Delphi protected variables and constructor

Question

I have written this simple code:

type
  TTestA = class
  strict protected
    a: integer;
  public
    f: integer;
    constructor Create(x: Integer);
  end;

type
  TTestB = class(TTestA)
  strict private
    c: integer;
  end;

I use strict because these classes are in the same unit as the TForm1 class. Since a is protected by definition, it should be accessible only in subclasses, but then why this code doesn't work?

procedure TForm1.Button1Click(Sender: TObject);
var
  K: TTestB;
begin
  k := TTestB.Create(3);
  value = k.a; //I cannot access a
end;

Also, the protected can be useful to create an abstract class. In C++, if I declare a constructor as protected, I cannot create an instance of the object and only subclasses can. Can Delphi do this?

I am having the same problem with the variable and the constructor.

You are asking two separate questions. That is against StackOverflow guidelines. Please move your abstract question to another post. — Remy Lebeau
– Remy Lebeau, Commented May 23, 2017 at 21:09
Regarding the first question: Do not make the mistake of confusing classes with units. TForm is not related to either of the classes TTestA or TTestB. But TTestB is a subclass (descendent) of TTestA, so they are related. — Rudy Velthuis
– Rudy Velthuis, Commented May 23, 2017 at 21:52

Deltics · Accepted Answer · 2017-05-23 21:09:03Z

7

protected access means "accessible in the class and any sub-class".

This means that a in your example will be accessible to methods in class TTestB but this does not extend to consumers of instances of TTestB (or TTestA).

The code in TForm1 is part of a class which is not a subclass of TTestA.

Put another way, TForm1 does not inherit from TTestA and therefore cannot access any private or protected members of k.

answered May 23, 2017 at 21:09

Deltics

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Remy Lebeau · Accepted Answer · 2017-05-23 21:13:28Z

5

The answer is in your own question:

Since a is protected by definition it should be accessible only in subclasses

Regardless of the fact that TForm1 uses a local k variable of type TTestB, since TForm1 itself is not a subclass of TTestA, it does not have access to the k.a member. TTestB is a subclass of TTestA, so internally it has access to a.

This is explained in more detail in Embarcadero's documentation:

Classes and Objects (Delphi): Visibility of Class Members

edited May 23, 2017 at 21:13

answered May 23, 2017 at 21:08

Remy Lebeau

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