如何检查 interface{} 是否为切片

Question

I'm noob in Go :) so my question may be stupid, but can't find answer, so.

I need a function:

func name (v interface{}) {
    if is_slice() {
        for _, i := range v {
            my_var := i.(MyInterface)
            ... do smth
        }
    } else {
        my_var := v.(MyInterface)
        ... do smth
    }
}

How can I do is_slice in Go? Appreciate any help.

Answer 1

In your case the type switch is the simplest and most convenient solution:

func name(v interface{}) {
    switch x := v.(type) {
    case []MyInterface:
        fmt.Println("[]MyInterface, len:", len(x))
        for _, i := range x {
            fmt.Println(i)
        }
    case MyInterface:
        fmt.Println("MyInterface:", x)
    default:
        fmt.Printf("Unsupported type: %T\n", x)
    }
}

The case branches enumerate the possible types, and inside them the x variable will already be of that type, so you can use it so.

Testing it:

type MyInterface interface {
    io.Writer
}

var i MyInterface = os.Stdout
name(i)
var s = []MyInterface{i, i}
name(s)
name("something else")

Output (try it on the Go Playground ):

MyInterface: &{0x1040e110}
[]MyInterface, len: 2
&{0x1040e110}
&{0x1040e110}
Unsupported type: string

For a single type check you may also use type assertion :

if x, ok := v.([]MyInterface); ok {
    // x is of type []MyInterface
    for _, i := range x {
        fmt.Println(i)
    }
} else {
    // x is not of type []MyInterface or it is nil
}

There are also other ways, using package reflect you can write a more general (and slower) solution, but if you're just starting Go, you shouldn't dig into reflection yet.

Answer 2

icza's answer is correct, but is not recommended by go creators :

interface{} says nothing

A better approach may be to define a function for each type you have:

func name(v MyInterface) {
    // do something
}

func names(vs []MyInterface) {
    for _, v := range(vs) {
        name(v)
    }
}

Answer 3

The is_slice method can be something like this:

func IsSlice(v interface{}) bool {
    return reflect.TypeOf(v).Kind() == reflect.Slice
}

Can also put additional condition of reflect.TypeOf(v).Kind() == reflect.Array if required.

Answer 4

From https://blog.golang.org/json

Decoding arbitrary data

Consider this JSON data, stored in the variable b:

b := []byte(`{"Name":"Wednesday","Age":6,"Parents":["Gomez","Morticia"]}`)

Without knowing this data's structure, we can decode it into an interface{} value with Unmarshal:

var f interface{} err := json.Unmarshal(b, &f) At this point the Go value in f would be a map whose keys are strings and whose values are themselves stored as empty interface values:

f = map[string]interface{}{
    "Name": "Wednesday",
    "Age":  6,
    "Parents": []interface{}{
        "Gomez",
        "Morticia",
    },
}

To access this data we can use a type assertion to access f's underlying map[string]interface{}:

m := f.(map[string]interface{})

We can then iterate through the map with a range statement and use a type switch to access its values as their concrete types:

for k, v := range m {
    switch vv := v.(type) {
    case string:
        fmt.Println(k, "is string", vv)
    case float64:
        fmt.Println(k, "is float64", vv)
    case []interface{}:
        fmt.Println(k, "is an array:")
        for i, u := range vv {
            fmt.Println(i, u)
        }
    default:
        fmt.Println(k, "is of a type I don't know how to handle")
    }
}

In this way you can work with unknown JSON data while still enjoying the benefits of type safety.