Why can\'t I take the address of a map value in Go?

Taking the Address of Map Values in Go

Question: Why can't I take the address of a map value?

Background:

The Go programming language restricts the use of the "&" operator to obtain the address of map values. This behavior is distinct from slice values, which can be addressed using "&".

Clarification of Misconceptions:

Contrary to the belief that maps are backed by memory structures like arrays, they are actually stored in dynamic, reorganizing bucket structures. This means that map values do not have a fixed memory location.

The Underlying Reason:

The primary reason Go prohibits address-taking of map values is due to the dynamic nature of map data structures. As map entries are created, updated, or deleted, the buckets can be reorganized to optimize performance and memory usage. If addresses were allowed for map values, they would become invalidated during these relocations.

Numeric Modifications in Maps:

Although operators like " " and " =" can be used to modify numeric map values in-place, this shorthand form actually performs a series of operations that do not directly access the underlying value.

Example:

func icandothis() {
    cmap := make(map[int]complex64)
    cmap[1] += complex(1, 0)
    fmt.Println(cmap[1])
}

Expanding the shorthand form reveals that the operation actually creates a new temporary value to perform the addition and stores it back into the map:

func icandothis() {
    cmap := make(map[int]complex64)
    v := cmap[1]
    v += complex(1, 0)
    cmap[1] = v
    fmt.Println(cmap[1])
}

Modifying Struct Values in Maps:

Unlike numeric values, modifying struct values in maps requires the following steps:

1. Retrieve the struct value from the map.
2. Modify the struct value.
3. Store the modified struct value back into the map.

Example:

type Complex struct {
    R float32
    I float32
}

func (x *Complex) Add(c Complex) {
    x.R += c.R
    x.I += c.I
}

func main() {
    cmap := make(map[int]Complex)
    c := cmap[1]
    c.Add(Complex{1, 0})
    cmap[1] = c
    fmt.Println(cmap[1])
}

By following these steps, the struct value can be modified in-place without requiring its address.

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