The Delphi Geek: Implementing Record Assignment Operator [1]

Friday, January 09, 2015

Implementing Record Assignment Operator [1]

Following the yesterday’s hunch, I did some research and I came up with a way to detect when a record is copied (simple, supported, working everywhere) and even to access both records (source and target) during that operation (unsafe, unsupported and currently working only for Win32).

Now I can write such code:

program RecordAssignment;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  FastMM4,
  System.SysUtils,
  GpRecAssignOverload in 'GpRecAssignOverload.pas';

type
  TRec = record
  private
    FData    : integer;
    FAssignOp: IInterface; //should be last field in the record!
  public
    constructor Create(data: integer);
  end;

{ TRec }

constructor TRec.Create(data: integer);
begin
  FData := data;
  Writeln('Record [', FData, '] created');

  FAssignOp := TAssignRec<TRec>.Create(
    procedure(var rLeft, rRight: TRec)
    begin
      Writeln('Record assignment');
      rLeft.FData := rLeft.FData + 1;
    end
  );
end;

procedure Test;
var
  rec1, rec2: TRec;
begin
  rec1 := TRec.Create(42);
  rec2 := rec1;
  Writeln('rec1.FData = ', rec1.FData);
  Writeln('rec2.FData = ', rec2.FData);
end;

begin
  try
    Test;
    Write('>'); Readln;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

And the output is:

Just to make something clear – yes, technically I can write this but I’m pretty sure that I don’t want to. Tomorrow I’ll show the implementation for TAssignRec and you’ll see why.

11 comments:

Jørn E. Angeltveit12:11
Could this be solved with operator overloading?
Like this example: https://plus.google.com/+Asbj%C3%B8rnHeid/posts/KnR9pUPTy7P

Or will Implicit/Explicit only be triggered when there's a typecast involved?
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Anonymous12:36
@Jorn: no, assignment can be overloaded in C++ but not in Delphi.

@Gabr: instead of using a dummy interface to intercept an assignment why not to move a record data into an instance implementing interface? this is essentially a basic idea of my TForge project. You get solid automatic memory management with clear and safe "user" code, and very interesting and highly effective "engine" code.
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Jørn E. Angeltveit15:50
If you go through pointers, you can have something like this.
(You need to add @ in front of the records you want to run through the custom assignment)

type
TRec = record
private
FData : integer;
public
constructor Create(iData: integer);
class operator Implicit(aSource: Pointer): TRec;
end;

//-----

class operator TRec.Implicit(aSource: Pointer): TRec;
type
TTempRec = type TRec;
begin
Result.FData := TTempRec(aSource^).FData + 1;
end;

//-----

procedure Test;
var
rec1, rec2, rec3: TRec;
begin
rec1 := TRec.Create(42);
rec2 := rec1; //Plain assignment
rec3 := @rec1; //Custom assignment

Writeln('rec1.FData = ', rec1.FData);
Writeln('rec2.FData = ', rec2.FData);
Writeln('rec3.FData = ', rec3.FData);
end;

//-----

Gives you:

rec1.FData = 42
rec2.FData = 42
rec3.FData = 43

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Unknown18:43
Dirty hack, but works :D

program project1;
type
Tfoo=record
a: string;
end;
Tbar = record
a: string;
end;
// I use two equivalent types

// and two assigment op's: bar->foo and foo->bar:
operator := (foo: TFoo): TBar;
begin
result.a:=foo.a;
end;
operator := (bar: TBar): TFoo;
begin
result.a := bar.a;
end;
var
a,b:Tfoo;
begin
a.a:='a';
b.a:='b';
a := b;
writeln(a.a);
readln;
end.
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