TStream warning in delphi

3
votes

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.

1
delphiinitializationtstream
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

1 Answers

10
votes

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;