Undocumented language enhancements

[vfbb 275]

Sometimes I come across some delphi language enhancements that are undocumented or are poorly documented. In the last few years docwiki has improved a lot, but there are still undocumented cases, such as accessing an item in an array just by declaring a property, without any method:

  TipGeolocation = record
  private
    FCoordinates: array[0..1] of Double;
  public
    property Latitude: Double read FCoordinates[0] write FCoordinates[0];
    property Longitude: Double read FCoordinates[1] write FCoordinates[1];
  end;

Share here any enhancements to the delphi language that you have discovered but are not documented.

[vfbb 275]

Virtual interfaces: to create an object that support an interface in run time. See System.Rtti.TVirtualInterface

[David Heffernan 2293]

49 minutes ago, vfbb said:

Share here any enhancements to the delphi language that you have discovered but are not documented.

The entire language is essentially undocumented. There is no formal specification, no documented grammar etc. 

[balabuev 97]

13 hours ago, vfbb said:

such as accessing an item in an array just by declaring a property, without any method

 

Similarly, should work with records:

type
  TMy = class
  private
    FPoint: TPoint;
  public
    property X: Integer read FPoint.X write FPoint.X;
    property Y: Integer read FPoint.Y write FPoint.Y;
  end;

 

This can even be mixed with static arrays:

type
  TMyData = record
    Points: array[0..1] of TPoint;
  end;

  TMy = class
  private
    FData: TMyData;
  public
    property X: Integer read FData.Points[1].X write FData.Points[1].X;
    property Y: Integer read FData.Points[1].Y write FData.Points[1].Y;
  end;

 

 

Edited February 17, 2021 by balabuev

[Der schöne Günther 306]

1 hour ago, David Heffernan said:

The entire language is essentially undocumented. There is no formal specification, no documented grammar etc. 

Not highly scientific, but better than nothing:

Delphi Language Reference - RAD Studio (embarcadero.com)

[David Heffernan 2293]

8 minutes ago, Der schöne Günther said:

Not highly scientific, but better than nothing:

Delphi Language Reference - RAD Studio (embarcadero.com)

Obviously I know this and have read it plenty of times. But it's way short of a formal specification. Just read the topic for properties. What does fieldOrMethod mean? It's not precisely defined anywhere.

 

The entire language documentation is like that. Just a little imprecise. 

Edited February 16, 2021 by David Heffernan

[Remy Lebeau 1257]

Undocumented intrinsic routines

Undocumented Delphi routines

Undocumented Delphi record alignment directive

Just to name a few.  I know there are other sites that document more, I just don't have the time to hunt them down.

[Fr0sT.Brutal 897]

"Static variables" inside procedures

function Counter: Integer;
{$J+}
const
  I: Integer = 0;
{$J-}
begin
  I := I + 1;
  Result := I;
end;

When you declare a const with a type, Delphi allocates memory for the constant. The program can change the value of the “constant,” just as though it were a variable. The difference between a typed constant and a local variable is that a typed constant in a subroutine keeps its value across subroutine calls.

If you disable the $WriteableConst directive, you can prevent any assignments to a typed constant, thereby making the constant truly constant. There is no substitute for typed constants in a subroutine, though, so disable $WriteableConst with care. Disable only specific constants whose value you want to protect, and do not disable this directive globally in a project or an entire file.

 

Short init of record constants

 

type 
  TR = record
    a: string;
    b: string;
  end;

Const TR_Empty: TR = ();
Const TR_A: TR = (a: 'foo');

 

(not undocumented but non-obvious ones)

Edited February 17, 2021 by Fr0sT.Brutal