[fpc-pascal] Quick Modern Object Pascal Introduction, for Programmers

[Marco van de Voort]

In our previous episode, Michalis Kamburelis said:
> 
> - A section about TPersistent.Assign. This is the "basic approach to
> cloning" that should probably be shown first. See it here:
> http://michalis.ii.uni.wroc.pl/~michalis/modern_pascal_introduction/modern_pascal_introduction.html#_cloning_tpersistent_assign

See also
http://stackoverflow.com/questions/4041760/correct-way-to-duplicate-delphi-object/4041906#4041906

That is for an own root class and pre class helpers, but you could do it also in or TPersistent
helper:

type tpersisthelp = class helper for TPersistent
    function Clone(t: TPersistentClass = nil): TPersistent; 
end;

function tpersisthelp.Clone(t: TPersistentClass = nil): TPersistent;
  begin
    if Assigned(t) then
      Result := t.Create
    else
      Result := TPersistentClass(Self.ClassType).Create;
    Result.Assign(Self);
  end;

Since TComponent declares a virtual destructor that might even be better.