Variable is passed by reference instead of value - Why?
lazarus / 21-General

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bgravity · 2022-11-08 21:26

Big picture: I am using a stringgrid for data entry. I want to
highlight cells with invalid entries. I am using the solution proposed
by TLama on stackoverflow here: 
https://stackoverflow.com/questions/9684266/how-to-toggle-the-cell-
colour-and-text-in-a-tstringgrid-on-and-off

Details: In addition to highlighting errors I need to reset the cell
color to default colors each time the user edits the cell so that when
the user corrects the error the cell returns to the default color. I
am using two variables to hold the cell colors, a form variable that
holds the default values and a local procedure variable that holds the
value for the current cell.
I am having two issues:
1. Changes to the local variable are affecting the form variable
2. I get an access violation when I free the memory for the form
variable.

I have tracked the problem to the fact that when I assigned the local
variable the default values, instead of copying the values from the
default to the local variable, the two variables are assigned the same
memory, meaning they are really just one variable not two.

My questions:
1. Is there a better way to change the stringgrid cell background than
the stackoverflow solution?
2. Why are acelldata and fdefaultcelldata set to the same memory
location instead of copying the values from default to acelldata. If I
use a record, I get the behavior I expect but I don't think I can
store a record in Tstringgrid.object.
3. Is there a way to get the behavior I expect?

Thanks in advance

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 unit
Unit1; {$mode objfpc}{$H+} interface uses  Classes, SysUtils, Forms,
Controls, Graphics, Dialogs, StdCtrls, celldata;  type  stcelldata =
record    validInput:Boolean;    foregroundColor:TColor;   
backgroundColor:Tcolor;  end;   { TForm1 }   TForm1 = class(TForm)   
Button1: TButton;    procedure Button1Click(Sender: TObject);   
procedure FormCreate(Sender: TObject);    procedure
FormDestroy(Sender: TObject);  private    fdefaultCelldata:TCellData; 
  fstdefaultCellData:stcelldata;  public   end; var  Form1:
TForm1; implementation {$R *.lfm} { TForm1 } procedure
TForm1.FormCreate(Sender: TObject);begin 
fdefaultCelldata:=TCellData.create(True,clBlack,clWindow); 
fdefaultTime:=2;  fstdefaultCellData.backgroundColor:=clWindow; 
fstdefaultCellData.foregroundColor:=clBlack; 
fstdefaultCellData.validInput:=True;end; procedure
TForm1.Button1Click(Sender: TObject);var  acelldata:Tcelldata; 
astcelldata:stcelldata;begin  acelldata :=
TCellData.create(True,clBlack,clWindow);  acelldata
:=fdefaultcelldata;                                          // This
assigns the memory of acelldata to the same location as
fdefaultcelldata  astcelldata := fstdefaultCelldata;                 
               // This behaves as I expect, copying the default values
to astcelldata  try    acelldata.ValidInput:=false;   
astcelldata.validInput:=false;  finally    acelldata.free;           
                                             // This call to acelldata
works  end;end; procedure TForm1.FormDestroy(Sender: TObject);begin 
fdefaultCelldata.Free;                                               
    // This call to free causes an access violationend;end.   unit
celldata; {$mode ObjFPC}{$H+} interface uses  Classes, SysUtils,
Forms, Controls, Graphics, Dialogs,
StdCtrls; type TCellData=class(TObject)private  fValidInput:Boolean; 
fForeground:TColor;  fBackground:TColor;public  property
ValidInput:Boolean read fValidInput write fValidInput;  property
ForegroundColor:TColor read fForeground write fForeground;  property
BackgroundColor:TColor read fBackground write fBackground;  
Constructor Create(Valid: Boolean; fg,
bg:TColor);end;  implementation { TCellData } constructor
TCellData.Create(Valid: Boolean; fg, bg:TColor);begin 
ValidInput:=Valid;  ForegroundColor:=fg; 
BackgroundColor:=bg;end; end.

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Leledumbo · 2022-11-08 21:54

Quote from: bgravity on November 08, 2022, 10:26:31 pm1. Is there a
better way to change the stringgrid cell background than the
stackoverflow solution?
Better is subjective. I might use an external in-memory matrix instead
of storing extended cell data that contains colors and such into the
grid's objects property, but that's just me. Basically I try to keep
actual data from rendering data for maintainability reason.
Quote from: bgravity on November 08, 2022, 10:26:31 pm2. Why are
acelldata and fdefaultcelldata set to the same memory location instead
of copying the values from default to acelldata. If I use a record, I
get the behavior I expect but I don't think I can store a record in
Tstringgrid.object.
TCellData is a class, therefore when you do:
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 x :=
TCellData.Create; it creates the instance somewhere on the heap then
makes x points to that instance. x holds the reference to the object,
not the object itself. So when you do:
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 y := x; you
make y and x both point at the same object, not making y a copy of
(the object) x (is pointing at).
Quote from: bgravity on November 08, 2022, 10:26:31 pm3. Is there a
way to get the behavior I expect?
Yes, make a deep copy method in TCellData. You might remember some
types, typically those that descend from TPersistent, have Assign
method that accept its own type (or a TPersistent) as the sole
parameter. Inside, it selectively copies the properties to make a
clone. Here's how TStrings implement its Assign method.