Figure 1 - Creating a default TPersistent field

{interface}
type DemoComponent =
       class(TComponent)
       private
       fGlyph:        TBitmap;
       fGlyphWritten: boolean;
       procedure SetGlyph(Glyph: TBitmap); {not shown}
       protected
       constructor Create(Owner: TComponent); override;
       procedure Loaded;		      override;
       public
       published
       property Glyph: TBitmap read fGlyph write SetGlyph;
       end;
{implementation}
constructor DemoComponent.Create(Owner: TComponent);
begin
  inherited Create(Owner);
  fGlyph := TBitmap.Create;
  {Be sure to fill in the field with an empty object}
end;
procedure DemoComponent.SetGlyph(Glyph: TBitmap);
begin
  if fGlyph <> Glyph then    { fGlyph = Glyph when SetGlyph is  }
    begin                    { called by the Loaded procedure   }
    fGlyph.Free;    {Assign can fail if the target is not Empty:}
    fGlyph := TBitmap.Create; {Free/Create/Assign is a lot safer}
    fGlyph.Assign(Glyph);
    end;
  {Extract any necessary data ... set the PropertyWritten flag}
  fGlyphWritten := True;
end;
procedure DemoComponent.Loaded;
begin
  inherited Loaded; {Don’t forget to do this!}
  if (not fGlyphWritten) and (not fGlyph.Empty) then
    SetGlyph(fGlyph); {Extract any necessary data from the bitmap}
end;