How to Get a Type Without an Instance in Go?

TypeOf Without an Instance: Passing Result to a Function

In Go, it is possible to obtain a "Type" without having an instance of that type. This can be achieved through the use of reflect.TypeOf(), but typically examples involve an instance.

Getting Type Without Instance

To acquire a "Type" without an instance, the trick is to start with a pointer to the type, which can be assigned a "typed" nil. Subsequently, .Elem() can be used to obtain the reflect.Type descriptor of the pointed type, known as the base type.

<code class="go">t := reflect.TypeOf((*int)(nil)).Elem()
fmt.Println(t)

t = reflect.TypeOf((*http.Request)(nil)).Elem()
fmt.Println(t)

t = reflect.TypeOf((*os.File)(nil)).Elem()
fmt.Println(t)</code>

Example Output:

int
http.Request
os.File

Passing Type Around

If you need to pass around types and utilize them in switches, create and store them in global variables for reference:

<code class="go">var (
    intType         = reflect.TypeOf((*int)(nil))
    httpRequestType = reflect.TypeOf((*http.Request)(nil))
    osFileType      = reflect.TypeOf((*os.File)(nil))
    int64Type       = reflect.TypeOf((*uint64)(nil))
)

func printType(t reflect.Type) {
    switch t {
    case intType:
        fmt.Println("Type: int")
    case httpRequestType:
        fmt.Println("Type: http.request")
    case osFileType:
        fmt.Println("Type: os.file")
    case int64Type:
        fmt.Println("Type: uint64")
    default:
        fmt.Println("Type: Other")
    }
}

func main() {
    printType(intType)
    printType(httpRequestType)
    printType(osFileType)
    printType(int64Type)
}</code>

Simplified Approach

If you're utilizing the types solely for comparison purposes, consider using constants instead of reflect.Type:

<code class="go">type TypeDesc int

const (
    typeInt TypeDesc = iota
    typeHttpRequest
    typeOsFile
    typeInt64
)

func printType(t TypeDesc) {
    switch t {
    case typeInt:
        fmt.Println("Type: int")
    case typeHttpRequest:
        fmt.Println("Type: http.request")
    case typeOsFile:
        fmt.Println("Type: os.file")
    case typeInt64:
        fmt.Println("Type: uint64")
    default:
        fmt.Println("Type: Other")
    }
}

func main() {
    printType(typeInt)
    printType(typeHttpRequest)
    printType(typeOsFile)
    printType(typeInt64)
}</code>

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