Chapter 12 of 20

Interfaces

Implicit implementation, the empty interface, type assertions

Interface Definition and Implicit Implementation

Go interfaces are defined by a method set; a type implements an interface automatically as long as it has every method in the set — no implements keyword.

package main

import "fmt"

type Stringer interface {
    String() string
}

type User struct{ Name string }

func (u User) String() string { return "User(" + u.Name + ")" }

func main() {
    var s Stringer = User{"Alice"}
    fmt.Println(s.String())
}

The Empty Interface and any

interface{} has no methods, so every type satisfies it. Since Go 1.18, any is a built-in alias for it.

package main

import "fmt"

func Print(v any) { fmt.Println(v) }

func main() {
    Print(1)
    Print("hi")
    Print([]int{1, 2})
}

Type Assertions and type switch

package main

import "fmt"

func describe(v any) {
    switch x := v.(type) {
    case int:
        fmt.Println("int", x)
    case string:
        fmt.Println("string", x)
    default:
        fmt.Printf("%T %v\n", x, x)
    }
}

func main() {
    describe(42)
    describe("hi")
    describe(3.14)
}

Interface Design Tips

The smaller the interface the better: the standard library's io.Reader and io.Writer each have just one method
Interfaces are defined by the consumer, not "declared as implemented" by the implementer
Return concrete types, accept interfaces (accept interfaces, return structs)