As of version 2.6 of Free pascal, the In operator can be overloaded as well. The first argument of the in operator must be the operand on the left of the in keyword. The following overloads the in operator for records:
{$mode objfpc}{$H+}
type
TMyRec = record A: Integer end;
operator in (const A: Integer; const B: TMyRec): boolean;
begin
Result := A = B.A;
end;
var
R: TMyRec;
begin
R.A := 10;
Writeln(1 in R); // false
Writeln(10 in R); // true
end.
The in operator can also be overloaded for other types than ordinal types, as in the following example:
{$mode objfpc}{$H+}
type
TMyRec = record A: Integer end;
operator in (const A: TMyRec; const B: TMyRec): boolean;
begin
Result := A.A = B.A;
end;
var
S,R: TMyRec;
begin
R.A := 10;
S.A:=1;
Writeln(S in R); // false
Writeln(R in R); // true
end.