how to call inherited constructor of TObjectDictionary in Delphi

Question

I created the following class, after reading about the significant performance improvement of TDictionary over TStringList:

 TAnsiStringList = class(TObjectDictionary<AnsiString,TObject>) public constructor Create(const OwnsObjects: Boolean = True); reintroduce; destructor Destroy; override; procedure Add(const AString: AnsiString); procedure AddObject(const AString: AnsiString; AObject: TObject); end;

I coded the constructor like this:

 { TAnsiStringList } constructor TAnsiStringList.Create(const OwnsObjects: Boolean = True); begin if OwnsObjects then inherited Create([doOwnsKeys,doOwnsValues]) else inherited Create; end;

...expecting that this TObjectDictionary constructor would be called:

 constructor Create(Ownerships: TDictionaryOwnerships; ACapacity: Integer = 0); overload;

...if the Ownerships parameter were specified. If the Ownerships parameter is not specified, I expected that the following inherited TDictionary constructor would be called:

 constructor Create(ACapacity: Integer = 0); overload;

The code compiles and runs, but when I call

 inherited Create([doOwnsKeys,doOwnsValues]) I get the following error:

Invalid class typecast

Does anyone see what I'm doing wrong, and is there a proper way to do this?

TIA

David Heffernan · Accepted Answer · 2013-04-12 15:35:05Z

The problem is that you are asking the container to call Free on your keys when items are removed. But your keys are not classes and so that is an invalid request. This is trapped at runtime rather than compile time because the ownership is not determined until runtime.

You only want doOwnsValues and should remove doOwnsKeys.

 if OwnsObjects then inherited Create([doOwnsValues]) else inherited Create;

For what it is worth, if you are trying to maked an AnsiString equivalent to TStringList, then your approach is flawed. A string list is an ordered container, but a dictionary is not. You won't be able to index by integer, iterate in order and so on. I also do not understand why you would want to force all consumers of this class to declare the objects to be of type TObject. You should leave that parameter free for the consumer to specify. That's the beauty of generics.

Perhaps you don't want an ordered container, in which case a dictionary is what you need. But in that case you simply don't need to create a new class. You can simply use TObjectDictionary as is.

If you are dead set on creating a sub class then I'd do it like this:

type TAnsiStringDict<T: class> = class(TObjectDictionary<AnsiString, T>)

That will allow the consumer of the class to decide which type of objects they put in the dictionary, and maintain compile time type safety.

So, when you want a dictionary of list boxes your declare a variable like this:

var ListBoxDict: TAnsiStringDict<TListBox>;

Thanks for your prompt reply. The reason I took this approach is simply due to the performance improvement when performing lookups on large lists. I've often used a stringlist as a text based indexing system on lists of objects. If I don't have a need to look up a string or object using the numeric index, why wouldn't it be a better approach to use TObjectDictionary, since it is so much faster on the lookup?
You can simply use TObjectDictionary as is. If you want a version that explicitly forces your ownership choice, and chooses AnsiString for the keys, at least leave the value type parameterized. Then you can have type safety. You might find that performance is worse with AnsiString. Unless you use AnsiString everywhere else and don't touch RTL/VCL methods that will convert to and from UTF-16.
If I don't have a need to look up a string or object using the numeric index That's fine. That's my last paragraph. But then your class name is wrong. Every Delphi coder that reads TAnsiStringList is going to assume an ordered array like container.
Incidentally, I put a small test app together to test the performance difference, and I found that adding items to a TDictionary is 30-50% slower than a TStringList. But it's so much faster to do lookups on a TDictionary that it offsets the performance loss. The reason I created this class the way I did is because our legacy software uses a similar class that doens't need the indexed ordering, and uses the old ".indexOf(string) > -1" approach, which is quite competently handled by TDictionary's ".ContainsKey(string)" function.
I totally get the performance trade-offs. String list lookup is greatly improved if you set Sorted to True. Because then it can use binary search. I think I answered the question in the first part of the answer. The rest was general advice. 1. I think the class is named incorrectly. 2. I think you can use TObjectDictionary as is without subclassing. 3. If you do subclass, leave the value parameter open for consumers to specify.

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