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Author Topic: Advanced records as function parameters  (Read 250 times)

avk

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Advanced records as function parameters
« on: September 30, 2020, 06:53:20 pm »
Hi!

A simple test
Code: Pascal  [Select][+][-]
  1. program adv_rec_test;
  2. {$mode objfpc}{$H+}
  3. {$MODESWITCH ADVANCEDRECORDS}
  4. uses
  5.   SysUtils;
  6. type
  7.   TIntArray = array of Integer;
  8.   TTest = record
  9.     Data: TIntArray;
  10.     function IsSame(const aRec: TTest): Boolean;
  11.     function IsSame2(constref aRec: TTest): Boolean;
  12.   end;
  13.  
  14. function TTest.IsSame(const aRec: TTest): Boolean;
  15. begin
  16.   WriteLn('IsSame:');
  17.   WriteLn('@aRec = ', Format('$%p', [@aRec]));
  18.   WriteLn('@Self = ', Format('$%p', [@Self]));
  19.   if @aRec = @Self then
  20.     exit(True);
  21.   Result := False;
  22. end;
  23.  
  24. function TTest.IsSame2(constref aRec: TTest): Boolean;
  25. begin
  26.   WriteLn('IsSame2:');
  27.   WriteLn('@aRec = ', Format('$%p', [@aRec]));
  28.   WriteLn('@Self = ', Format('$%p', [@Self]));
  29.   if @aRec = @Self then
  30.     exit(True);
  31.   Result := False;
  32. end;
  33.  
  34. var
  35.   t: TTest;
  36.  
  37. begin
  38.   t.Data := [1, 2, 3, 4];
  39.   WriteLn(t.IsSame(t));
  40.   WriteLn(t.IsSame2(t));
  41. end.
  42.  
gives an interesting result for i386_win-32(fpc-3.3.1r46955):
Code: Text  [Select][+][-]
  1. IsSame:
  2. @aRec = $0141FF48
  3. @Self = $00419010
  4. FALSE
  5. IsSame2:
  6. @aRec = $00419010
  7. @Self = $00419010
  8. TRUE
  9.  

What am I missing?
« Last Edit: September 30, 2020, 07:03:38 pm by avk »

Martin_fr

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Re: Advanced records as function parameters
« Reply #1 on: September 30, 2020, 07:10:42 pm »
If you pass a record (advanced or not) as parameter, then this is passing the actual data => you hand over a copy.

Records (unlike classes) are not passed by reference.
Though apparently advanced record in "self" is passed by ref (and needs to be => it is a "var param")

"const arec: TTEst"
allows the compiler to choose between copy or ref. Apparently for the specific record a copy was passed.

"constref" forces a ref. (same as "var param")

WooBean

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Re: Advanced records as function parameters
« Reply #2 on: September 30, 2020, 07:13:04 pm »
I think the answer is here: https://www.freepascal.org/docs-html/ref/refsu66.html

It is not limited to advanced records or a particular target platform.

WB
Win7/64, Lazarus 1.8 win64-win64, FPC 3.0.4

avk

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Re: Advanced records as function parameters
« Reply #3 on: September 30, 2020, 08:27:10 pm »
Thanks to everyone who answered, the question was pretty silly. Hence the simple conclusion: at night a person should sleep.

 

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