:23

 

Alex

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• Go has no type casting. You can't cast anything. – Flimzy Dec 25 '18 at 13:11

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4 Answers

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54

 

concreteCat,_ := reflect.ValueOf(cat).Interface().(Cat)

see http://golang.org/doc/articles/laws_of_reflection.html fox example

type MyInt int
var x MyInt = 7
v := reflect.ValueOf(x)
y := v.Interface().(float64) // y will have type float64.
fmt.Println(y)

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edited Jun 14 '14 at 10:18

Benjamin

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answered Dec 19 '13 at 8:05

 

sharewind

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• 15

  That's if you actually have knowledge of the type. In the case you would like to apply kind of a blind type assertion, I'm actually not sure it is possible. – Taye Aug 17 '14 at 11:00

• One solution seems to be to leave the reflect.Value as-is and simply move all your calculations to using the reflect package without hope of ever converting to back to a regular struct. You can see this in gongular. Related: play.golang.org/p/Ok_q3lx490h – Xeoncross Mar 1 '18 at 4:28

• 2

  It doesn't work. panic: interface conversion: interface {} is main.MyInt, not float64 – TomSawyer May 3 '18 at 19:47

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33

 

Ok, I found it

reflect.Value has a function Interface() that converts it to interface{}

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edited Jun 23 '13 at 16:37

fuz

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answered Jun 23 '13 at 15:32

Alex

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• 19

  That converts it to interface{}. How do we convert it to the actual type? – Matt Dec 4 '13 at 16:40

• 1

  It's not possible in Go, @Matt. You can't reinvent generics without unsafe or manual type conversions (in Go 1 at least). – Tejas Manohar Jan 31 '17 at 22:44

• concreteCat, _ := cat.Interface().(Cat) is worked for me instead of reflect.ValueOf(cat).Interface().(Cat) – yountae.kang Aug 10 '18 at 3:01

• @yountae.kang I'm not sure you found out why that happens, but either way the reason why cat.Interface().(Cat) works for you is because in the above use case cat is of type reflect.Value and there is no need to convert it to the reflect.Value type again. – hbn1991 Feb 20 '19 at 14:38

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2

 

This func auto-converts types as needed. It loads a config file values into a simple struct based on struct name and fields:

import (
    "fmt"
    toml "github.com/pelletier/go-toml"
    "log"
    "os"
    "reflect"
)
func LoadConfig(configFileName string, configStruct interface{}) {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("LoadConfig.Recovered: ", r)
        }
    }()
    conf, err := toml.LoadFile(configFileName)
    if err == nil {
        v := reflect.ValueOf(configStruct)
        typeOfS := v.Elem().Type()
        sectionName := getTypeName(configStruct)
        for i := 0; i < v.Elem().NumField(); i++ {
            if v.Elem().Field(i).CanInterface() {
                kName := conf.Get(sectionName + "." + typeOfS.Field(i).Name)
                kValue := reflect.ValueOf(kName)
                if (kValue.IsValid()) {
                    v.Elem().Field(i).Set(kValue.Convert(typeOfS.Field(i).Type))
                }
            }
        }
    } else {
        fmt.Println("LoadConfig.Error: " + err.Error())
    }
}

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answered Jan 2 at 18:13

 

rribas

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0

 

Seems the only way would be to do a switch statement similar to (code below) (also, something like the commented line would've-been nice though doesn't work (:()):

 

func valuesFromStruct (rawV interface{}) []interface{} {
    v := reflect.ValueOf(rawV)
    out := make([]interface{}, 0)
    for i := 0; i < v.NumField(); i += 1 {
        field := v.Field(i)
        fieldType := field.Type()
        // out = append(out, field.Interface().(reflect.PtrTo(fieldType)))
        switch (fieldType.Name()) {
        case "int64":
            out = append(out, field.Interface().(int64))
            break`enter code here`
        case "float64":
            out = append(out, field.Interface().(float64))
            break
        case "string":
            out = append(out, field.Interface().(string))
            break
        // And all your other types (here) ...
        default:
            out = append(out, field.Interface())
            break
        }
    }
    return out
}