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Results.Assign crashes. Why?

 
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David Reid
Guest
PostPosted: Fri Nov 21, 2003 1:05 am    Post subject: Results.Assign crashes. Why? Reply with quote


Hello,
Does anybody know why the following code crashes on the line
"Results.Assign(myFont)" in the GetFont function? Please see the following
code.

Thank-you,
David Reid


<< Code Start >>
function TForm1.GetFont:TFont;
var
myFont:TFont;
begin
myFont := TFont.Create;
try
myFont.Size := 10;
Result.Assign(myFont);
finally
FreeAndNil(myFont);
end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
Label1.Font.Assign(GetFont);
end;
<< Code End >>


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Bryan Crotaz
Guest
PostPosted: Fri Nov 21, 2003 1:05 am    Post subject: Re: Results.Assign crashes. Why? Reply with quote


Because you haven't created the result font object.

try this:

function TForm1.GetFont:TFont;
begin
Result := TFont.Create;
try
Result.Size := 10;
except
on Exception do
begin
Result.Free;
Raise;
end;
end;
end;


"David Reid" <dreid (AT) racltd (DOT) com> wrote

Quote:
Hello,
Does anybody know why the following code crashes on the line
"Results.Assign(myFont)" in the GetFont function? Please see the
following
code.

Thank-you,
David Reid


Code Start
function TForm1.GetFont:TFont;
var
myFont:TFont;
begin
myFont := TFont.Create;
try
myFont.Size := 10;
Result.Assign(myFont);
finally
FreeAndNil(myFont);
end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
Label1.Font.Assign(GetFont);
end;
Code End





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Harley Pebley
Guest
PostPosted: Fri Nov 21, 2003 1:11 am    Post subject: Re: Results.Assign crashes. Why? Reply with quote


Quote:
Does anybody know why the following code crashes on the line
"Results.Assign(myFont)" in the GetFont function?

Yes.

Quote:
Please see the following code.

Code Start
function TForm1.GetFont:TFont;
var
myFont:TFont;
begin
myFont := TFont.Create;
try
myFont.Size := 10;
Result.Assign(myFont);
finally
FreeAndNil(myFont);
end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
Label1.Font.Assign(GetFont);
end;
Code End

I assume you want an answer as to why. <g>

Result does not point to an object. You need to assign an object to it
before calling Assign. I assume you want to put a Font property on an
object. Generally it's done like so:

TMyObject = class(TObject)
private
fFont: TFont;
protected
procedure SetFont(const aValue: TFont);
function GetFont: TFont;
public
procedure AfterConstruction; override;
procedure BeforeDestruction; override;
property Font: TFont read GetFont write SetFont;
end;

procedure TMyObject.AfterConstruction;
begin
inherited;
fFont := TFont.Create;
end;

procedure TMyObject.BeforeDestruction;
begin
fFont.Free;
inherited;
end;

procedure TMyObject.SetFont(const aValue: TFont);
begin
fFont.Assign(aValue);
end;

function TMyObject.GetFont: TFont;
begin
result := fFont;
end;

If you want to set default values, like the point size, that would be done
in the AfterConstruction method. If you have a method called GetFont that
creates a TFont and assigns it to result (i.e. result := TFont.Create),
then it will leak memory when used as in Button1Click.

HTH,
Harley Pebley

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Joanna Carter (TeamB)
Guest
PostPosted: Fri Nov 21, 2003 7:58 am    Post subject: Re: Results.Assign crashes. Why? Reply with quote
Harley Pebley wrote:

Quote:
If you want to set default values, like the point size, that would be
done in the AfterConstruction method. If you have a method called
GetFont that creates a TFont and assigns it to result (i.e. result :=
TFont.Create), then it will leak memory when used as in Button1Click.

Harley, unfortunately, your explanation does not answer David's question; he
was asking about returning an object from a function.

Also you do not need to override the AfterConstruction and BeforeDestruction
methods to do what you demonstrated, the constructor and destructor are
there for that kind of thing and it is much more normal to override them.

:-)

Joanna

--
Joanna Carter (TeamB)

Consultant Software Engineer
TeamBUG support for UK-BUG
TeamMM support for ModelMaker



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Harley Pebley
Guest
PostPosted: Fri Nov 21, 2003 7:27 pm    Post subject: Re: Results.Assign crashes. Why? Reply with quote
Quote:
| If you want to set default values, like the point size, that would be
| done in the AfterConstruction method. If you have a method called
| GetFont that creates a TFont and assigns it to result (i.e. result :=
| TFont.Create), then it will leak memory when used as in Button1Click.

Harley, unfortunately, your explanation does not answer David's question; he
was asking about returning an object from a function.

Oh, I think my explanation did answer his question. But it wasn't the quote
above. ;-)

Quote:
Also you do not need to override the AfterConstruction and BeforeDestruction
methods to do what you demonstrated, the constructor and destructor are
there for that kind of thing and it is much more normal to override them.

Hmm, normal describes such a broad range. <g> Of course one can implement
constructors and destructors which do this, but, if they're static they
don't work too well with factories. AfterConstruction and BeforeDestruction
work in both cases. (I know you know all this, this is for the lurkers. :-)

Cheers,
Harley Pebley

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Martin James
Guest
PostPosted: Fri Nov 21, 2003 7:46 pm    Post subject: Re: Results.Assign crashes. Why? Reply with quote

AfterConstruction and BeforeDestruction
Quote:
work in both cases. (I know you know all this, this is for the lurkers.
:-)


My dog shows no interest in returning objects from functions.

Martin




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Bob Dawson
Guest
PostPosted: Fri Nov 21, 2003 9:56 pm    Post subject: Re: Results.Assign crashes. Why? Reply with quote
"Martin James" wrote
Quote:

My dog shows no interest in returning objects from functions.

I take it your dog's not into factory patterns.

bobD



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Joanna Carter (TeamB)
Guest
PostPosted: Fri Nov 21, 2003 10:02 pm    Post subject: Re: Results.Assign crashes. Why? Reply with quote
Bob Dawson wrote:

Quote:
I take it your dog's not into factory patterns.

But I bet he has a fascination for trees :-)

Joanna

--
Joanna Carter (TeamB)

Consultant Software Engineer
TeamBUG support for UK-BUG
TeamMM support for ModelMaker



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Bryan Crotaz
Guest
PostPosted: Sat Nov 22, 2003 12:12 am    Post subject: Re: Results.Assign crashes. Why? Reply with quote
Quote:

AfterConstruction and BeforeDestruction
work in both cases. (I know you know all this, this is for the lurkers.
:-)


My dog shows no interest in returning objects from functions.

That's lurchers you're a-thinkin' of.

Bryan



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Woody (TMW)
Guest
PostPosted: Sat Nov 22, 2003 11:04 pm    Post subject: Re: Results.Assign crashes. Why? Reply with quote
"Bryan Crotaz" <bryan@%nospam%ivisionsystems.com> wrote

Quote:

AfterConstruction and BeforeDestruction
work in both cases. (I know you know all this, this is for the
lurkers.
:-)


My dog shows no interest in returning objects from functions.

That's lurchers you're a-thinkin' of.

"You rang"

Lurch - Addams Family


--
Woody (TMW)
Freeware Page: http://users.eonet.net/woodytmw

All computers wait at the same speed.



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