Delphi self keyword

Question

I am learning Delphi reading Marco Cantu\'s book and it\'s super complete. It\'s very clear but I have a doubt about the keyword self. I already have experience

Answer 1

Self is very similar to this in Java, C++, or C#. However it is a little bit more invoked, as the following code will show.

In Delphi, you can have class methods that are not static but also have a Self pointer, which then obviously does not point to an instance of the class but to the class type itself that the method is called on.

See the output of this program:

program WhatIsSelf;

{$APPTYPE CONSOLE}

type
  TAnimal = class
    procedure InstanceMethod;
    class procedure ClassMethod;
    class procedure StaticClassMethod; static;
  end;

  TDog = class(TAnimal)
  end;

class procedure TAnimal.ClassMethod;
begin
  Writeln(Self.ClassName);
end;

procedure TAnimal.InstanceMethod;
begin
  Writeln(Self.ClassName);
end;

class procedure TAnimal.StaticClassMethod;
begin
//  Writeln(Self.ClassName); // would not compile with error: E2003 Undeclared identifier: 'Self'
end;

var
  t: TAnimal;
begin
  t := TDog.Create;
  t.InstanceMethod;
  t.ClassMethod;

  TAnimal.ClassMethod;
  TDog.ClassMethod;

  Readln;
end.

Answer 2

Yes, Delphi's Self is the direct analogue of Java's this.

Answer 3

Formally speaking, Self is a normal identifier (that is automatically predeclared in some circumstances).

Self is automatically defined in the implementation of methods and class methods and its purpose there is similar to this in Java as already mentioned. The underlying technology is analogous to the Result variable in ordinary functions.

In other words, there is no self keyword (that's why it's typically written with an uppercase S). Whenever you want (and can ^[*]), you may introduce your own Self variable, method or class.

type
  // TForm1 ommitted

  Self = class
    function Self: Integer;
  end;

function Self.Self: Integer;
begin
  Result := SizeOf(Self);
end;

You'll have of course difficulties to instantiate an object of this type within a method, in these cases, you have to use it through an adapter function (or procedure):

function UseSelf: Integer;
var
  S: Self;
begin
  S := Self.Create;
  Result := S.Self;
  S.Free;
end;

function VarSelf: Integer;
var
  Self: Integer;
begin
  Self := SizeOf(Result);
  Result := Self;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(IntToStr(UseSelf)+' '+IntToStr(VarSelf));
end;

---

^[*] You cannot declare a Self variable within methods, because there is already such a declaration and that's exactly what the error says:

Identifier redeclared: 'Self'

Answer 4

self is a special variable that points to the object that owns the currently executing code and it gives access to the current object. In Ruby programming language there is self too, however equivalent of it in JavaScript, C++, C# and Java is this and in VB is Me.