What are the uses of freeandnil

• Nov 16, 2004

• #2

towerbase

Programmer

Jul 31, 2002

1,053

GB

FreeAndNil properly disposes of a (populated) TStringList.

The merit of using FreeAndNil is that you can use IsAssigned on the object reference.

In the following code example, by calling FreeAndNil (test) instead of test.Free, IsAssigned (test) gives the expected answer of FALSE. However, if test.Free was called instead of FreeAndNil then IsAssigned (test) gives the unexpected answer of TRUE and an Access Violation occurs in the ShowMessage.

Code:

procedure TForm1.Button1Click(Sender: TObject);
var
  test: TStringList;
begin
  test := TStringList.Create;
  try
    test.Add( '1' );
    test.Add( '2' );
  finally
    FreeAndNil ( test );
  end;
  if Assigned ( test ) then
    ShowMessage ( test[1] );
end;

Andrew
Hampshire, UK

 

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• Nov 16, 2004

• #3

hilfy

Active member

Oct 31, 2003

2,564

US

In general, I prefer using FreeAndNil to .Free. Before D4, if there was any chance that you would access an object after it had been freed, you also had to explicitly set it to NIL.

Code:

Item.Free;
Item := NIL;

If you didn't do this, you could not tell if the object was already instantiated or not so you risked an access violation error if you tried to access the object after it had been freed. With FreeAndNil, you don't have to remember to do this in your code because it's handled for you.

Yes, it is enough to dispose of a TStringList unless you have used the objects property of the string list to store something. In that case, you have to iterate through the string list and destroy each object and then use FreeAndNil on the string list.

-Dell
-Dell

 

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• Nov 16, 2004

• #4

Stretchwickster

Programmer

Apr 30, 2001

1,746

GB

I concur with Andrew, and use FreeAndNil in conjunction with the Assigned method. By the way, there is no IsAssigned method. I think Andrew has just muddled up the name as he refers to Assigned in his code snippet.

Clive
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"To err is human, but to really foul things up you need a computer."
Paul Ehrlich
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To get the best answers from this forum see: faq102-5096

 

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• Nov 16, 2004

• #5

towerbase

Programmer

Jul 31, 2002

1,053

GB

You're right Clive. There is no IsAssigned method.



Andrew IsMuddled
Hampshire, UK

 

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• Nov 16, 2004

• #6

Stretchwickster

Programmer

Apr 30, 2001

1,746

GB

So you've been lying to me about your surname!!! ;-)

Clive
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"To err is human, but to really foul things up you need a computer."
Paul Ehrlich
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To get the best answers from this forum see: faq102-5096

 

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