Advanced records as function parameters
lazarus / 62-General

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avk · 2020-09-30 16:53

Hi!

A simple test
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
adv_rec_test;{$mode objfpc}{$H+}{$MODESWITCH ADVANCEDRECORDS}uses 
SysUtils;type  TIntArray = array of Integer;  TTest = record    Data:
TIntArray;    function IsSame(const aRec: TTest): Boolean;    function
IsSame2(constref aRec: TTest): Boolean;  end; function
TTest.IsSame(const aRec: TTest): Boolean;begin  WriteLn('IsSame:'); 
WriteLn('@aRec = ', Format('$%p', [@aRec]));  WriteLn('@Self = ',
Format('$%p', [@Self]));  if @aRec = @Self then    exit(True);  Result
:= False;end; function TTest.IsSame2(constref aRec: TTest):
Boolean;begin  WriteLn('IsSame2:');  WriteLn('@aRec = ', Format('$%p',
[@aRec]));  WriteLn('@Self = ', Format('$%p', [@Self]));  if @aRec =
@Self then    exit(True);  Result := False;end; var  t: TTest; begin 
t.Data := [1, 2, 3, 4];  WriteLn(t.IsSame(t)); 
WriteLn(t.IsSame2(t));end.  gives an interesting result for
i386_win-32(fpc-3.3.1r46955):
Code: Text  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
IsSame:@aRec = $0141FF48@Self = $00419010FALSEIsSame2:@aRec =
$00419010@Self = $00419010TRUE 
What am I missing?

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Martin_fr · 2020-09-30 17:10

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")

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WooBean · 2020-09-30 17:13

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

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avk · 2020-09-30 18:27

Thanks to everyone who answered, the question was pretty silly. Hence
the simple conclusion: at night a person should sleep.