How to Prevent Types from Being Used as Map Keys in Go?

Preventing Types from Functioning as Map Keys

In certain scenarios, it may be undesirable for a specific type to be employed as a map key. Despite possessing a private member, such types can still be used in this capacity. This article explores methods to prevent such occurrences.

The fundamental condition for a type to serve as a map key is that all its fields must be comparable. One effective solution is to include a non-comparable field in the type structure. Types such as slices, maps, and functions cannot be compared, so adding a field of this type effectively precludes the use of the enclosing type as a map key.

For instance:

type MyType struct {
   A *A
   b b
   preventUseAsKey []int
}

An attempt to utilize MyType as a map key:

m := map[MyType]int{}

would result in the following compile-time error:

invalid map key type MyType

However, it's important to note that this approach carries a potential drawback: it renders the type non-comparable. This means comparison operators (e.g., ==, !=) can no longer be applied to the modified type, which may impact certain use cases.

To preserve comparability while restricting map key usage, a wrapper type can be employed. The wrapper type embeds the original type but adds a non-comparable field. This allows comparison operations to be performed on the embedded type while still prohibiting the wrapper type from being used as a map key.

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