How to overload assign operator for record in Delphi

Question

I want to make the type of record that uses dynamic arrays.

Using the variables A and B of this type I want to be able to perform operations A: = B (and other) and be able to modify the content of A without modification B like in snipped code below:

    type
      TMyRec = record
        Inner_Array: array of double;
      public
        procedure SetSize(n: integer);
        class operator Implicit(source: TMyRec): TMyRec;
      end;

    implementation

    procedure TMyRec.SetSize(n: integer);
    begin
      SetLength(Inner_Array, n);
    end;

    class operator TMyRec.Implicit(source: TMyRec): TMyRec;
    begin
    //here I want to copy data from source to destination (A to B in my simple example below)
    //but here is the compilator error
    //[DCC Error] : E2521 Operator 'Implicit' must take one 'TMyRec' type in parameter or result type
    end;


    var
      A, B: TMyRec;
    begin
      A.SetSize(2);
      A.Inner_Array[1] := 1;
      B := A;
      A.Inner_Array[1] := 0;
//here are the same values inside A and B (they pointed the same inner memory)

There are two problems:

when I don't use overriding assigning operator in my TMyRec, A:=B means A and B (their Inner_Array) are pointing the same place in memory.
to avoid problem 1) I want to overload assign operator using:

class operator TMyRec.Implicit(source: TMyRec): TMyRec;

but compilator (Delphi XE) says:

[DCC Error] : E2521 Operator 'Implicit' must take one 'TMyRec' type in parameter or result type

How to resolve this problems. I read several similar posts on stackoverflow but they don't work (if I understood they well) on my situation.

Artik

You should make your `Inner_Array` immutable; code like `A.Inner_Array[1] := 1;` will be forbidden - any write to `Inner_Array` should create a new array instance. Read also http://sergworks.wordpress.com/2013/04/10/on-the-operator-overloading-in-delphi/ for some more hints. — kludg, Aug 01 '13 at 02:25
@user246408 Immutable vector/matrix types are usually inconvenient and lead to poor performance. For example, many matrix algorithms operate in place. — David Heffernan, Aug 01 '13 at 09:54
[There is a way to catch assignment A:=B and copy data from A to B](https://stackoverflow.com/a/47320631/7579632), if you are willing to abuse the language. — Nasreddine Galfout, Nov 16 '17 at 12:31
https://www.thedelphigeek.com/2015/01/implementing-record-assignment-operator_9.html — Gabriel, Dec 21 '19 at 19:26
Related question: https://stackoverflow.com/questions/953151/can-you-overload-the-assignment-operator-for-delphi-records — Gabriel, Jun 09 '22 at 10:02

score 6 · Accepted Answer · edited May 25 '21 at 07:46

6

It is not possible to overload the assignment operator. This means that what you are attempting to do is not possible.

Edit: It is possible now - http://docwiki.embarcadero.com/RADStudio/Sydney/en/Custom_Managed_Records#The_Assign_Operator

edited May 25 '21 at 07:46

Kromster

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answered Jul 31 '13 at 20:14

David Heffernan

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Does it mean, there are no way to catch assignment A:=B and copy data from A to B? – Artik Jul 31 '13 at 20:25
2

It is possible to introduce a COW functionality on dynamic arrays. This will take care of the cloning when writing to the array, just as the `String` type works. It's a bit messy, but I managed to make it work. I will publish the code soon. Main thing that disturbs me is that intrinsic functions like `SetLength`,`Length`, etc are not possible to introduce as operator overloads. – LU RD Jul 31 '13 at 20:46

score 0 · Answer 2 · answered Oct 26 '18 at 07:01

The only known way to do it is to use pointers, which is not safe, so you must understand what you're doing.

It's something like this:

type
  PMyRec = ^TMyRec;
  TMyRec = record
    MyString : string;
    class operator Implicit(aRec : PMyRec) : TMyRec;
  end;
....
class operator TMyRec.Implicit(aRec : PMyRec) : TMyRec;
begin
  if aRec = nil then // to do something...
    raise Exception.Create('Possible bug is here!');
  Result.MyString := aRec^.MyString;
end;

And the call example should look like this:

var
  aRec1, aRec2 : TMyRec;
begin
  aRec1.MyString := 'Hello ';
  aRec2.MyString := 'World';
  writeln(aRec1.MyStr, aRec2.MyStr);
  aRec2 := @aRec1;
  writeln(aRec1.MyStr, aRec2.MyStr);
end.

How to overload assign operator for record in Delphi

2 Answers2