Explain type assertion and type switch with real-world examples.

Question

Accepted Answer

var i interface{} = "hello"

// Single assertion — panics if wrong type
s := i.(string)
fmt.Println(s) // "hello"

// Safe assertion with ok check
s, ok := i.(string)
if ok {
    fmt.Println("string:", s)
} func handleError(err error) (int, string) {
    switch e := err.(type) {
    case *ValidationError:
        return 400, e.Message
    case *NotFoundError:
        return 404, "resource not found: " + e.Resource
    case *AuthError:
        return 401, "unauthorized"
    default:
        return 500, "internal server error"
    }
}

// Type switch on interface{} — common when decoding JSON
func printValue(v interface{}) {
    switch val := v.(type) {
    case string:
        fmt.Println("string:", val)
    case float64:
        fmt.Println("number:", val)
    case bool:
        fmt.Println("bool:", val)
    case map[string]interface{}:
        fmt.Println("object with keys:", len(val))
    case nil:
        fmt.Println("null")
    }
} Real-world use: encoding/json unmarshals into interface{} where numbers become float64, objects become map[string]interface{}, and arrays become []interface{}. Type switches are essential for processing such dynamic data safely.

Explain type assertion and type switch with real-world examples.

Type assertion extracts the concrete type from an interface value. A type switch checks multiple possible types in sequence, and is commonly used when handling JSON or errors.

Scenario Questions

Follow-up Questions