Go nil pointer dereference — runtime error: invalid memory address

What causes this error

In Go, pointer types, interfaces, maps, slices, channels, and function values default to nil. Accessing a field, calling a method, or indexing a nil value causes a runtime panic with 'invalid memory address or nil pointer dereference'. Common sources: a function returning `*T, error` where the caller ignores the error and uses the nil `*T`; an interface value holding a nil concrete type; an uninitialized struct with pointer fields.

How to fix it

1. 01

   step 1

   Read the panic stack trace to find the exact line

   Go's panic output includes a full goroutine stack trace with file names and line numbers. The line causing the dereference is the topmost frame in your own package (skip the runtime frames).

   # Panic output example:
   # goroutine 1 [running]:
   # main.greet(...)
   #     /app/main.go:15 +0x...  ← this is your line
   # main.main()
   #     /app/main.go:8 +0x...
   
   # Run with full stack trace
   GOTRACEBACK=all go run main.go

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2. 02

   step 2

   Check if an error was ignored after a function returning (*T, error)

   The most common cause: `obj, _ := getObject()` where `getObject` returned `nil, err`. The underscore discards the error and `obj` is nil. Always check the error before using the result.

   // BAD — ignoring error, using potentially nil obj
   obj, _ := repo.FindByID(id)
   fmt.Println(obj.Name)  // panic if not found
   
   // GOOD — check error first
   obj, err := repo.FindByID(id)
   if err != nil {
       return fmt.Errorf("find user: %w", err)
   }
   if obj == nil {
       return fmt.Errorf("user %d not found", id)
   }
   fmt.Println(obj.Name)

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3. 03

   step 3

   Initialize struct pointer fields before use

   A struct with pointer fields has them all set to nil by default. Initialize them before accessing.

   type Config struct {
       DB     *DBConfig
       Server *ServerConfig
   }
   
   // BAD — Config{} leaves DB and Server as nil
   cfg := Config{}
   fmt.Println(cfg.DB.Host)  // panic
   
   // GOOD — initialize all pointer fields
   cfg := Config{
       DB:     &DBConfig{Host: "localhost", Port: 5432},
       Server: &ServerConfig{Port: 8080},
   }

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4. 04

   step 4

   Use the value type instead of a pointer where possible

   If you don't need the semantics of nil (optional value), use a value type instead of a pointer. Value types can never be nil.

   // BAD — pointer required no reason
   type Handler struct {
       logger *log.Logger  // can be nil
   }
   
   // GOOD — value type, never nil
   type Handler struct {
       logger log.Logger  // always initialized
   }

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How to verify the fix

• The panic no longer occurs when running the code path.
• Nil checks are in place for all pointer results from functions that can return nil.
• Tests cover the nil/not-found case explicitly.