There are 2 features, I am missing.
And I can't see a way to add them via subclassing, as the internal
FItems array is private, and neither TItem, nor TPair can be gotten by
the integer index into that array.
1) AddOrGetPointerToEmpty
This would actually just be to try and see if it gives any benefit in
performance.
There is TryAdd()
But, the caller must already have a new value, that it can pass for that
key.
However, if the dictionary already has a value for that key, then the
existing value may be enough,
So the caller ends up
NewVal := CreaterNewValSpendingTime;
if not Dictionary.TryAdd(TheKey, NewVal) then
NewVal.Destroy;
Alternatively the caller can do
If not Dictionary.ContainsKey(TheKey) then ...
But the latter then means, that "TheKey" need to be hashed twice, if a
new value needs to be added (Even though the hash could already be known).
If there was a function
GetItemPointer(AKey: TKey; out APtr: PValue): boolean;
which returns True, if Akey was found and therefore APtr points to
existing Data, while otherwise APtr points to the empty slot, where the
data can be copied to.
2) Key and Data to be the same.
Lets say I have lots of data, to which I have pointers (e.g. instances
of some object).
Now when I have such a pointer, I want to know if I had a previous
pointer to the same data. (even if the new pointer points to a copy of
the data at a new location).
I could do TDictionary<TMyClass, TMClass>
I need to have my own IEqualityComparer, so I don't compare the
pointers, but the data in the object. (That I can do).
But now I need to do
MyDict.Add(val, val);
Which means the same value is stored as pointer and data => doubling mem
requirements.
What I would need is
- set TData to "record end" (zero size)
- be able to retrieve the existing key, for another key that returns
equal to the stored key
So instead of
MyVal := Dict.Items[AKey];
I need
ExsitingKey := Dict.Keys[ANewKey];
Obviously since the key is the data, my IEqualityComparer can do proper
comparison, so even if the hash clashes, the pointed to data can be
compared.
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