Nil Pointer Dereference

Why This Happens

In Go, a nil pointer holds no address. When you try to read a field or call a method on a nil pointer, the runtime panics. This commonly occurs when a function returns a pointer and an error, and you use the pointer without checking the error first.

The Problem

type User struct {
    Name string
}

func findUser(id int) *User {
    return nil
}

func main() {
    u := findUser(42)
    fmt.Println(u.Name) // panic: nil pointer dereference
}

The Fix

func main() {
    u := findUser(42)
    if u == nil {
        fmt.Println("user not found")
        return
    }
    fmt.Println(u.Name)
}

Step-by-Step Fix

1. 1

   Identify the nil pointer

   Read the stack trace to find the exact line where the dereference occurs and identify which variable is nil.

2. 2

   Trace the origin

   Follow the variable back to where it was assigned. Check if the function that returned it can return nil.

3. 3

   Add a nil check

   Add an explicit nil check before using the pointer, or ensure the function always returns a valid pointer.