How to expose the built in enumerator for a private static array field in Delphi?

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I am trying to expose the build in TEnumerator for a private static array.

Delphi itself allows to enumerate a static array directly (see below) so I suspect that Delphi creates an enumerator in the background for the static array and I am hoping that I will be able to create and expose the same enumerator in GetEnumerator method.

(I am using Delphi XE2).

program Project6;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils,
  System.Generics.Collections;

type
  TMyEnum = (meA, meB);

  TMyClass = class
  private
    FItems: array[TMyEnum] of Integer;
  protected
  public
    function GetEnumerator: TEnumerator<Integer>;
  end;

{ TMyClass }

function TMyClass.GetEnumerator: TEnumerator<Integer>;
begin
  // What is the simplies way of creating this enumerator?
end;

var
  myObj: TMyClass;
  i: Integer;

begin
  myObj := TMyClass.Create;
  try
    // This works but only in the same unit
    for i in myObj.FItems do
      WriteLn(i);

    for i in myObj do
      WriteLn(i);

  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  ReadLn;
end.

Note that I can write a custom emulator like below. But I am trying to avoid this and expose the built in one.

TStaticArrayEnumerator<T> = class(TEnumerator<T>)
  private
    FCurrent: Pointer;
    FElementAfterLast: Pointer;
  protected
    function DoGetCurrent: T; override;
    function DoMoveNext: Boolean; override;
  public
    constructor Create(aArray: Pointer; aCount: Integer);
  end;

{ TStaticArrayEnumerator<T> }

constructor TStaticArrayEnumerator<T>.Create(aArray: Pointer; aCount: Integer);
begin
  // need to point Current before the first element (see comment in DoMoveNext)
  FCurrent := Pointer(NativeInt(aArray) - SizeOf(T));
  FElementSize := aElementSize;
  FElementAfterLast := Pointer(NativeInt(aArray) + aCount * SizeOf(T))
end;

function TStaticArrayEnumerator<T>.DoGetCurrent: T;
begin
  Result := T(FCurrent^);
end;

function TStaticArrayEnumerator<T>.DoMoveNext: Boolean;
begin
  // This method gets called before DoGetCurrent gets called the first time
  FCurrent := Pointer(NativeInt(FCurrent) + SizeOf(T));

  Result := not (FCurrent = FElementAfterLast);
end;
2 Answers
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