[fpc-pascal]Dynamic array of objects

memsom at interalpha.co.uk memsom at interalpha.co.uk
Fri May 30 11:46:15 CEST 2003

> Hi, I need something like a dynamic array of objects,
> easy to manipulate each object and field. Is TStream
> and TCollection what I need, or maybe TList is enough,
> maybe another Type?


the unit called 'contnrs' has a TObjectList class. I would advocate it's use. 
The main reason is that you can construct a TObjectList like so:

  MyObjectList := TObjectList.Create(True);

and this then means that the TObjectList 'owns' all objects added to it, and it 
will free then when they are deleted without you having to do any work 

  MyObjectList.Delete( idx ); 

  //same as

    tmp: TObject;
    tmp := TObject ( MyList[idx] );  
      { might also need the line 'MyList.Pack;'}

You can extend it or wrap it to allow you classes to be accessed directly, but 
something like:

  tmp: TMyClass;
  idx: integer;
  tmp := TMyClass.Create;

  tmp.SomeStringField := 'test';

  idx := MyObjectList.Add(tmp);

  //accessing the stored class...
  writeln( TMyClass( MyObjectList.Objects[idx] ).SomeStringField );
  //or with type checking 

  writeln( ( MyObjectList.Objects[idx] as TMyClass ).SomeStringField );

  //verifying that the class is of type TMyClass or Descends from it

  if ( MyObjectList.Objects[idx] is TMyClass ) then
    writeln( 'it is a TMyClass and it''s actual class type is ', 
             TMyClass( MyObjectList.Objects[idx] ).ClassName ); 
             //^^^  no need for 'as' because we know it *is* a TMyClass
             //     or descendent instance.


The one thing I do suggest is not mixing unrelated classes in the list. Classes 
that all descend from a base class are fine, but putting unrelated TObject 
descendents in the same list can make juggling the type casting a little more 
work. It's better to have a base class with common functionality that you 
override in the decendents than disparate classes. You may need to reengineer 
some things, or to use multiple lists, but it will work out better in the long 
run imho.

There is also a TComponentList and a TClassList and a TInterfaceList too. All 
with similar properties.

Hope that helps...


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