类型断言落在引用/指针上

Question

I am trying to get a map out of the reference and the assertion seems to be failing:

session.AddFlash(gin.H{
    "username": username,
    "password": password,
}, "_post")

...

flashes := session.Flashes("_post")
if flashes != nil {
    flash = flashes[0]
    fmt.Printf("%T\n", flash)
    if post_data, ok := flash.(gin.H); ok {
        fmt.Println("post_data:", post_data)
        data = post_data
    } else {
        fmt.Println("post_data(failed):", post_data)
    }
}

However I always get the following output, which the assert fails:

*gin.H
post_data(failed): map[]

I assume its due to the assertion I am doing so I've tried:

if post_data, ok := (*flash).(gin.H); ok {

But then I get invalid indirect of flash (type interface {})

I have also tried:

if post_data, ok := *flash.(gin.H); ok {

But that gives me invalid indirect of flash.(gin.H) (type gin.H)

Answer 1

You're putting the * at the wrong place. It should be flash.(*gin.H) .

Answer 2

Type assertions require the type you want to assert to be of the exact same type of the assertion part.

From your code, flash is a type interface{} , and the underlying type is *gin.H (printed by fmt.Printf("%T", flash) ). Therefore the assertion should be flash.(*gin.H)

Here are a few examples about of to use type assertions:

package main

import r "reflect"

type A struct {
    Name string
}

func main() {
    // No pointer
    aa := A{"name"}
    var ii interface{} = aa

    bb := ii.(A)
    // main.A

    // Pointer
    a := &A{"name"}
    var i interface{} = a

    b := *i.(*A)
    // main.A

    c := i.(*A)
    // *main.A

    d := r.Indirect(r.ValueOf(i)).Interface().(A)
    // main.A
}

}