Receiver types in Go - Mini Project: Build & Apply

Understanding Receiver Types in Go

📖 Scenario: Imagine you are creating a simple program to manage a bank account. You want to add money and check the balance using methods attached to your account.

🎯 Goal: Build a Go program that defines a BankAccount struct and uses both pointer and value receiver methods to modify and read the account balance.

📋 What You'll Learn

Create a struct called BankAccount with a field balance of type float64

Write a method Deposit with a pointer receiver that adds an amount to the balance

Write a method Balance with a value receiver that returns the current balance

Create a main function to demonstrate using both methods

💡 Why This Matters

🌍 Real World

Managing bank accounts or any data where you need to update or read values using methods.

💼 Career

Understanding receiver types is essential for writing clear and efficient Go code, especially in backend development and systems programming.

1

Create the BankAccount struct

Create a struct called BankAccount with a field balance of type float64.

Go

type BankAccount struct {
    // Your code here
}

Hint

Use the type keyword to define a struct and add a field named balance with type float64.

2

Add a Deposit method with pointer receiver

Add a method called Deposit with a pointer receiver (acc *BankAccount) that takes a parameter amount float64 and adds it to acc.balance.

Go

package main

type BankAccount struct {
    balance float64
}

// Your code here

Hint

Use a pointer receiver so the method can change the original balance field.

3

Add a Balance method with value receiver

Add a method called Balance with a value receiver (acc BankAccount) that returns the current balance as float64.

Go

package main

type BankAccount struct {
    balance float64
}

func (acc *BankAccount) Deposit(amount float64) {
    acc.balance += amount
}

// Your code here

Hint

Use a value receiver because this method only reads data and does not modify it.

4

Use the methods in main and print the balance

In the main function, create a variable account of type BankAccount. Call Deposit(100.0) on account. Then print the result of account.Balance().

Go

package main

func main() {
    // Your code here
}

Hint

Create the account variable, call Deposit with 100.0, then print account.Balance().

Practice

1. What is the main difference between a value receiver and a pointer receiver in Go methods?

easy

A. Value receivers are faster than pointer receivers in all cases.

B. Value receivers can only read data; pointer receivers can only write data.

C. Pointer receivers create a new copy of the data each time.

D. Value receivers get a copy of the data; pointer receivers get the original data to modify.

Solution

1. Step 1: Understand receiver types

   Value receivers receive a copy of the original data, so changes inside the method do not affect the original.

2. Step 2: Pointer receivers behavior

   Pointer receivers receive the address of the original data, allowing methods to modify the original value.

3. Final Answer:

   Value receivers get a copy of the data; pointer receivers get the original data to modify. -> Option D

4. Quick Check:

   Receiver type = copy vs original [OK]

Hint: Pointer receivers allow modifying original data, value receivers do not [OK]

Common Mistakes:

• Thinking value receivers modify original data
• Believing pointer receivers create copies
• Assuming pointer receivers are always slower

2. Which of the following is the correct syntax to define a method with a pointer receiver for a struct Person?

easy

A. func (p Person) Greet() string { return "Hi" }

B. func (p *Person) Greet() string { return "Hi" }

C. func Greet(p *Person) string { return "Hi" }

D. func (p Person*) Greet() string { return "Hi" }

Solution

1. Step 1: Identify pointer receiver syntax

   Pointer receivers use an asterisk before the type in the receiver: (p *Person).

2. Step 2: Check method declaration format

   The method must be declared with receiver in parentheses before the method name.

3. Final Answer:

   func (p *Person) Greet() string { return "Hi" } -> Option B

4. Quick Check:

   Pointer receiver syntax = (p *Type) [OK]

Hint: Pointer receiver syntax uses * before type inside parentheses [OK]

Common Mistakes:

• Missing parentheses around receiver
• Placing * after variable name instead of type
• Defining method without receiver

3. What will be the output of this Go program?

package main
import "fmt"
type Counter struct { count int }
func (c Counter) Increment() { c.count++ }
func (c *Counter) IncrementPtr() { c.count++ }
func main() {
  c := Counter{count: 5}
  c.Increment()
  fmt.Println(c.count)
  c.IncrementPtr()
  fmt.Println(c.count)
}

medium

A. 5
6

B. 5
5

C. 6
6

D. 6
7

Solution

1. Step 1: Analyze Increment method with value receiver

   Increment uses a value receiver, so it works on a copy. The original count remains 5 after calling Increment.

2. Step 2: Analyze IncrementPtr method with pointer receiver

   IncrementPtr uses a pointer receiver, so it modifies the original count from 5 to 6.

3. Final Answer:

   5 6 -> Option A

4. Quick Check:

   Value receiver no change, pointer receiver changes [OK]

Hint: Value receiver copies data; pointer receiver changes original [OK]

Common Mistakes:

• Assuming value receiver changes original
• Confusing method calls with pointer vs value receivers
• Expecting both increments to change count

4. Identify the error in this Go code snippet:

type Rectangle struct { width, height int }
func (r Rectangle) Scale(factor int) {
  r.width = r.width * factor
  r.height = r.height * factor
}
func main() {
  rect := Rectangle{2, 3}
  rect.Scale(2)
  fmt.Println(rect.width, rect.height)
}

medium

A. The struct fields width and height must be pointers.

B. Scale method must return a new Rectangle instead of modifying fields.

C. Scale method should have a pointer receiver to modify Rectangle fields.

D. The Scale method should be defined outside the Rectangle type.

Solution

1. Step 1: Check receiver type in Scale method

   Scale uses a value receiver, so it works on a copy and does not modify the original Rectangle.

2. Step 2: Fix receiver to pointer to modify original

   Changing receiver to pointer (r *Rectangle) allows Scale to modify the original struct fields.

3. Final Answer:

   Scale method should have a pointer receiver to modify Rectangle fields. -> Option C

4. Quick Check:

   Modify struct fields = pointer receiver needed [OK]

Hint: Use pointer receiver to change struct fields inside methods [OK]

Common Mistakes:

• Using value receiver expecting changes to persist
• Trying to make struct fields pointers unnecessarily
• Thinking methods must be outside struct

5. You want to create a Go method Reset for a struct Timer that sets its seconds field to zero. Which receiver type should you use and why?

hard

A. Use a pointer receiver because you need to modify the original Timer's seconds field.

B. Use a value receiver because it is simpler and does not modify the original.

C. Use a value receiver and return a new Timer with seconds zeroed.

D. Use a pointer receiver but do not modify any fields inside the method.

Solution

1. Step 1: Understand the goal of Reset method

   Reset must change the Timer's seconds field to zero, modifying the original struct.

2. Step 2: Choose receiver type to allow modification

   Pointer receiver is needed to modify the original Timer instance inside the method.

3. Final Answer:

   Use a pointer receiver because you need to modify the original Timer's seconds field. -> Option A

4. Quick Check:

   Modify original struct = pointer receiver [OK]

Hint: Modify original struct? Use pointer receiver [OK]

Common Mistakes:

• Using value receiver expecting original to change
• Returning new struct instead of modifying original
• Using pointer receiver but not modifying fields