Why Can't I Directly Address Map Values in Go?

Map Values Non-Addressability in Go

In Go, map values cannot be addressed directly as in C . This becomes evident when attempting to take the address of an entry in a map. Consider the code below:

import "fmt"

func main() {
    var mymap = make(map[int]string)
    mymap[1] = "One"
    var myptr *string = &mymap[1]
    fmt.Println(*myptr) // Generates error
}

This code triggers an error stating "cannot take the address of map value". However, if an intermediate string variable is used to store the value before taking its address, the code runs without issue:

import "fmt"

func main() {
    var mymap = make(map[int]string)
    mymap[1] = "One"
    mystring := mymap[1]
    var myptr *string = &mystring
    fmt.Println(*myptr) // Works
}

Why are Map Values Not Addressable?

The prohibition against directly addressing map values stems from the internal implementation of maps in Go. Most likely, they are implemented as a type of hash table. The dynamic nature of hash tables allows insertions and deletions, potentially invalidating addresses of existing entries.

To protect against such errors, Go prevents the addressability of map values. This ensures that if the underlying hash table is reorganized, the retrieved value remains valid.

Addressing Values Indirectly

While direct addressing of map values is disallowed, indirect addressing is allowed. This means that a variable can hold the value of a map entry without being able to modify the value directly. Consider the following example:

var mystring string = mymap[1] // Indirect addressing

This line assigns the value of the entry with the key 1 in mymap to mystring. Changes to mymap will be reflected in mystring. However, mystring is immutable, and attempts to modify its value will result in an error.

Conclusion

The non-addressability of map values in Go is a design choice intended to ensure that the values retrieved from a map remain valid, even if the underlying data structure is reorganized. This restriction prevents errors that could occur if the address of a map entry were saved and the map were subsequently modified. While it may not offer the same flexibility as in some other programming languages, it contributes to the stability and predictability of Go's map implementation.

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