Overload Add operator for three extended records

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;
开发者_开发问答
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.)

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

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.