Go map with user-defined key with user-defined equality?

Question

Suppose I have a struct type in Go that I want to use as a key in a map, but I don\'t want to use Go\'s builtin equality operation. What\'s the best way to build s

Answer 1

Go has strict comparable semantics for values used as map keys. As such, you cannot define your own hash code and equality functions for map keys as you can in many other languages.

However, consider the following workaround. Instead of using the struct instances directly as a keys, use a derived attribute of the struct which is intrinsically usable as a key and has the equality semantics you desire. Often it is simple to derive an integer or string value as a hash code which serves as the identity for an instance.

For example:

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

Of course, immutability is a critical concern with this approach. In the example above, if you modify the field a then the instance can no longer be used as a hash key because its identity has changed.

Answer 2

This is not possible in Go. There is no operator overloading or 'Equality' method you can override (due to not inheriting from a common base class like in .NET which your example reminds me of). This answer has more information on equality comparisons if you're interested; Is it possible to define equality for named types/structs?

As mentioned in the comments if you want to make something like this work I would recommend using a property on the object as a key. You can define equality based on how you set the value of that property (like it could be a checksum of the objects bytes or something if you're looking for memberwise equality).