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Delphi/Lazarus: Difference between restrict, private, and protected Access Modifiers

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The difference between private, protected and public is pretty straightforward:

*Private members/methods are only visible within the class that declares them.

*Protected members/methods are visible within the class, and to all subclasses.

*Public members and methods are visible to all other classes.

In Delphi there's a "bug" that makes the visibility of all members public within the same unit. The strict keyword corrects this behaviour, so that private is actually private, even within a single unit. For good encapsulation I would recommend always using the strict keyword.

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  1. type
  2.   TFather = class
  3.   private
  4.     FPriv : integer;
  5.   strict private
  6.     FStrPriv : integer;
  7.   protected
  8.     FProt : integer;
  9.   strict protected
  10.     FStrProt : integer;
  11.   public
  12.     FPublic : integer;
  13.   end;
  14.  
  15.   TSon = class(TFather)
  16.   public
  17.     procedure DoStuff;
  18.   end;
  19.  
  20.   TUnrelated = class
  21.   public
  22.     procedure DoStuff;
  23.   end;
  24.  
  25. procedure TSon.DoStuff;
  26. begin
  27.   FProt := 10;       // Legal, as it should be. Accessible to descendants.
  28.   FPriv := 100;      // Legal, even though private. This won't work from another unit!
  29.   FStrictPriv := 10; // <- Compiler Error, FStrictPrivFather is private to TFather
  30.   FPublic := 100;    // Legal, naturally. Public members are accessible from everywhere.
  31. end;
  32.  
  33. procedure TUnrelated.DoStuff;
  34. var
  35.   F : TFather;
  36. begin
  37.   F := TFather.Create;
  38.   try
  39.     F.FProt := 10;     // Legal, but it shouldn't be!
  40.     F.FStrProt := 100; // <- Compiler error, the strict keyword has "made the protection work"
  41.     F.FPublic := 100;  // Legal, naturally.
  42.   finally
  43.     F.Free;
  44.   end;
  45. end;

URL: http://stackoverflow.com/questions/1516493/difference-between-strict-private-and-protected-access-modifiers-in-delphi

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