Delphi panics when returning a custom class in a function?

[jamesp0tter]

I have defined a custom class:

type
  TWhtv = class
    one: string;
    two: string;
  end;

and the following code

function abc: TWhtv;
var
  u: TWhtv;
begin
  u.one := 'first';
  u.two := 'second';
  result := u;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  o: TWhtv;         
  s: string;
begin
  o := abc;
  s := o.one + o.two;
  caption := s;
end;

When I execute this code, caption changes to 'firstsecond' as expected, but when I exit the application it panics with an Access Violation!

I tried only with

procedure TForm1.Button1Click(Sender: TObject);
var
  o: TWhtv;         
  s: string;
begin
  o := abc;
  s := o.one;
  caption := s;
end;

where caption will only become 'first' (asking only one of the variables of TWhtv), and the caption changes accordingly and NO access violation!

So, Delphi panics when I ask for more than one var of the returned class? Or am I doing something wrong?

Thanks!

PS: using Delphi 7

jamesp0tter

 

[Zathras]

My Delphi is getting rusty, but shouldn't you be using something like

function abc: TWhtv;
var
  u: TWhtv;
begin
  [b][COLOR=blue]u := TWhtv.Create[/color][/b];
  u.one := 'first';
  u.two := 'second';
  result := u;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  o: TWhtv;         
  s: string;
begin
  o := abc;
  s := o.one + o.two;
  caption := s;
  [b][COLOR=blue]o.Free;[/color][/b]
end;

Personally, I don't like creating things in functions that have to be destroyed elsewhere. I would do something more like this:

Procedure abc (var u:TWhtv);
begin
  u.one := 'first';
  u.two := 'second';
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  o: TWhtv;         
begin
  o := TWhtv.Create;
  abc(o);
  caption := o.one + o.two;
  o.Free;
end;

Looks a bit cleaner, doesn't it?

 

[whosrdaddy]

whenever you are dealing with classes, you need to call the constructor.

so let's take your example :

type
  TWhtv = class(TObject) //this not really necessary, but it is more explicit
  private 
    one: string;
    two: string;
  public 
    construtor Create; 
  end;

implementation
// class implementation

constructor TWhtv.Create;
begin
 // use the constructor to init your class vars
 One:='';
 Two:='';
end;

correct class utilization:

CODE
function abc: TWhtv;
begin
 Result:=TWhtv.Create;// very important
 Result.one := 'first';
 Result.two := 'second';
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  o: TWhtv;         
  s: string;
begin
  o := abc;
  s := o.one + o.two;
  caption := s;
  FreeAndNil(o); // don't forget to deallocate self created objects
end;

Hope this helps,
Daddy

-----------------------------------------------------
What You See Is What You Get
Never underestimate tha powah of tha google!

 

[whosrdaddy]

Damn you zathras!

I guess I need to speed up my typing



-----------------------------------------------------
What You See Is What You Get
Never underestimate tha powah of tha google!

 

[Zathras]

I usually like to test my code before posting here, but in this case I didn't bother.

BTW, do you agree with me that it is preferable to keep the construction and destruction of objects close together. I realize that the function is just returning a pointer to the memory where the class resides and will work just fine, but somehow it just feels wrong to me.

 

[whosrdaddy]

yeah, I do agree.
I prefer creating the class, then try to do something with the class and finally free the class. with this workmethod I never have memoryleaks. if this is not possible, I use objectlists.

/Daddy

-----------------------------------------------------
What You See Is What You Get
Never underestimate tha powah of tha google!

 

[jamesp0tter]

Nice, thanks, that didn't occur to me

jamesp0tter