Why Can't I Directly Modify Struct Fields in Go Maps, and What's the Alternative?

Understanding Map Value Manipulation in Go

In Go, it is often desirable to store data structures, such as structs, within maps. However, some users encounter difficulty when trying to modify fields of these struct values directly within the map. This question explores the reasons behind this limitation.

Why Can't I Directly Modify Map Value Fields?

When you store a struct by value in a map, accessing that struct retrieves a copy of the value, rather than a reference. As a result, any modifications made to this copy do not affect the original struct in the map. To modify the original struct, you must first retrieve the copy, make the changes, and then write the modified copy back into the map.

Alternatives to Direct Modification

Although direct field modification is not allowed for map values, there is an alternative approach: storing pointers to structs instead. By using pointers, you can modify the underlying struct directly without having to read and write it back to the map.

Consider the following example:

type dummy struct {
    a int
}

x := make(map[int]*dummy)
x[1] = &dummy{a: 1}
x[1].a = 2

In this scenario, the map stores pointers to dummy structs. When you access x[1].a, you are directly modifying the original struct referenced by the pointer. This allows seamless field modification without the need for reading and writing the struct copies.

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