Structs and Methods

Receivers decide what changes

A struct groups fields; methods attach behaviour to a type. A method with a value receiver (a Account) gets a copy, so changes do not affect the caller's value; a pointer receiver (a *Account) can modify the original and avoids copying large structs. Go calls pointer methods on addressable values automatically. Use pointer receivers when a method mutates state or the struct is large, and be consistent within a type.

Composition over inheritance

Go models data with structs, behaviour with methods and small interfaces, and reuse with generics.

A value receiver that silently changes nothing, run

I ran this with Go 1.27.1 (go run . in a module named demo, standard library only). BrokenDeposit modifies a copy, so the balance stays 100; Deposit uses a pointer receiver and the balance becomes 150.

package main

import "fmt"

type Account struct {
	Owner   string
	Balance int
}

func (a Account) Describe() string { // value receiver: works on a copy
	return fmt.Sprintf("%s has %d", a.Owner, a.Balance)
}

func (a *Account) Deposit(n int) { // pointer receiver: modifies the original
	a.Balance += n
}

func (a Account) BrokenDeposit(n int) {
	a.Balance += n // changes only the copy
}

func main() {
	acc := Account{Owner: "Asha", Balance: 100}
	acc.BrokenDeposit(50)
	fmt.Println(acc.Describe())
	acc.Deposit(50)
	fmt.Println(acc.Describe())
}

Output:

Asha has 100
Asha has 150

Mutating methods need pointer receivers

If a method should change the receiver, use a pointer receiver; a value receiver compiles fine but changes only a copy.

त्वरित जाँच: Why did BrokenDeposit not change the balance?

• The balance field is unexported
• Go structs are immutable
• It has a value receiver, so it modified a copy
• Methods cannot change fields