Cannot Assign to Struct Field in Map

Why This Happens

When a map holds struct values, you cannot directly modify a field of the struct stored in the map. This is because map elements are not addressable in Go, so the compiler cannot give you a pointer to modify in place. You must read the value, modify the copy, and store it back.

The Problem

type User struct {
    Name string
    Age  int
}

func main() {
    users := map[string]User{
        "alice": {Name: "Alice", Age: 30},
    }
    users["alice"].Age = 31 // cannot assign to struct field
}

The Fix

func main() {
    users := map[string]User{
        "alice": {Name: "Alice", Age: 30},
    }
    u := users["alice"]
    u.Age = 31
    users["alice"] = u
}

Step-by-Step Fix

1. 1

   Identify the direct field assignment

   Find the line where you are trying to modify a struct field directly through a map access.

2. 2

   Copy the struct value

   Read the struct from the map into a local variable.

3. 3

   Modify and write back

   Modify the local copy and assign it back to the map. Alternatively, use a map of pointers: map[string]*User.