Why Are Map Values Not Addressable in Go?

Map Value Addressability in Go

While working with Go, developers may encounter the interesting observation that map values are not addressable. This is apparent when attempting to directly retrieve the address of a map value, as demonstrated in the following code:

var mymap map[int]string = make(map[int]string)
mymap[1] = "One"
var myptr *string = &mymap[1]
fmt.Println(*myptr)

This code generates the error:

mapaddressable.go:7: cannot take the address of mymap[1]

In contrast, if we assign the value of the map entry to a new variable, we can successfully obtain its address:

var mymap map[int]string = make(map[int]string)
mymap[1] = "One"
mystring := mymap[1]
var myptr *string = &mystring
fmt.Println(*myptr)

This distinction raises questions about why map values in Go are not addressable. Is this an intentional design choice or an inherent limitation of the language?

To understand the rationale behind this decision, it's important to consider the underlying implementation of maps in Go. Maps are typically implemented using hash tables, which allocate memory dynamically based on the number of entries. As new entries are added or removed, the hash table may be resized and reorganized to maintain a specific load factor.

If map values were addressable, it would be possible to obtain the address of a value and modify it directly. However, if the hash table is subsequently resized or reorganized, the original address may become invalid. To prevent such inconsistencies, Go restricts the addressability of map values.

This design decision in Go is a trade-off between efficiency and simplicity. Allowing addressable map values could potentially lead to errors and memory issues if hash table modifications are not handled correctly. By prohibiting direct addressability, Go ensures the integrity of the map data structure at the expense of certain flexibility.

This is in contrast to languages like C , where map values are addressable. However, this added flexibility comes with increased responsibility for ensuring that map modifications are handled safely without invalidating pointers.

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