TObject and TClass The TObject class, declared in the
System
unit, is the ultimate ancestor of all other classes. TObject defines only
a handful of methods, including a basic constructor and destructor. In addition to TObject, the
System
unit declares
the class reference type TClass:
TClass = class of TObject;
If the declaration of a class type doesn't specify an ancestor, the class inherits directly from TObject. Thus
type TMyClass = class
...
end;
is equivalent to
type TMyClass = class(TObject)
...
end;
The latter form is recommended for readability.
Compatibility of Class Types A class type is assignment-compatible with its ancestors. Hence a variable of a class type can reference an instance
of any descendant type. For example, given the declarations
type
TFigure = class(TObject);
TRectangle = class(TFigure);
TSquare = class(TRectangle);
var
Fig: TFigure;
the variable
Fig
can be assigned values of type
TFigure
,
TRectangle
, and
TSquare
.
Object Types The Win32 Delphi compiler allows an alternative syntax to class types, which you can declare object types using
the syntax
type objectTypeName = object (ancestorObjectType)
memberList
end;
where objectTypeName is any valid identifier, (ancestorObjectType) is optional, and memberList declares fields,
methods, and properties. If (ancestorObjectType) is omitted, then the new type has no ancestor. Object types cannot
have published members.
Since object types do not descend from TObject, they provide no built-in constructors, destructors, or other methods.
You can create instances of an object type using the
New
procedure and destroy them with the
Dispose
procedure,
or you can simply declare variables of an object type, just as you would with records.
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Object types are supported for backward compatibility only. Their use is not recommended on Win32, and they have
been completely deprecated in the Delphi for .NET compiler.
