如何打开reflect.Type？

Question

I have managed to do this, but it does not look efficient:

var t reflect.Type
switch t {
case reflect.TypeOf(([]uint8)(nil)):
    // handle []uint8 array type
}

Answer 1

First question, are you sure you want to switch on reflect.Type and not use a type switch? Example:

switch x := y.(type) {
case []uint8:
  // x is now a []uint8
}

Assuming that will not work for your situation, my recommendation is to make those package variables. Example:

var uint8SliceType = reflect.TypeOf(([]uint8)(nil))

func Foo() {
    var t reflect.Type
    switch t {
    case uint8SliceType:
        // handle []uint8 array type
    }

}

Answer 2

you may not need reflect if you are just trying to detect type.

switch t := myVar.(type){
  case []uint8:
    // t is []uint8
  case *Foo:
    // t is *Foo
  default:
    panic("unknown type")
}

What are you actually trying to accomplish?

Answer 3

The answer to the initial question How to switch on reflect.Type? is: You can't. However, you can do it with reflect.Value .

• Given a variable v interface{} you can call reflect.TypeOf(v) and reflect.ValueOf(v) , which return a reflect.Type or reflect.Value , resp.
  • If the type of v is not interface{} then these function calls will convert it to interface{} .
• reflect.Type contains various run-time information about the type, but it does not contain anything usable to retrieve the type of v itself as needed in a type switch.
• Hovewer, reflect.Value provides it through its Interface() method, which returns the underlying value as interface{} . This you can use in a type switch or type assertion.

import "fmt"
import "reflect"

var v int
var rt reflect.Type = reflect.TypeOf(v)
fmt.Println(rt.String(), " has awesome properties: Its alignment is",
    rt.Align(), ", it has", rt.Size(), "bytes, is it even comparable?",
    rt.Comparable())
// … but reflect.Type won’t tell us what the real type is :(
// Let’s see if reflect.Value can help us.
var rv reflect.Value = reflect.ValueOf(v)
// Here we go:
vi := rv.Interface()
switch vi.(type) {
// Mission accomplished.
}

Perhaps it helps to clarify a few points which may cause confusion about dynamic typing in Go. At least I was confused by this for quite some time.

reflect vs. interface{}

In Go there are two systems of run-time generics:

• In the language : interface{} , useful for type switches/assertions,
• In the library : The reflect package, useful for inspection of run-time generic types and values of such.

These two systems are separated worlds, and things that are possible with one are impossible with the other. For example, Given an interface{} , it is in plain Go (with safe code) impossible to, say, if the value is an array or slice, regardless of its element type, then get the value of the i-th element. One needs to use reflect in order to do that. Conversely, with reflect it is impossible to make a type switch or assertion: convert it to interface{} , then you can do that.

There are only very few points of an interface between these systems. In one direction it is the TypeOf() and ValueOf() functions which accept interface{} and return a reflect struct. In the other direction it is Value.Interface() .

It is a bit counter-intuitive that one needs a Value , not a Type , to do a type switch. At least this is somewhat consistent with the fact that one needs a value construct a Type by calling TypeOf() .

reflect.Kind

Both reflect.Type and reflect.Value have a Kind() method. Some suggest using the value these methods return, of type reflect.Kind , to imitate a type switch.

While this may be useful in certain situations, it is not a replacement for a type switch. For example, using Kind one cannot distinguish between int64 and time.Duration because the latter is defined as

type Duration int64

Kind is useful to tell if a type is any kind of struct, array, slice etc., regardless of the types it is composed of. This is not possible to find out with a type switch.

(Side note. I had the same question and found no answer here helpful so I went to figure it out myself. The repeated counter-question “why are you doing this?”, followed by unrelated answers did not help me either. I have a good reason why I want to do it precisely this way.)

Answer 4

This might work.

switch t := reflect.TypeOf(a).String() {
   case "[]uint8":
   default:
}

Answer 5

As others have said, it's not clear what you are trying to achieve by switching on reflect.Type However, I came across this question when probably trying to do something similar, so I will give you my solution in case it answers your question.

As captncraig said , a simple type switch could be done on a interface{} variable without needing to use reflect.

func TypeSwitch(val interface{}) {
    switch val.(type) {
        case int:
            fmt.Println("int with value", val)
        case string:
            fmt.Println("string with value ", val)
        case []uint8:
            fmt.Println("Slice of uint8 with value", val)
        default:
            fmt.Println("Unhandled", "with value", val)
    }
}

However, going beyond this, the usefulness of reflection in the context of the original question could be in a function that accepts a struct with arbitrarily typed fields, and then uses a type switch to process the field according to its type. It is not necessary to switch directly on reflect.Type, as the type can be extracted by reflect and then a standard type switch will work. For example:

type test struct {
    I int
    S string
    Us []uint8
}

func (t *test) SetIndexedField(index int, value interface{}) {
    e := reflect.ValueOf(t).Elem()
    p := e.Field(index)
    v := p.Interface()
    typeOfF := e.Field(index).Type()
    switch v.(type) {
        case int:
            p.SetInt(int64(value.(int)))
        case string:
            p.SetString(value.(string))
        case []uint8:
            p.SetBytes(value.([]uint8))
        default:
            fmt.Println("Unsupported", typeOfF, v, value)
    }
}

The following examples demonstrate the use of this function:

var t = test{10, "test string", []uint8 {1, 2, 3, 4}}
fmt.Println(t)
(&t).SetIndexedField(0, 5)
(&t).SetIndexedField(1, "new string")
(&t).SetIndexedField(2, []uint8 {8, 9})
fmt.Println(t)

(A few points on reflection in go:

1. It is necessary to export the struct fields for reflect to be able to use them, hence the capitalisation of the field names
2. In order to modify the field values, it would be necessary to use a pointer to the struct as in this example function
3. Elem() is used to "dereference" the pointer in reflect

)

Answer 6

Well, I did this by first transfer it to interface and then use the .(type)

    ty := reflect.TypeOf(*c)
    vl := reflect.ValueOf(*c)
    for i:=0;i<ty.NumField();i++{
        switch vl.Field(i).Interface().(type) {
        case string:
            fmt.Printf("Type: %s Value: %s \n",ty.Field(i).Name,vl.Field(i).String())
        case int:
            fmt.Printf("Type: %s Value: %d \n",ty.Field(i).Name,vl.Field(i).Int())
        }
    }