add-a-method-to-a-struct.md

Add A Method To A Struct

Given a struct in Go, we can attach a method to that struct. Put another way, we can define a method whose receiver is that struct. Then with an instance of that struct, we can call the method.

Let's say we are modeling a turtle that can move around a 2D grid. A turtle has a heading (the direction it is headed) and a location (its current X,Y coordinate).

type Heading string

const (
	UP    Heading = "UP"
	RIGHT Heading = "RIGHT"
	DOWN  Heading = "DOWN"
	LEFT  Heading = "LEFT"
)

type Turtle struct {
	Direction Heading
	X         int
	Y         int
}

We can then add a method like so by specifying the receiver as the first part of the declaration:

func (turtle *Turtle) TurnRight() {
	switch turtle.Direction {
	case UP:
		turtle.Direction = RIGHT
	case RIGHT:
		turtle.Direction = DOWN
	case DOWN:
		turtle.Direction = LEFT
	case LEFT:
		turtle.Direction = UP
	}
}

The receiver is a pointer to a Turtle. The method is called TurnRight. There are no parameters or return values.

Here are a sequence of calls to demonstrate how it works:

func main() {
	turtle := Turtle{UP, 5, 5}

	fmt.Println("Turtle Direction:", turtle.Direction)
	//=> Turtle Direction: UP

	turtle.TurnRight()

	fmt.Println("Turtle Direction:", turtle.Direction)
	//=> Turtle Direction: RIGHT

	turtle.TurnRight()

	fmt.Println("Turtle Direction:", turtle.Direction)
	//=> Turtle Direction: DOWN
}

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