没有实例并将结果传递给func的TypeOf [英] TypeOf without an instance and passing result to a func

问题描述

是否可以在没有实例的情况下获取类型"?我看过一些利用 reflect.TypeOf()的示例，但是它们都处理一个实例.

Is it possible in go to get a "Type" without an instance? I've seen some examples that utilize reflect.TypeOf() but they all deal with an instance.

下面是我正在尝试做的一小段:

Below is a snippet of what I am attempting to do:

import (
    "net/http"
)

type ParamReader struct {
    // The request from which to extract parameters
    context *http.Request
}

// Initialize the ParamReader with a specific http request. This serves
// as the 'context' of our param reader. All subsequent calls will validate
// the params that are present on this assigned http.Request
func (p *ParamReader) Context(r *http.Request) {
    p.context = r
}

// Validate that a given param 's' is both present and a valid
// value of type 't'. A value is demeed valid if a conversion from 
// its string representation to 't' is possible
func(p *ParamReader) Require(s string, t Type) {
    // if context not have 's'
    //      addError('s' is not present)
    //      return


    if( t == typeof(uint64)) {
        // If not s -> uint64
        //      addError('s' is not a valid uint64)
    } else if (t == typeof(uint32)) {
        // ....
    } / ....
}

我的用法示例是

func (h *Handler) OnRequest(r *http.Request) {
  h.ParamReader.Context(r)
  h.ParamReader.Require("age", uint16)
  h.ParamReader.Require("name", string)
  h.ParamReader.Require("coolfactor", uint64)
  h.ParamReader.Optional("email", string, "unspecified")
  h.ParamReader.Optional("money", uint64, "0")

  if h.ParamReader.HasErrors() {
    // Iterate or do something about the errors
  } else {
    coolness := h.ParamReader.ReadUint64("coolfactor")
    email := h.ParamReader.ReadString("email")
    money := h.ParamReader.ReadUint64(0)
  }
}

注意，在写完这些之后，我意识到我可以提供"RequireUint64" ，"RequireUint32" 等.也许这就是Go的方式吗?

Note, after writing this out, I realize I could provide a "RequireUint64", "RequireUint32", etc.. perhaps that would be the Go way?

推荐答案

是的，有可能.诀窍是从指向类型的指针开始(其值可以是 typed nil ，这完全可以)，然后使用 Type.Elem()以获得指向类型( base 类型)的 reflect.Type 描述符.

Yes, it's possible. The trick is to start from a pointer to the type (whose value can be a typed nil, that's perfectly OK), and then use Type.Elem() to get the reflect.Type descriptor of the pointed type (the base type).

查看一些示例:

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)

输出(在转到游乐场上尝试):

int
http.Request
os.File

查看相关问题:

Golang反映:从名称中获取类型表示吗?

如何获取一种类型?

如果要传递类型并在 switch es中使用它们，可以像这样一次创建并存储在全局变量中，并引用全局变量:

If you want to pass around the types and use them in switches, you can create and store them in global variables once like this, and refer to the global vars:

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)
}

上述内容的输出(在转到游乐场上尝试)

Output of the above (try it on the Go Playground):

Type: int
Type: http.request
Type: os.file
Type: uint64

但是，老实说，如果您以这种方式使用它，并且没有使用 reflect.Type 的方法，那么创建常量将变得更加容易和高效.看起来可能像这样:

But honestly, if you're using it like this way and you're not using reflect.Type's methods, then creating constants is much easier and more efficient. It could look like this:

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)
}

输出是相同的.在去游乐场上尝试.

Output is the same. Try it on the Go Playground.

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