TStream warning in delphi

Question

I have the following code snippet

 Procedure TFrm.Retrieve(mystring : string);
  var 
   bs : TStream;
   ...
  begin
    ...
    bs:=nil;
    //bs:= TStream.create; 
    try
     bs := CreateBlobStream(FieldByName('Picture'), bmRead);
    finally
     bs.Free;
    end;
  ... 
  end;

I have a problem understanding the initialisation of bs variable.

If I dont initialize it , a but obvious warning i get.

 Variable 'bs' might not have been initialized.

Now if I do it as the commented part i.e.

 bs:= TStream.create;

I get the following warning.

Constructing instance of 'TStream' containing abstract method 'TStream.Read'
Constructing instance of 'TStream' containing abstract method 'TStream.Write'

and finally it works totally fine if I use

 bs:=nil;

Am I Doing it correct by assigning it to Nil?

Any views appreciated.

This hint meant to inform you what if exception will happen at line 10, then `bs` variable will be still uninitialized at line 12. Since Delphi compiler provides no means for initialization of local variable such flow will cause an exception. So, you decision to initialize to nil is perfect. — OnTheFly, Dec 04 '11 at 18:28

score 10 · Accepted Answer · edited Dec 01 '11 at 13:41

10

TStream is abstract so you shouldn't instantiate it (calling an abstract method causes a runtime error). Instead, you should instantiate a non-abstract descendant. When you're done you should Free the instance.

For example:

var
  Stream: TStream;
begin
  try
    Stream := CreateBlobStream(FieldByName('Picture'), bmRead);
    try
      // ...
    finally
      Stream.Free;
    end;
  except 
    // handle exceptions
  end;
end;

edited Dec 01 '11 at 13:41

gabr

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answered Dec 01 '11 at 13:24

Ondrej Kelle

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I want to have it inside the `try` block for handling exceptions if any. How do I go about it? – Shirish11 Dec 01 '11 at 13:28
2

As I've shown you. The pattern is `try``finally``end;` – Ondrej Kelle Dec 01 '11 at 13:32
If you want to handle exceptions, wrap everything in try..except. I've updated TOndrej's answer to demonstrate this. – gabr Dec 01 '11 at 13:41
1

@Shirish11 To handle exceptions, see @gabr's edit. Put your handling code in the `except`..`end;` block as the comment suggests. – Ondrej Kelle Dec 01 '11 at 13:46

TStream warning in delphi

1 Answers1