Defining methods in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a method in Go?

A method in Go is a function with a special receiver argument. It is like a function tied to a specific type, allowing you to call it using the dot notation on values of that type.

intermediate

How do you define a method with a pointer receiver in Go?

You define a method with a pointer receiver by specifying the receiver as a pointer type in parentheses before the method name, for example:

func (p *Person) UpdateName(newName string) { p.Name = newName }

This allows the method to modify the original value.

intermediate

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

A value receiver gets a copy of the value, so changes inside the method do not affect the original. A pointer receiver gets the address, so changes inside the method affect the original value.

intermediate

Can methods be defined on any type in Go?

No, methods can only be defined on named types declared in the same package. You cannot define methods on built-in types or types from other packages.

beginner

Example: Define a method called Greet for a struct Person that prints a greeting message.

<pre>package main

import "fmt"

type Person struct { Name string }

func (p Person) Greet() {
    fmt.Println("Hello, my name is", p.Name)
}</pre>

What does the receiver in a Go method specify?

A The type the method belongs to

B The return type of the method

C The package where the method is defined

D The name of the method

The receiver specifies the type the method is associated with, allowing the method to be called on values of that type.

Which receiver type allows a method to modify the original value?

A Value receiver

B Interface receiver

C Constant receiver

D Pointer receiver

Pointer receivers receive the address of the value, so changes inside the method affect the original value.

Can you define a method on the built-in type int in Go?

A No, never

B Yes, always

C Only if int is declared in your package

D Only if int is a pointer

You cannot define methods on built-in types like int directly.

How do you call a method named Print on a variable p of type Person?

A Print(p)

B p.Print()

C Person.Print(p)

D Print.Person(p)

Methods are called using dot notation on the variable: p.Print()

What happens if you define a method with a value receiver and modify the receiver inside the method?

A The program will not compile

B The original value is changed

C Only the copy inside the method changes

D The method runs twice

With a value receiver, the method works on a copy, so changes do not affect the original value.

Explain how to define a method in Go and the role of the receiver.

Think about how methods are like functions tied to a type.

Describe the difference between pointer and value receivers and when to use each.

Consider if you want the method to change the original data or not.

Practice

1. What is the purpose of defining a method with a receiver in Go?

easy

A. To import external packages into the program

B. To create a standalone function unrelated to any type

C. To associate a function with a specific type so it can act on its data

D. To declare a global variable accessible everywhere

Solution

1. Step 1: Understand what a receiver does

   A receiver connects a method to a type, allowing the method to access or modify that type's data.

2. Step 2: Compare with other options

   Options A, B, and C describe unrelated concepts: imports, standalone functions, and global variables.

3. Final Answer:

   To associate a function with a specific type so it can act on its data -> Option C

4. Quick Check:

   Receiver links method to type = D [OK]

Hint: Receiver connects method to type's data [OK]

Common Mistakes:

• Thinking methods are standalone functions
• Confusing methods with variables
• Believing receivers import packages

2. Which of the following is the correct syntax to define a method named Speak for a struct type Dog?

easy

A. func (d Dog) Speak() { }

B. func Speak(d Dog) { }

C. func Dog.Speak() { }

D. func Speak() Dog { }

Solution

1. Step 1: Recall Go method syntax

   Methods use the form: func (receiver Type) MethodName() { }.

2. Step 2: Match syntax to options

   func (d Dog) Speak() { } matches this pattern exactly. func Speak(d Dog) { } is a function, not a method. func Dog.Speak() { } is invalid syntax. func Speak() Dog { } is a function returning Dog, not a method.

3. Final Answer:

   func (d Dog) Speak() { } -> Option A

4. Quick Check:

   Method syntax = func (receiver Type) Name() [OK]

Hint: Method syntax always has receiver in parentheses before name [OK]

Common Mistakes:

• Omitting receiver parentheses
• Using dot notation in function name
• Confusing methods with functions

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

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

medium

A. 6

B. 5

C. 0

D. Compilation error

Solution

1. Step 1: Understand receiver type in Increment method

   The method uses a value receiver (c Counter), so it works on a copy of the struct, not the original.

2. Step 2: Trace the method call and print

   Increment increases the copy's value, but original c.value remains 5. So printing c.value outputs 5.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Value receiver does not modify original = 5 [OK]

Hint: Value receiver copies data; original stays unchanged [OK]

Common Mistakes:

• Assuming value receiver changes original data
• Confusing pointer and value receivers
• Expecting incremented output

4. Identify the error in this method definition:

package main
import "fmt"
type Person struct { name string }
func (p *Person) SetName(newName string) {
    p.name = newName
}
func main() {
    p := Person{name: "Alice"}
    p.SetName("Bob")
    fmt.Println(p.name)
}

medium

A. No error; code runs and prints 'Bob'

B. Cannot call method with pointer receiver on value variable

C. Method must return a value

D. Missing import of fmt package

Solution

1. Step 1: Check pointer receiver usage

   In Go, methods with pointer receivers can be called on value variables; Go automatically takes address.

2. Step 2: Verify imports and method behavior

   Code imports fmt, method sets name correctly, so p.name becomes "Bob" and prints it.

3. Final Answer:

   No error; code runs and prints 'Bob' -> Option A

4. Quick Check:

   Pointer receiver works on value variable = B [OK]

Hint: Pointer receiver methods can be called on values directly [OK]

Common Mistakes:

• Thinking pointer receiver requires pointer variable
• Forgetting Go auto-addressing
• Expecting compile error

5. You want to define a method Reset for a struct Timer that sets its seconds field to zero. Which receiver type should you use and why? Options: A) Pointer receiver, because it modifies the original struct's field B) Value receiver, because it copies the struct and resets the copy C) Value receiver, because methods cannot have pointer receivers D) Pointer receiver, but it cannot modify fields inside the struct

hard

A. Pointer receiver, but it cannot modify fields inside the struct

B. Value receiver, because it copies the struct and resets the copy

C. Value receiver, because methods cannot have pointer receivers

D. Pointer receiver, because it modifies the original struct's field

Solution

1. Step 1: Understand effect of receiver types on struct modification

   Value receivers work on copies, so changes do not affect original struct fields.

2. Step 2: Choose pointer receiver to modify original

   Pointer receivers allow methods to change the original struct's fields, so Reset can set seconds to zero effectively.

3. Final Answer:

   Pointer receiver, because it modifies the original struct's field -> Option D

4. Quick Check:

   Pointer receiver modifies original struct = A [OK]

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

Common Mistakes:

• Using value receiver expecting original change
• Believing methods can't have pointer receivers
• Thinking pointer receiver can't modify fields