Pointer vs Value Receivers

Part of the Object Oriented Programming section of Coddy's GO journey — lesson 8 of 107.

Methods can have either a value receiver or a pointer receiver. A value receiver works on a copy of the struct. A pointer receiver works on the original struct and can modify its fields.

Value receiver — works on a copy

type Counter struct {
    Count int
}

func (c Counter) GetCount() int {
    return c.Count
}

// This does NOT modify the original
func (c Counter) IncrementWrong() {
    c.Count++ // Only changes the copy!
}

Pointer receiver — modifies the original

// This DOES modify the original
func (c *Counter) Increment() {
    c.Count++
}

func (c *Counter) Reset() {
    c.Count = 0
}

Using both together

func main() {
    counter := Counter{Count: 0}

    counter.Increment()
    counter.Increment()
    fmt.Println(counter.GetCount()) // Output: 2

    counter.Reset()
    fmt.Println(counter.GetCount()) // Output: 0
}

Use a value receiver when the method only reads data. Use a pointer receiver (*StructName) when the method needs to modify the struct's fields. Go automatically handles the referencing — you call both the same way.

Challenge

Medium

Add methods to the Wallet struct using the correct receiver types:

GetBalance — value receiver, returns the current balance
Deposit — pointer receiver, adds amount to balance
Withdraw — pointer receiver, subtracts amount only if sufficient funds

Cheat sheet

Methods can have either a value receiver or a pointer receiver. A value receiver works on a copy of the struct. A pointer receiver works on the original struct and can modify its fields.

Value receiver — works on a copy:

type Counter struct {
    Count int
}

func (c Counter) GetCount() int {
    return c.Count
}

Pointer receiver — modifies the original:

func (c *Counter) Increment() {
    c.Count++
}

Use a value receiver when the method only reads data. Use a pointer receiver (*StructName) when the method needs to modify the struct's fields.

Try it yourself

package main

import "fmt"

func main() {
    var balance, deposit, withdraw float64
    fmt.Scan(&balance, &deposit, &withdraw)

    w := Wallet{Balance: balance}

    fmt.Printf("Balance: %.2f\n", w.GetBalance())

    w.Deposit(deposit)
    fmt.Printf("After deposit: %.2f\n", w.GetBalance())

    w.Withdraw(withdraw)
    fmt.Printf("After withdrawal: %.2f\n", w.GetBalance())

    fmt.Printf("Final balance: %.2f\n", w.GetBalance())
}