Delphi 'in' operator overload on a set - delphi

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

For the in operator to work the right operand must be of the record type since it's a set operator and not a binary operator. In your case it is the left operand.
So the following will work:
type
MyRecord = record
Value: MyEnum;
class operator In(const A: MyRecord; const B: MySet): Boolean;
end;
MyRecord2 = record
Value: MySet;
class operator In(const A: MyRecord; const B: MyRecord2): Boolean;
class operator In(const A: MyEnum; const B: MyRecord2): Boolean;
end;
class operator MyRecord.In(const A: MyRecord; const B: MySet): Boolean;
begin
Result := A.Value in B;
end;
class operator MyRecord2.In(const A: MyRecord; const B: MyRecord2): Boolean;
begin
Result := A.Value in B.Value;
end;
class operator MyRecord2.In(const A: MyEnum; const B: MyRecord2): Boolean;
begin
Result := A in B.Value;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
R: MyRecord;
R2: MyRecord2;
begin
R.Value := value1;
R2.Value := [value1, value2];
if R in R2 then;
if value1 in R2 then;
end;

Answer 2

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.