Why can an empty interface in Go be assigned to any type?

The empty interface interface{} contains no methods. Any type in Go can satisfy an interface that requires no methods. Therefore, the value of any type can be assigned to a variable of type interface{} or passed to a function accepting interface{}. Essentially, interface{} acts as a container for a value of any type, storing both the value itself and its type.

package main

import "fmt"

func main() {
	var i interface{} // Declare a variable of empty interface type

	i = 42            // Assign an int
	fmt.Printf("Value: %v, Type: %T\n", i, i)

	i = "hello"       // Assign a string
	fmt.Printf("Value: %v, Type: %T\n", i, i)

	i = true          // Assign a bool
	fmt.Printf("Value: %v, Type: %T\n", i, i)

	i = struct{}{}    // Assign an empty struct
	fmt.Printf("Value: %v, Type: %T\n", i, i)
}

Casting to a specific type from interface{} is possible using type assertion or a type switch.

package main

import "fmt"

func main() {
	var i interface{} = "world"

	// Type assertion
	s, ok := i.(string) // Check if the value is a string
	if ok {
		fmt.Println("Type assertion successful:", s)
	} else {
		fmt.Println("Type assertion failed")
	}

	// Type switch
	switch v := i.(type) {
	case int:
		fmt.Println("Type is int, value:", v)
	case string:
		fmt.Println("Type is string, value:", v)
	default:
		fmt.Println("Unknown type")
	}
}