How Can I Use Custom Types as Keys in Go Maps?

Custom Key and Equality for Go Maps

In Go, maps use strict equality for comparing keys. This means that to use a custom type as a map key, it must implement the standard equality operator (==). However, there are workarounds to this limitation.

Using Derived Keys

Instead of using struct instances as keys directly, consider using a derived attribute that serves as an identity for the instance. This attribute should be intrinsically usable as a key and have the desired equality semantics. Common options include integer hash codes or string representations.

type Key struct {
  a *int
}

func (k *Key) HashKey() int {
  return *(*k).a
}

k1, k2 := Key{intPtr(1)}, Key{intPtr(2)}
m := map[int]string{}
m[k1.HashKey()] = "one"
m[k2.HashKey()] = "two"

Caution: It's crucial to ensure that derived keys only collide when they truly represent semantic identity. Modifying fields in a derived key invalidates its use as a key because its identity has changed.

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