Structs and Interfaces

Go Methods: Value and Pointer Receivers

Go has no classes. Attach methods to a type and choose the right receiver.

Intermediate
4 min read
1 runnable example

What you'll learn

Methods and receivers
Value vs pointer receiver
Constructor functions

Run code →

Receiver → what the method can do

func (r Rectangle) Area()read a copy

func (r *Rectangle) Scale(f)change the original

A function with a receiver

A method is a function with a receiver: the value it is called on, written before the method name. Go has no classes, so methods on structs are how types get behavior.

Area has a value receiver and works on a copy, which is fine for reading. Scale has a pointer receiver, so it changes the original rectangle. Go takes the address for you when you call rect.Scale(2). NewSquare is an ordinary function used as a constructor.

Key points

A value receiver works on a copy.
A pointer receiver can change the original.
NewSquare is a normal function used as a constructor.

Runnable Go example

Run code →

main.go

package main

import "fmt"

type Rectangle struct {
	Width, Height float64
}

func NewSquare(size float64) Rectangle {
	return Rectangle{Width: size, Height: size}
}

// Area has a value receiver: it only reads the rectangle.
func (r Rectangle) Area() float64 {
	return r.Width * r.Height
}

// Scale has a pointer receiver: it changes the rectangle.
func (r *Rectangle) Scale(factor float64) {
	r.Width *= factor
	r.Height *= factor
}

func main() {
	rect := Rectangle{Width: 3, Height: 4}
	fmt.Println("area:", rect.Area())

	rect.Scale(2)
	fmt.Println("scaled:", rect.Width, "x", rect.Height, "=", rect.Area())
	fmt.Println("square:", NewSquare(5).Area())
}

Get a Run button for this example Result of running the code

area: 12
scaled: 6 x 8 = 48
square: 25

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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