Unlocking Go’s Empty Interface: Store Any Type in One Variable

Empty Interface

An empty interface is an interface that defines no methods. Because it has no methods, any Go value satisfies it, allowing any object to be stored in a variable of type interface{}.

Definition

type empty_int interface{}

It is often abbreviated directly as interface{}.

Usage Examples

Declaring an empty interface variable: var i interface{} Using an empty interface as a function parameter, e.g., the fmt package’s Println:

func Println(a ...interface{}) (n int, err error)

Empty Interface in Data Structures

You can create slices, maps, or structs that hold interface{} values, enabling storage of heterogeneous types.

Example of a slice of empty interfaces:

package main

import "fmt"

func main() {
    any := make([]interface{}, 5)
    any[0] = 11
    any[1] = "hello world"
    any[2] = []int{11, 22, 33, 44}
    for _, v := range any {
        fmt.Println(v)
    }
}

The output demonstrates that the slice can hold integers, strings, and other slices.

Copying to an Empty Interface Slice

Directly assigning a typed slice to an []interface{} slice causes a compile‑time error because the memory layout differs (each empty interface element occupies two machine words). To copy, iterate and append each element:

var any []interface{}
for _, v := range testSlice {
    any = append(any, v)
}

This approach ensures each empty interface element references the underlying value correctly.