Methods and Receivers

Overview

Methods are functions with a receiver argument, allowing you to define behavior on types.

Method Syntax

type Rectangle struct {
    Width, Height float64
}

func (r Rectangle) Area() float64 {
    return r.Width * r.Height
}

rect := Rectangle{10, 5}
area := rect.Area()  // 50

Value vs Pointer Receivers

Value Receiver

func (r Rectangle) Scale(factor float64) Rectangle {
    return Rectangle{r.Width * factor, r.Height * factor}
}

// Original unchanged
r := Rectangle{10, 5}
scaled := r.Scale(2)  // New rectangle

Pointer Receiver

func (r *Rectangle) ScaleInPlace(factor float64) {
    r.Width *= factor
    r.Height *= factor
}

// Original modified
r := Rectangle{10, 5}
r.ScaleInPlace(2)  // r is now {20, 10}

When to Use Pointer Receivers

Use pointer receiver when: - Method needs to modify the receiver - Receiver is a large struct (avoid copying) - Consistency: if any method needs pointer, use pointer for all

type Buffer struct {
    data []byte
}

func (b *Buffer) Write(p []byte) {
    b.data = append(b.data, p...)
}

func (b *Buffer) String() string {
    return string(b.data)  // Even though it doesn't modify, use * for consistency
}

Methods on Any Type

type MyInt int

func (m MyInt) Double() MyInt {
    return m * 2
}

n := MyInt(5)
n.Double()  // 10

Automatic Dereferencing

Go automatically handles * and & for method calls:

r := Rectangle{10, 5}
(&r).ScaleInPlace(2)  // Explicit pointer
r.ScaleInPlace(2)     // Go handles it automatically

ptr := &Rectangle{10, 5}
(*ptr).Area()         // Explicit dereference
ptr.Area()            // Go handles it automatically

Embedding and Method Promotion

type Animal struct {
    Name string
}

func (a Animal) Speak() string {
    return "..."
}

type Dog struct {
    Animal  // Embedded
    Breed string
}

d := Dog{Animal: Animal{Name: "Rex"}, Breed: "Labrador"}
d.Speak()  // Promoted method: "..."
d.Name     // Promoted field: "Rex"

Method Override

func (d Dog) Speak() string {
    return "Woof!"
}

d.Speak()         // "Woof!" (Dog's method)
d.Animal.Speak()  // "..." (Animal's method)

Summary

Receiver	Use Case
`(t T)`	Read-only, small types
`(t *T)`	Modify state, large types

Feature	Description
Auto-deref	Go handles `*` and `&`
Embedding	Methods are promoted
Override	Inner type’s method shadows embedded