Go Methods in Depth
A method set is the group of methods one value can answer. The receiver kind decides which values hold which methods.
A value receiver leaves the original, a pointer receiver changes it
Counter has one method of each receiver kind. Both methods add one, and the prints show which change stays.
package main
import "fmt"
type Counter struct {
N int
}
func (c Counter) BumpCopy() {
c.N++
}
func (c *Counter) Bump() {
c.N++
}
func main() {
c := Counter{N: 5}
c.BumpCopy()
fmt.Println(c.N)
c.Bump()
fmt.Println(c.N)
}An interface holding a pointer answers a pointer method
Speaker asks for a Speak method, and Robot defines Speak on a pointer receiver. An interface holding the pointer can call it.
package main
import "fmt"
type Speaker interface {
Speak() string
}
type Robot struct {
Name string
}
func (r *Robot) Speak() string {
return r.Name + " speaks"
}
func main() {
var s Speaker = &Robot{Name: "Beep"}
fmt.Println(s.Speak())
}An interface holding a value is refused
The same interface now holds a plain Robot value. The value lacks the pointer method, and the compiler refuses the program.
package main
import "fmt"
type Speaker interface {
Speak() string
}
type Robot struct {
Name string
}
func (r *Robot) Speak() string {
return r.Name + " speaks"
}
func main() {
var s Speaker = Robot{Name: "Beep"}
fmt.Println(s.Speak())
}Methods on a named slice type
Names is a named slice type with two methods. The methods read the slice like any other receiver value.
package main
import "fmt"
type Names []string
func (n Names) First() string {
return n[0]
}
func (n Names) Count() int {
return len(n)
}
func main() {
names := Names{"Ana", "Bo", "Cy"}
fmt.Println(names.First())
fmt.Println(names.Count())
}Example
package main
import "fmt"
type Thermostat struct {
Temp int
}
func (t Thermostat) Read() int {
return t.Temp
}
func (t *Thermostat) Heat() {
t.Temp += 2
}
func (t *Thermostat) Cool() {
t.Temp--
}
func main() {
th := Thermostat{Temp: 20}
fmt.Println(th.Read())
th.Heat()
fmt.Println(th.Read())
th.Cool()
fmt.Println(th.Read())
}Key points
- A method with a value receiver receives a copy of the value.
- A change inside a value receiver method leaves the original unchanged.
- A method with a pointer receiver receives the pointer, so its changes reach the original.
- The method set of a pointer includes the methods of both receiver kinds.
- The method set of a value holds only the value receiver methods.
- An interface value can call only the methods in the method set of the value it holds.
- A method can sit on any named type, not only on a struct type.
In this example, the thermostat starts at 20. The Read method has a value receiver, so it reports the current value without changing it. The Heat method has a pointer receiver, and it raises Temp by 2. The next read prints 22. The Cool method lowers Temp by 1, and the last read prints 21.
Note
A pointer value in an interface keeps the full method set, so it answers pointer receiver methods.
Try it Yourself
Edit the code, then press Run.
Running is simulated · Go Playground
Exercise
A method must change the struct it is called on. Which receiver should the method use?