Understanding constructor visibility

Question

Here's two simple classes, initially both have no keywords (virtual, overload, override, reintroduce):

TComputer = class(TObject)
public
   constructor Create(Teapot: Integer);
end;

TCellPhone = class(TComputer)
public
   constructor Create(Teapot: Integer; Handle: string);
end;

i will represent these above defintions as the slightly shorter:

TComputer = class(TObject)
   constructor Create(Teapot: Integer);

TCellPhone = class(TComputer)
   constructor Create(Teapot: Integer; Handle: string);

And when constructing TCellPhone there is only one constructor (int, string) - because the ancestor constructor has been hidden. i will indicate the visible constructors of TCellPhone as:

• Teapot: Integer; Handle: string

---

Now for the question, the first 3 cases make sense, the 4th does not:

1. Ancestor constructor is hidden by descendant:

TComputer = class(TObject)
   constructor Create(Teapot: Integer);

TCellPhone = class(TComputer)
   constructor Create(Teapot: Integer; Handle: string);

• Teapot: Integer; Handle: string

This makes sense, the ancestor constructor is hidden because i've declared a new constructor.

2. Ancestor virtual constructor is hidden by descendant:

TComputer = class(TObject)
   constructor Create(Teapot: Integer); virtual;

TCellPhone = class(TComputer)
   constructor Create(Teapot: Integer; Handle: string);

• Teapot: Integer; Handle: string

This makes sense, the ancestor constructor is hidden because i've declared a new constructor.

Note: Because the ancestor is virtual: Delphi will warn you that you're hiding the virtual ancestor (in the previous example of hiding a static constructor: nobody cares, so no warning). The warning can be suppressed (meaning "Yeah yeah yeah, i'm hiding a virtual constructor. i meant to do that.") by adding reintroduce:

    TComputer = class(TObject)
       constructor Create(Teapot: Integer); virtual;

    TCellPhone = class(TComputer)
       constructor Create(Teapot: Integer; Handle: string); reintroduce;

3. Ancestor constructor not hidden in descendant because of overloading:

TComputer = class(TObject)
   constructor Create(Teapot: Integer);

TCellPhone = class(TComputer)
   constructor Create(Teapot: Integer; Handle: string); overload;

• Teapot: Integer; Handle: string
• Teapot: Integer

This makes sense, since the descendant constructor is an overload of the ancestor, so both are allowed to be present. The ancestor constructor is not being hidden.

4. Virtual ancestor constructor not hidden in descendant because overloading - but still get a warning:

This is the case that makes no sense:

TComputer = class(TObject)
   constructor Create(Teapot: Integer); virtual;

TCellPhone = class(TComputer)
   constructor Create(Teapot: Integer; Handle: string); overload;

• Teapot: Integer; Handle: string

• Teapot: Integer

  Method 'Create' hides virtual method of base type 'TComputer'

This makes little sense. Not only is the ancestor not hidden, the descendant is overloaded; it shouldn't even be complaining.

What gives?

Answer 1

Delphi's documentation says:

If you overload a virtual method, use the reintroduce directive when you redeclare it in descendant classes. For example,

type
  T1 = class(TObject)
    procedure Test(I: Integer); overload; virtual;
  end;
  T2 = class(T1)
    procedure Test(S: string); reintroduce; overload;
  end;

Without the reintroduce directive, it still works, as you've noticed, but you'll get the warning.

Also, you are actually hiding TObject.Create, but it has nothing to do with the warning. If you think you might want access to TObject.Create also, do this:

type
  TComputer = class(TObject)
    constructor Create(Teapot: Integer); reintroduce; overload; virtual;
  end;

type
  TCellPhone = class(TComputer)
    constructor Create(Teapot: Integer; Handle: String); reintroduce; overload;
  end;