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Receivers and Methods as Pointers and Values

Receivers and Methods as Pointers and Values

In this lesson, you'll study the difference between the value and the pointer type in Go to receive a result or call a method.

We'll cover the following...

Pointer or value as a receiver

The recv is most often a pointer to the receiver-type for performance reasons (because we don’t make a copy of the value, as would be the case with call by value); this is especially true when the receiver type is a struct. Define the method on a pointer type if you need the method to modify the data the receiver points to. Otherwise, it is often cleaner to define the method on a normal value type.

This is illustrated in the following example :

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package main
import (
"fmt"
)
type B struct {
  thing int
}
func (b *B) change() { b.thing = 1 }
func (b B) write() string { return fmt.Sprint(b) }
func main() {
  var b1 B // b1 is a value
  b1.change()
  fmt.Println(b1.write())
  b2 := new(B) // b2 is a pointer
  b2.change()
  fmt.Println(b2.write())
}

In the above code, at line 6, we make a struct of type B with one integer field thing. Look at the header of change() method at line 10: func (b *B) change(). The part (b *B) shows that only the pointer to B type object can call this method. This method is changing its internal field thing by assigning the value of 1 to it. Now, look at the header of the write() method at line 12: func (b B) write() string. The part (b B) shows that only the B type object can call this method. This method is returning its internal field thing after converting it to type string.

Note: The function Sprint() from fmt package returns the string without printing them on the console.

Now, look at main. We make a variable b1 of type B using var keyword at line 15. In the next line, we call change() method on b1. After returning from this method, thing of b1 will hold value 1. At line 17, we are ...