Interfaces

An interface contains a collection of method signatures.
Interfaces are implemented implicitly. There is no implements keyword.

// Declaration
type name interface{}

type Shape interface {
    Area() float64
}

// Struct for implementation
type rectangle struct {
    width, height float64
}

func (r rectangle) Area() float64 {
    return r.Width * r.Height
}

The empty interface

An empty interface (interface{} or any with Go 1.18+) is implemented by all types.

Type assertions

Access the underlying concrete type of an interface with type assertion.

var i interface{} = "text"

// panic if type is wrong
var s string = i.(string)

// check type
s, ok := i.(string)

Type switches

Instead of the specific type, the keyword type is used.

var i interface{} = 1

switch v := i.(type) {
case int:
    // type of v = int
case string:
    // type of v = string
default:
    // type of v = same type as i
}