I want to create a list in which every element must have 3 fields: KeyType, KeyName, Value. The type of Value field should be: String, Cardinal, Integer, Byte, Boolean... depending on value of the KeyType. I need this to make something like windows registry. It is possible ?
2 Answers
I'd use an off the shelf generic linked list. I'm sure Spring has some, but if you don't want to take on all the dependencies that would entail you could use Chris Rolliston's simple linked list: https://code.google.com/p/ccr-exif/source/browse/blogstuff/CCR.SimpleLinkedList.pas?r=34
Then you just need to decide on the type of the payload. Use a variant type like, for instance, TValue or Variant, although the latter would constrain you to Windows. Or you could make your own bespoke variant type:
type
TMyValue = record
StringValue: string; // managed type must be outside variant part of record
case DataType: TDataType of
dtInteger:
(IntegerValue: Integer);
dtCardinal:
(CardinalValue: Cardinal);
....
end;
You'd then make a node type like this:
type
TNode = record
Name: string;
Value: TValue; // or TMyValue
end;
Finally, your linked list is simply TSimpleLinkedList<TNode> and you are done.
In my opinion it is important to use a generic container here for the sake of consistency. Doing so allows you to keep separate the aspects of the container and the element.
answered May 12, 2015 at 17:05
David Heffernan
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1 Comment
Johan
You can't use string in a variant record. The compiler does not allow managed types there.
There are two options.
You can use a nornal record/class with a variant type, something like:
type
TDataType = (dtBoolean, dtString, ....);
PNode = ^TNode;
TNode = record
Prev, Next: PNode;
Keyname: string;
DataType: TDataType;
Data: variant; //or TValue
end;
Or ue a variant record
TNode = record
Prev, next: PNode;
DataType: TDataType;
Keyname: string;
Datastring:string;
case DataType of
dtCardinal: (datacardinal: Cardinal);
dtBoolean: (databoolean: boolean);
....
end;
Note that managed types such as interfaces and strings cannot be included in the variant part of the record, so you'll have to put those in the normal part before that.
Registry cannot be captured in a linked list
Note that the registry is a tree, you you'll need a tree instead of a linked list, that means you'll need 3 links: root, left, right.
And you'll need to use a tree structure. Any tree can be mapped to a binary tree, so 3 is all you need (two if you leave out the root node).
TDictionary<string, TNode>
Will also work. In that case TNode does not include Prev/next members because the dictionary takes care of that.
1 Comment
David Heffernan
I think the reference to the registry was less about the tree structure, and more about the ability to hold variant types
TValue.array of TDataTypefor index-related access andTStringList.AddObjectfor key-related access. There are alsoTHashadStringList,TDictionaryand so on. Just determine your goal.