Is it possible to use a class declared in Implementation section from Interface section

Question

If I understand correctly, the interface section is visible to other units, and the implementation section is visible only in the current .pas file.

I have two classes, class TA should be visible to the outside, other class TB should not, but I need a field of type TB in TA.

interface
    
type
  TA = class
    //something
    B : TB;
  end;
    
//something
    
implementation
    
type
  TB = class
    //something
  end;

It doesn't work like that. I also cannot use a forward declaration. Is there a way?

Or, is there a way to declare TB in the interface section but make it kind-of private?

Answer 1

A type cannot be used before it is declared (in terms of line numbers). In particular, this means that you cannot use a type declared in the implementation section in the interface section.

However, consider the following example:

unit VisibilityTest;

interface

type
  TFrog = class
  strict private type
    TFrogMetabolism = class
      procedure DoAnabolismStuff;
      procedure DoCatabolismStuff;
    end;
  strict private
    FMetabolism: TFrogMetabolism;
  public
    procedure Croak;
    procedure Walk;
    procedure Jump;
  end;

implementation

{ TFrog.TFrogMetabolism }

procedure TFrog.TFrogMetabolism.DoAnabolismStuff;
begin

end;

procedure TFrog.TFrogMetabolism.DoCatabolismStuff;
begin

end;

{ TFrog }

procedure TFrog.Jump;
begin

end;

procedure TFrog.Croak;
begin

end;

procedure TFrog.Walk;
begin

end;

end.

Here the TFrog class is visible to other units, as well as its Croak, Walk, and Jump methods.

And it does have a (strict private in this example) field of type TFrogMetabolism, a type which can only be used inside TFrog -- and therefore only inside this unit -- because of the preceding strict private specification.

This should give you some ideas. A few variants are possible:

• If you remove strict from strict private type, the TFrogMetabolism class can be used everywhere inside this particular unit, and not only in TFrog.

• If you replace private with protected, the class can also be used in classes that aren't TFrog but are derived from TFrog.