What does "either addressable nor a map index expression" mean?

The error message "either addressable nor a map index expression" generally arises in programming languages like Go (Golang). It indicates that you are trying to perform an operation (usually assigning a value) on something that cannot be addressed directly in memory or accessed using map indexing.

To break this down:

1. Addressable: In this context, "addressable" means that the memory location of a variable or value can be obtained directly. Variables declared in a straightforward manner (e.g., x := 10) are usually addressable. You can obtain their memory address using the & operator (e.g., &x).

2. Map Index Expression: This refers to accessing elements within a map using their keys. For instance, in Go, if you have myMap := map[string]int{"a": 1, "b": 2}, myMap["a"] is a map index expression.

Therefore, "either addressable nor a map index expression" implies that the target of your assignment or operation is neither a standard variable with a direct memory address nor an element accessible via map indexing.

Common scenarios where this error can occur:

- Trying to modify the result of a function call directly. For example:

package main
import "fmt"

func getValue() int {
  return 10
}

func main() {
  getValue() = 20 // This will cause the error
  fmt.Println(getValue())
}

The issue here is that you're trying to assign a value (20) to the result of getValue(), which isn't addressable.

- Trying to modify a field of a struct that's returned by value, not by pointer.

package main
import "fmt"

type MyStruct struct {
  Value int
}

func getStruct() MyStruct {
  return MyStruct{Value: 10}
}

func main() {
  getStruct().Value = 20 // This will cause the error
  fmt.Println(getStruct())
}

In this case, getStruct() returns a copy of the MyStruct struct. You can't modify a copy and expect the original to change. To fix it, the function should return a pointer to the struct.

The fix for the above example would be:

package main
import "fmt"

type MyStruct struct {
  Value int
}

func getStruct() MyStruct {
  return &MyStruct{Value: 10}
}

func main() {
  s := getStruct()
  s.Value = 20 // This is now valid
  fmt.Println(s)
}

Debugging this error involves carefully examining the line of code indicated in the error message and determining why the left-hand side of the assignment is neither addressable nor a map index expression. Is it a temporary value? Is it a copy of a struct rather than a pointer to a struct? Understanding the difference between values and pointers is crucial in addressing this error.

Learn more about this