Question

In Delphi XE2, I’m trying to overload the in operator on a record to allow me to check whether the value represented by the record is part of a set. My code looks like this:

type
  MyEnum = (value1, value2, value3);
  MySet = set of MyEnum;
  MyRecord = record
    Value: MyEnum;
    class operator In(const A: MyRecord; B: MySet): Boolean;
  end;

class operator MyRecord.In(const A: MyRecord; B: MySet): Boolean;
begin
  Result := A.Value in B;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  R: MyRecord;
  S: MySet;
begin
  R.Value := value1;
  S := [value1, value2];
  Button1.Caption := BoolToStr(R in S);
end;

The code fails to compile. For the statement R in S the compiler says: Incompatible types MyRecord and MyEnum.

How can I overload the In operator on MyRecord so that R in S will evaluate to True in the above code?

Answer 1

Well, you can almost do this, but you may not want to. AFAIK, class operators only work on the class (or record) they are defined within, so both R and S in your code have to be TMyRecord. With some injudicious use of implicit casting, we get the following:

unit Unit2;
interface
type
  MyEnum = (value1, value2, value3);
  MySet = set of MyEnum;
  MyRecord = record
    Value: MyEnum;
    ValueSet: MySet;
    class operator Implicit(A: MyEnum): MyRecord;
    class operator Implicit(A: MySet): MyRecord;
    class operator In (Left,Right:MyRecord): Boolean;
  end;

implementation

class operator MyRecord.Implicit(A: MyEnum): MyRecord;
begin
  Result.Value := A;
end;

class operator MyRecord.Implicit(A: MySet): MyRecord;
begin
  Result.ValueSet := A;
end;

class operator MyRecord.In(Left, Right: MyRecord): Boolean;
begin
  Result:= left.Value in Right.ValueSet;
end;
end.

The following will now complile, and even work:

procedure TForm1.Button1Click(Sender: TObject);
var
  R: MyRecord;
  S: MyRecord;
begin
  R.Value := value1;
  S := [value1,value2,value3];
  Button1.Caption := BoolToStr(R In S,true);
end;

Which, I’m sure we will all agree, is much more elegant than ‘BoolToStr(R.Value in S)’.
However the following will also compile, but give the wrong result:

procedure TForm1.Button1Click(Sender: TObject);
var
  R: MyRecord;
  S: MyRecord;
begin
  R.Value := value1;
  S := [value1,value2,value3];
  Button1.Caption := BoolToStr(S In R,true);
end;

So, as Dorin commented, better to just have dull, staid old ‘BoolToStr(R.Value in S)’. Unless of course you are being paid per line of code. And a bonus for bug-fixing.