On Sunday, 31 July 2016 at 16:39:59 UTC, Jack Stouffer wrote:
But D provides a default toHash for every type if it's not
defined. I was wondering why not just rely on that version.
If two objects are equal, their hashes must also be equal.
Consider this example:
struct Nullable(T) {
bool isNull;
T value;
bool opEquals(Nullable!T other) {
if(this.isNull != other.isNull) return false;
// Any two nulls are equal.
if(isNull) return true;
return this.value == other.value;
}
}
auto n1 = Nullable!int(true, 3);
auto n2 = Nullable!int(true, 4);
writeln(n1 == n2); // true
writeln(n1.hashOf == n2.hashOf); // false = BAD!
Now that I think about it, maybe this should be in the language
docs; I don't think they mention it.
(https://dlang.org/spec/operatoroverloading.html#equals)
