How Can I Implement Custom Key Comparison in Go Maps?

Custom Key Comparison in Go Maps

In Go, maps require comparable keys, but sometimes a custom equality operation is desired. This scenario arises when a custom data structure, such as a user-defined struct, is used as the map key.

To circumvent this limitation, a workaround involves deriving a separate attribute from the struct that serves as the map key. This derived attribute should have the desired equality semantics and should be intrinsically usable as a key.

Consider the following example:

type Key struct {
    a *int
}

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

In this example, the HashKey method derives an integer value that represents the identity of the Key. The map can then be constructed using this derived attribute:

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

fmt.Println(m) // Outputs: map[1:one 2:two]

Note that the key comparison relies solely on the HashKey() method, allowing for custom equality semantics.

Immutability Considerations

However, immutability is crucial when using this approach. If the fields of the original struct are modified, the instance can no longer be used as a map key because its identity has changed.

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