Methods

A method is like a function, but it’s attached to a specific type (usually a struct). The type becomes the receiver of the method.

func (receiver TypeName) methodName(parameters) returnType {
    // method body
}

• Receiver → variable that represents the type the method belongs to.
• You can use value receivers (t TypeName) or pointer receivers (t *TypeName).

Example of Method

type Rectangle struct {
    width, height int
}

// Method with value receiver
func (r Rectangle) area() int {
    return r.width * r.height
}

func main() {
    rect := Rectangle{width: 10, height: 5}
    fmt.Println("Area:", rect.area()) // calling the method
}

Pointer Receiver Example (Modifying data)

func (r *Rectangle) resize(newWidth, newHeight int) {
    r.width = newWidth
    r.height = newHeight
}

func main() {
    rect := Rectangle{width: 10, height: 5}
    rect.resize(20, 15) // modifies the original struct
    fmt.Println("New Area:", rect.area())
}

Functions + Methods:

// Define a struct (custom type)
type Rectangle struct {
    width, height int
}

// Method: calculate area (belongs to Rectangle)
func (r Rectangle) area() int {
    return r.width * r.height
}

// Method: resize the rectangle (pointer receiver so it changes the original)
func (r *Rectangle) resize(newWidth, newHeight int) {
    r.width = newWidth
    r.height = newHeight
}

// Function: compare areas of two rectangles (not tied to Rectangle)
func compareArea(r1, r2 Rectangle) string {
    if r1.area() > r2.area() {
        return "First rectangle is larger."
    } else if r1.area() < r2.area() {
        return "Second rectangle is larger."
    }
    return "Both rectangles have the same area."
}

func main() {
    // Create two rectangles
    rect1 := Rectangle{width: 10, height: 5}
    rect2 := Rectangle{width: 8, height: 6}

    fmt.Println("Rectangle 1 area:", rect1.area()) // method call
    fmt.Println("Rectangle 2 area:", rect2.area()) // method call

    // Compare using a function
    fmt.Println(compareArea(rect1, rect2))

    // Resize rect1
    rect1.resize(12, 7)
    fmt.Println("Rectangle 1 new area:", rect1.area())

    // Compare again
    fmt.Println(compareArea(rect1, rect2))
}

1. We define a struct Rectangle → this represents the shape with width and height.
2. We attach two methods:
   • area() → calculates and returns the rectangle’s area.
   • resize() → changes the rectangle’s width and height.
3. We write a normal function compareArea() that works with any rectangles given to it.
4. In main():
   • We create two rectangles.
   • We call methods (area and resize) directly on them.
   • We call the function compareArea() to compare their sizes.

Function vs Method

Feature	Function	Method
Belongs to	Independent, not tied to a type	Tied to a specific type (via receiver)
Syntax	func name(...) ... {}	func (receiver Type) name(...) ... {}
Used for	General tasks	Behavior related to a specific type
Can modify data?	Only if passed a pointer to data	Yes, if receiver is a pointer

• Use functions for general-purpose logic.
• Use methods when you want to define behavior for a type (like attaching abilities to an object).