How to Modify Struct Fields in Maps in Go?

Assigning to Struct Fields in Maps

When working with maps in Go, it's crucial to understand the implications of assigning to struct fields within those maps. The error "cannot assign to struct field in a map" arises when an attempt is made to directly modify a struct field of a value stored in a map. This error occurs due to the way maps are implemented in Go.

In Go, a map is a collection of pairs where the key type is comparable and the element type can be any type, including structs. However, when a struct value is placed into a map as an element, it does not retain its addressability. This means that attempts to modify its fields as if it were a pointer will trigger the aforementioned error.

Consider the following code:

type Snapshot struct {
  Key   string
  Users []Users
}

snapshots := make(map[string] Snapshot, 1)

// Initialization
snapshots["test"] = Snapshot {
  Key: "testVal",
  Users: make([]Users, 0),
}

Now, an attempt to add new users to the Users slice within the Snapshot value stored in the map results in the dreaded error:

snapshots["test"].Users = append(snapshots["test"].Users, user)

To overcome this challenge, there are two viable solutions:

Method 1:

Before modification, create a local copy of the struct value stored in the map and then reassign the map entry with the modified copy:

tmp := snapshots["test"]
tmp.Users = append(tmp.Users, user)
snapshots["test"] = tmp

Method 2:

Alternatively, define the map with a pointer type, which allows for modifications to the struct fields:

snapshots := make(map[string] *Snapshot, 1)

// Initialization
snapshots["test"] = &Snapshot {
  Key: "testVal",
  Users: make([]Users, 0),
}

snapshots["test"].Users = append(snapshots["test"].Users, user) is now valid and will not encounter the error. However, it is important to note that the second solution may have implications for memory management and should be used with caution.

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