Golang重映射接口Go-cache

Question

I've got a struct like below:

type Page struct {
    title string
    url   string
}

and a map of structs:

 var mostViewed = make(map[int]Page)

With go-cache, I store the map with a TTL time.

c.Set("data", mostViewed, 60*time.Minute)    

But, once I recover "data" key, how could I assing it back to a map?

a, _ := c.Get("data")
fmt.Printf("%+v
", a)

out: map[17:{title:xxx, url:yyy}]

I tried something like:

 z := map[int]Page{a}

Any clue? It's like "remapping" a mapped string.

Answer 1

You get an interface{} type back, but you know what it is, so you need to use a type assertion to cast it back to a map[int]Page. Here is a quick external resource. https://newfivefour.com/golang-interface-type-assertions-switch.html

Here is an example

https://play.golang.org/p/lGseg88K1m

type Page struct {
    title string
    url   string
}

func main() {
    m := make(map[int]Page)
    m[1] = Page{"hi", "there"}
    iface := makeIface(m)

    // use type assertion to cast it back to a map
    if mapAgain, ok := iface.(map[int]Page); ok {
        // it really is a map[int]Page
        fmt.Printf("%+v
", mapAgain)
    } else {
        // its not actually a map[int]Page
        fmt.Println("oops")
    }

    // alternatively use a type-switch if it could be multiple types
    switch v := iface.(type) {
        case map[int]Page:
            //yay
            fmt.Printf("%+v
", v)
        case string:
            // its a string
            fmt.Println(v)
        default:
            // something we didn't account for

    }

}

func makeIface(m map[int]Page) interface{} {
    return m
}

Edit: as a side note, you probably want to make your map type map[int]*Page because if you did something like this:

page := m[1]
page.url = "different"
fmt.Println(page) // prints url="different"
fmt.Println(m[1].url) // remains unchanged

Because page would be a copy of what is in the map, not the Page in the map itself.