[fpc-devel] Multiple inheritance

[Ondrej Pokorny]

Hello!

Has anybody thought about multiple inheritance in object pascal? I am 
now working on a client-server service that entirely uses ORM. I have 
several patterns that repeat all over and some of them do not match into 
the direct inheritance schema.

For now I solved multiple inheritance with include file templates (like 
rtl/objpas/sysutils/finah.inc and fina.inc). But it would be much nicer 
to have it built-in:

TBase = class(TObject)
private
   FBaseField: Integer;
protected
   procedure LoadFromNode(const aNode: TXMLNode); virtual;
public
   property BaseField: Integer read FBaseField write FBaseField;
end;

TAddressExtension = class extension for TBase
private
   FAddress: string;
protected
   procedure LoadFromNode(const aNode: TXMLNode); override;
public
   property Address: string read FAddress write FAddress;
end;

TTag = (black, gray, white, green, blue);
TTags = set of TTag;

TTagsExtension = class extension for TBase
private
   FTags: TTags;
protected
   procedure LoadFromNode(const aNode: TXMLNode); override;
public
   property Tags: TTags read FTags write FTags;
end;

TCustomer = class(TBase, TAddressExtension, TTagsExtension)
protected
   procedure LoadFromNode(const aNode: TXMLNode); override;
end;

TCustomer gets all fields and properties from TBase, TAddressExtension 
and TTagsExtension. The LoadFromNode virtual method chain is:

TCustomer -> TTagsExtension -> TAddressExtension -> TBase.

Any thoughs on it? Is it sensible? How do you solve multiple inheritance?

+ If such a concept was in FPC, the LCL could get rid of the multiple 
inheritance hacks in widgetset bindings :)

Ondrej