Thread variable lost / changed after passing thread object to another class? - multithreading

Question

I wrote a little program that shows my problem:
unit Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private-Deklarationen }
public
{ Public-Deklarationen }
end;
TThread_A = class(TThread)
private
stupidvariable : integer;
protected
procedure Execute; override;
public
property getstupidvar : integer read stupidvariable;
constructor Create;
end;
TSomeClass = class
private
m_Obj : ^TThread_A;
procedure readVar;
public
constructor Create(obj: TThread_A);
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
constructor TSomeClass.Create(obj: TThread_A);
begin
m_Obj := #obj;
readVar;
end;
procedure TSomeClass.readVar;
begin
showmessage(inttostr(m_Obj.getstupidvar));
end;
constructor TThread_A.Create;
begin
inherited Create(false);
FreeOnTerminate := True;
end;
procedure TThread_A.Execute;
begin
stupidvariable := 100;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
threadA : TThread_A;
someClass : TSomeClass;
begin
threadA := TThread_A.Create;
someClass := TSomeClass.Create(threadA);
end;
end.
What happens here exactly?
I thought I am passing the object "threadA" to someClass and assign the address of "threadA" to "m_Obj".
Why is the object lost?

Answer 1

constructor TSomeClass.Create(obj: TThread_A);
begin
m_Obj := #obj;
readVar;
end;
Here obj is (in essence) a local variable, and so its lifetime ends when the function returns. Therefore you have remembered the address of something that no longer exists.
In fact you have too much indirection. Because TThread_A is a class, and classes are reference types, it is already a pointer. Change
m_Obj: ^TThread_A;
to
m_Obj: TThread_A;
and
m_Obj := #obj;
to
m_Obj := obj;
Now you are taking a copy of the reference to the instance, which is what I believe that you mean to do.
Not even that will leave you with a working program though. Because you set FreeOnTerminate the thread can be destroyed at any time. That means that you must not hold a reference to it since that reference can become invalid behind your back. So, you should also set FreeOnTerminate to False.
Not even that will leave you with predicatable outcome though. The thread procedure executes independently from the main thread. When you read the variable from the main thread, the thread may, or may not, have modified the variable. This is known as a data race. If you wish to wait until after the variable has been modified then you could use, for instance, an event object to allow the thread to signal that the variable is ready to be read.