Interfaces in Go
In Go, an interface
is a set of method signatures. A variable of interface type can hold any value that implements those methods.
Basics
You can declare an interface
like the following code. It is basically just a list of methods.
type NAME_OF_INTERFACE interface {
method_name1(return_type1)
method_name2(return_type2)
..........
method_namex(return_typex)
}
Any type that provides the methods named in an interface is treated as an implementation of that interface. No explicit declaration is required.
The empty interface
The interface type that does not specify any method is known as the empty interface
. It can be defined as the following code.
interface {}
A variable of empty interface type can hold values of any type.
package main
import "fmt"
func main(){
var intface interface{}
intface = 1
fmt.Println(intface) //=> 1
intface = "string" //=> string
fmt.Println(intface)
intface = []string{"Go", "Ruby", "JS"}
fmt.Println(intface) //=>[Go Ruby JS]
}
Interface values
A interface value
is represented as a pair of a concrete value and a dynamic type.
[Value, Type]
You can use %v
to print the concrete value and %T
to print the dynamic type.
package main
import "fmt"
func main(){
var intface interface{}
intface = 1
fmt.Printf("%v %T\n", intface, intface) //=>1 int
intface = "string"
fmt.Printf("%v %T\n", intface, intface) //=> string string
intface = []string{"Go", "Ruby", "JS"}
fmt.Printf("%v %T\n", intface, intface) //=> [Go Ruby JS] []string
}
Type assertions
A type assertion
provides access to an interface value's underlying concrete value.
concrete_value := Interface_value.(TYPE)
This statement above asserts that Interface_value
holds the concrete type TYPE
and assigns the underlying TYPE
value to variable
.
To check whether an interface value holds a specific type, a type assertion
can return two values. the underlying value
and a boolean value
that reports whether the assertion succeeded.
concrete_value, ok := Interface_value.(TYPE)
package main
import "fmt"
func main(){
var intface interface{} = "Hello World"
t := intface.(string)
fmt.Println(t) //=> Hello World
t2, ok := intface.(string)
fmt.Println(t2, ok) //=> Hello World true
t3, ok := intface.(float64)
fmt.Println(t3, ok) //=> 0 false
}
Type switches
A type switch
is a construct that permits several type assertions in series.
You can declare a type switch
like the following code.
switch v := x.(type) {
case TYPE1:
//here v has TYPE1
case TYPE2:
//here: v has TYPE2
...
default: ...
}
In the follwing code, the switch statement tests whether the interface value i
holds a value of type int
or string
. In each of the int
and string
cases, the variable v
will be of type int
or string
respectively and hold the value held by i
.
In the default case (where there is no match), the variable v
is of the same interface type and value as i
.
package main
import "fmt"
func typeSwitch(i interface{}) {
switch v := i.(type) {
case int:
fmt.Printf("Twice %v is %v\n", v, v*2)
case string:
fmt.Printf("%q is %v bytes long\n", v, len(v))
default:
fmt.Printf("I don't know about type %T!\n", v)
}
}
func main() {
typeSwitch(21) //=> Twice 21 is 42
typeSwitch("Hello World") //=> "Hello World" is 11 bytes long
typeSwitch(true)//=> I don't know about type bool!
}
Implementing interfaces
Interfaces
can be implemented as methods on structs like the following code.
package main
import "fmt"
type People interface {
intro()
}
type Person struct {
name string
}
func (rarg Person) intro() string{
return "Hello" + " " + "I'm" + " " + rarg.name
}
func main() {
bob := Person{"Bob"}
fmt.Println(bob.intro()) //=> Hello I'm Bob
}
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