Overload Add operator for three extended records

Question

Delphi 2006 introduced operator overloading which was then bugfixed in Delphi 2007. This is about Delphi 2007.

Why does the following not compile:

type
  TFirstRec = record
    // some stuff
  end;

type
  TSecondRec = record
    // some stuff
  end;

type
  TThirdRec = record
    // some stuff
    class operator Add(_a: TFirstRec; _b: TSecondRec): TThirdRec;
end;

class operator TThirdRec.Add(_a: TFirstRec; _b: TSecondRec): TThirdRec;
begin
   // code to initialize Result from the values of _a and _b
end;

var
  a: TFirstRec;
  b: TSecondRec;
  c: TThirdRec;
begin
  // initialize a and b

  c := a + b; // <== compile error: "Operator not applicable to this operand type"
end.

Since I have declared an operator that adds two operands a of type TFirstRec and b of type TSecondRec resulting in a TThirdRec, I would have expected this to compile.

(If you need something less abstract, think of TMyDate, TMyTime and TMyDateTime.)

Answer 1

When I tried to compile the code in Delphi 2009 I have got the compiler error

[Pascal Error] Project1.dpr(21): E2518 Operator 'Add' must take least one 'TThirdRec' type in parameters

at line

class operator Add(_a: TFirstRec; _b: TSecondRec): TThirdRec;

so the answer is - at least one of the arguments (_a; _b) must be of type TThirdRec

Answer 2

Serg is right. This does compile:

program Project51;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
  TThirdRec = record
    // some stuff
  end;

  TFirstRec = record
    // some stuff
  end;

  TSecondRec = record
    // some stuff
    class operator Add(_a: TFirstRec; _b: TSecondRec): TThirdRec;
  end;

class operator TSecondRec.Add(_a: TFirstRec; _b: TSecondRec): TThirdRec;
begin
   // code to initialize Result from the values of _a and _b
end;

var
  a: TFirstRec;
  b: TSecondRec;
  c: TThirdRec;
begin
  // initialize a and b

  c := a + b; // <== compile error: "Operator not applicable to this operand type"
end.

That can be a problem if you have to declare Add for all possible combinations of TFirstRec, TSecondRec and TThirdRec, as there is no forward declaration for records in Delphi.