Struct pointers in Go - Mini Project: Build & Apply

Struct pointers

📖 Scenario: You are creating a simple program to manage a book's information using Go structs and pointers. This helps you understand how to work with pointers to structs, which is common in Go programming.

🎯 Goal: Build a Go program that creates a struct for a book, uses a pointer to modify the book's title, and then prints the updated title.

📋 What You'll Learn

Create a struct type called Book with a field Title of type string.

Create a variable myBook of type Book with the title "Go Programming".

Create a pointer variable bookPtr that points to myBook.

Use the pointer bookPtr to change the Title field to "Advanced Go Programming".

Print the updated Title of myBook.

💡 Why This Matters

🌍 Real World

Working with pointers to structs is common in Go programs that manage data efficiently, such as in web servers, databases, or games.

💼 Career

Understanding struct pointers is essential for Go developers to write memory-efficient and clear code, which is highly valued in backend and systems programming jobs.

1

Create the Book struct and a variable

Create a struct type called Book with a field Title of type string. Then create a variable called myBook of type Book with the title set to "Go Programming".

Go

type Book struct {
    Title string
}

// Create myBook variable with Title "Go Programming"
// Your code here

Hint

Use type Book struct { Title string } to define the struct. Then create myBook with Book{Title: "Go Programming"}.

2

Create a pointer to the Book variable

Create a pointer variable called bookPtr that points to the variable myBook.

Go

package main

import "fmt"

type Book struct {
    Title string
}

func main() {
    myBook := Book{Title: "Go Programming"}
    // Create a pointer bookPtr to myBook
    // Your code here
}

Hint

Use the & operator to get the address of myBook and assign it to bookPtr.

3

Change the Title using the pointer

Use the pointer variable bookPtr to change the Title field of myBook to "Advanced Go Programming".

Go

package main

import "fmt"

type Book struct {
    Title string
}

func main() {
    myBook := Book{Title: "Go Programming"}
    bookPtr := &myBook
    // Change the Title using bookPtr
    // Your code here
}

Hint

Use bookPtr.Title = "Advanced Go Programming" to update the title through the pointer.

4

Print the updated Title

Print the updated Title field of the variable myBook using fmt.Println.

Go

package main

import "fmt"

type Book struct {
    Title string
}

func main() {
    myBook := Book{Title: "Go Programming"}
    bookPtr := &myBook
    bookPtr.Title = "Advanced Go Programming"
    // Print the updated Title
    // Your code here
}

Hint

Use fmt.Println(myBook.Title) to print the updated title.

Practice

1. What does a struct pointer in Go hold?
type Person struct { Name string }

easy

A. Only the struct's first field value

B. A copy of the struct data

C. The memory address of a struct instance

D. A string representation of the struct

Solution

1. Step 1: Understand what a pointer stores

   A pointer stores the memory address of a variable, not the variable's value itself.

2. Step 2: Apply to struct pointers

   A struct pointer holds the address of the struct instance, allowing access or modification of the original struct.

3. Final Answer:

   The memory address of a struct instance -> Option C

4. Quick Check:

   Struct pointer = memory address [OK]

Hint: Pointers hold addresses, not copies. Remember & for address [OK]

Common Mistakes:

• Thinking pointer holds a copy of the struct
• Confusing pointer with struct value
• Assuming pointer holds only first field

2. Which of the following is the correct way to declare a pointer to a struct Person in Go?

easy

A. var *p Person

B. var p Person*

C. var p &Person

D. var p *Person

Solution

1. Step 1: Recall Go pointer syntax

   In Go, a pointer type is declared by placing * before the type name, e.g., *Person.

2. Step 2: Match correct declaration

   var p *Person uses var p *Person, which is the correct syntax for a pointer variable.

3. Final Answer:

   var p *Person -> Option D

4. Quick Check:

   Pointer declaration = *Type [OK]

Hint: Pointer type syntax: *TypeName, not after variable [OK]

Common Mistakes:

• Placing * after variable name
• Using & in declaration instead of *
• Incorrect order of * and type

3. What is the output of this Go program?

package main
import "fmt"
type Point struct { X, Y int }
func main() {
 p := Point{1, 2}
 ptr := &p
 ptr.X = 10
 fmt.Println(p.X)
}

medium

A. 1

B. 10

C. 0

D. Compilation error

Solution

1. Step 1: Understand pointer modification

   ptr points to p. Changing ptr.X modifies p.X because ptr references p's memory.

2. Step 2: Check printed value

   After setting ptr.X = 10, p.X becomes 10. So, printing p.X outputs 10.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Pointer changes original struct field [OK]

Hint: Changing pointer field changes original struct field [OK]

Common Mistakes:

• Thinking ptr.X is separate copy
• Expecting original p.X to remain 1
• Confusing pointer dereference syntax

4. Identify the error in this Go code snippet:

package main
import "fmt"
type User struct { Name string }
func updateName(u User) {
 u.Name = "Alice"
}
func main() {
 user := User{"Bob"}
 updateName(&user)
 fmt.Println(user.Name)
}

medium

A. Passing pointer to function expecting value

B. Missing import for fmt package

C. Cannot assign string to Name field

D. Function parameter should be a pointer to User

Solution

1. Step 1: Check function parameter type

   Function updateName expects a User value, but main passes &user (a *User pointer).

2. Step 2: Identify the type mismatch

   This causes a compilation error because the argument type (*User) does not match the parameter type (User).

3. Final Answer:

   Passing pointer to function expecting value -> Option A

4. Quick Check:

   Function param must match argument type [OK]

Hint: Match function param type with argument (& for pointer) [OK]

Common Mistakes:

• Passing pointer to value parameter without pointer receiver
• Ignoring need to modify original struct
• Assuming string assignment causes error

5. Given this struct and function:

package main
import "fmt"
type Counter struct { Count int }
func increment(c *Counter) {
 c.Count++
}
func main() {
 c := Counter{Count: 5}
 increment(&c)
 increment(c)
 fmt.Println(c.Count)
}

What will happen when running this program?

hard

A. Compilation error at increment(c)

B. Output is 7

C. Output is 6

D. Runtime panic due to invalid pointer

Solution

1. Step 1: Analyze function calls

   increment expects a *Counter pointer. The first call increment(&c) is correct.

2. Step 2: Check second call increment(c)

   Passing c (a value) instead of *Counter causes a compilation error because types don't match.

3. Final Answer:

   Compilation error at increment(c) -> Option A

4. Quick Check:

   Pointer param requires pointer argument [OK]

Hint: Pointer params need pointer args; value won't auto-convert [OK]

Common Mistakes:

• Assuming Go auto-converts value to pointer
• Expecting output 7 without error
• Ignoring type mismatch in function call