methods.md

Object-Oriented Programming

Methods

Given our previous definition of an object, a custom struct type can contain the state (data) of an object. But what about the behaviour? That behaviour is provided by methods.

Let's create a rectangle type, with a width and a height. From this, it's simple to calculate the area, so let's write a function to do that.

type rectangle struct {
	width  int
	height int
}

func area(r rectangle) int {
	return r.width * r.height
}

func main() {
	r := rectangle{3, 4}
	fmt.Println(area(r))    // area 12
}

It works as expected, but now we need to either repeat the calculation or copy the function everywhere we want to calculate the area. We can solve this by turning the function into a method.

A method is a function that is bound to a receiver.

Go does not have classes, but you can define methods on any type. The method receiver appears in its own argument list between the func keyword and the method name.

type rectangle struct {
	width  int
	height int
}

func (r rectangle) area() int {
	return r.width * r.height
}

func main() {
	r := rectangle{3, 4}
	fmt.Println(r.area())   // area 12
}

Comparing the two versions, it's a very simple, yet powerful change.

// function, called with Area(r)
func area(r rectangle) int {
	return r.width * r.height
}

// method, called with r.Area()
func (r rectangle) area() int {
	return r.width * r.height
}

It's actually worth pointing out that the method version is effectively just syntactic sugar: under the hood it takes the receiver and puts it as the first argument, just as happens in the function version.

Exercise

Declare a new struct type to hold information about a tennis player, including the number of matches played and the number won. Add a method to this type that calculates the win ratio for the player. Create a new player, and output the win ratio for them.

• Exercise template: source / playground
• Example solution: source / playground