Extended record enumerators

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### 9.2 Extended record enumerators

Extended records can have an enumerator. To this end, a function returning an enumerator record must be defined in the extended record:

{$mode objfpc}
{$modeswitch advancedrecords}
type
TIntArray = array[0..3] of Integer;
TEnumerator = record
private
FIndex: Integer;
FArray: TIntArray;
function GetCurrent: Integer;
public
function MoveNext: Boolean;
property Current: Integer read GetCurrent;
end;
TMyArray = record
F: array[0..3] of Integer;
function GetEnumerator: TEnumerator;
end;
function TEnumerator.MoveNext: Boolean;
begin
inc(FIndex);
Result := FIndex < Length(FArray);
end;
function TEnumerator.GetCurrent: Integer;
begin
Result := FArray[FIndex];
end;
function TMyArray.GetEnumerator: TEnumerator;
begin
Result.FArray := F;
Result.FIndex := -1;
end;

After these definitions, the following code will compile and enumerate all elements in F:

var
Arr: TMyArray;
I: Integer;
begin
for I in Arr do
WriteLn(I);
end.

The same effect can be achieved with the enumerator operator:

{$mode objfpc}
{$modeswitch advancedrecords}
type
TIntArray = array[0..3] of Integer;
TEnumerator = record
private
FIndex: Integer;
FArray: TIntArray;
function GetCurrent: Integer;
public
function MoveNext: Boolean;
property Current: Integer read GetCurrent;
end;
TMyArray = record
F: array[0..3] of Integer;
end;
function TEnumerator.MoveNext: Boolean;
begin
inc(FIndex);
Result := FIndex < Length(FArray);
end;
function TEnumerator.GetCurrent: Integer;
begin
Result := FArray[FIndex];
end;
operator Enumerator(const A: TMyArray): TEnumerator;
begin
Result.FArray := A.F;
Result.FIndex := -1;
end;

This will allow the code to run as well.

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