Go — Methods

Value receivers

package main

import "fmt"

type Rectangle struct {
    Width, Height float64
}

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

func (r Rectangle) String() string {
    return fmt.Sprintf("Rectangle(%.1f x %.1f)", r.Width, r.Height)
}

func main() {
    r := Rectangle{Width: 5, Height: 3}
    fmt.Println(r.Area())
    fmt.Println(r)
}

Pointer receivers

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

func (r *Rectangle) SetWidth(w float64) {
    r.Width = w
}

func main() {
    r := Rectangle{Width: 5, Height: 3}
    r.Scale(2)
    fmt.Println(r.Area()) // 30
}

When to use pointer receivers

• Method modifies the receiver
• Method is expensive to copy (large struct)
• Consistency — if any method needs pointer, use pointer for all

Interface implementation

type Stringer interface {
    String() string
}

// Rectangle implements Stringer via String() method
func (r Rectangle) String() string {
    return fmt.Sprintf("Rectangle(%.1f x %.1f)", r.Width, r.Height)
}

Method embedding

type Point struct{ X, Y float64 }

func (p Point) Distance(q Point) float64 {
    dx := p.X - q.X
    dy := p.Y - q.Y
    return math.Sqrt(dx*dx + dy*dy)
}

type Circle struct {
    Center Point
    Radius float64
}

func main() {
    c := Circle{Center: Point{0, 0}, Radius: 5}
    fmt.Println(c.Center.Distance(Point{3, 4}))
}

Mini Practice

Write Go code that:

1. Creates a struct with value and pointer receiver methods
2. Implements the Stringer interface
3. Uses method embedding
4. Shows when to use pointer vs value receivers

Up Next

In the next lesson, you'll learn about Interfaces — Go's implicit interfaces.