Receiver types in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a receiver in Go methods?

A receiver is a special parameter in Go that allows a function to be called as a method on a type. It appears before the method name and defines which type the method belongs to.

beginner

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

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 can modify the original value.

intermediate

Why would you use a pointer receiver instead of a value receiver?

Use a pointer receiver to modify the original data, avoid copying large structs for efficiency, or to ensure consistency when methods need to share the same data.

intermediate

Can methods with pointer receivers be called on value types in Go?

Yes. Go automatically takes the address of the value to call pointer receiver methods, so you can call pointer receiver methods on values directly.

intermediate

What happens if you define a method with a value receiver and call it on a pointer type?

Go automatically dereferences the pointer to call the method with a value receiver, so the method works on the value the pointer points to.

In Go, what does a pointer receiver allow you to do?

A Copy the value to a new variable

B Only read the value without modification

C Prevent the method from being called

D Modify the original value inside the method

Pointer receivers allow methods to modify the original value by working with its memory address.

Which receiver type copies the value when a method is called?

A Value receiver

B Channel receiver

C Interface receiver

D Pointer receiver

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

Can you call a method with a pointer receiver on a value variable in Go?

A No, it causes a compile error

B Yes, Go automatically takes the address

C Only if the value is a pointer type

D Only if the method is exported

Go automatically takes the address of the value to call pointer receiver methods.

Why might you avoid using value receivers for large structs?

A Because value receivers cause runtime errors

B Because value receivers cannot modify the struct

C Because copying large structs is inefficient

D Because value receivers require pointers

Copying large structs can be slow and use more memory, so pointer receivers are preferred.

What does Go do if you call a value receiver method on a pointer variable?

A Automatically dereferences the pointer

B Throws a compile-time error

C Converts the pointer to an interface

D Calls the method on the pointer itself

Go automatically dereferences the pointer to call the value receiver method on the underlying value.

Explain the difference between value receivers and pointer receivers in Go methods.

Think about whether the method can change the original data or just a copy.

Describe how Go handles calling methods with pointer receivers on value variables and vice versa.

Go helps by converting between pointers and values when calling methods.

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