Brad White wrote:

> 
> I'm open to being persuaded, but the only example
> I've seen was Allen's.  While he did fine job demonstrating
> some very cool things that you could do with generics
> and an anonymous procedure, it made the code much
> less readable, IMHO.
> 

+1. Entirely.

> Do you have an example where it would increase redability
> or quality?
> 
> Thanks,
> Brad.

In a perfect world it would be something like:


function GetUserName: string;
begin
	Result:='';


	using temp:=TStringList.Create do
	begin
		temp.Add('Please be so kind');
		temp.Add('in order to enter');
		temp.Add('your beloved name');
		Result:=InputBox(temp.Text, '');
	end;


	if Result='' then //some bogus processing
	begin
		ShowMessage('You did not enter a name');
		Result:='guest'; //whatever
	end;
end;


There are some very interesting things to note however:


1. The above construction resembles (from *user* POV) to 'with' aliasing 
which is (AFAIS from QC and from here) the community's preferred 
solution for the 'Infamous "with" resolution'. IOW something like:


//for ex. some very big name here
with tbl:=MainCustomerDataModule.ProblematicOrderDataSource.Dataset do
begin
	tbl.SQL:='SELECT * FROM ORDERS WHERE STATUS=8'; //whatever
	tbl.Open;
end;


- or -


with temp:=TStringList.Create do
begin
	temp.Add('blah!'); //do something
	ShowMessage(temp.text);
	temp.Free;	//<---!
end;


As you see the main difference between 'with' aliasing and 'using' is 
that 'with' doesn't manage the lifetime of the aliased object.


In the 'using' case this gives the (at least theoretical) the 
possibility for some checks for the compiler such as:


var
	nastyRef: TStringList; //global


procedure Foo;
begin
   using temp:=TStringList.Create do
   begin
	temp.Add('blah!'); //do something
	nastyRef:=temp; //Compiler warning like: 'nastyRef is outside'
	ShowMessage(temp.text);	
   end;
   //temp destroyed here. So nastyRef is dangling.
end;


And this cannot be checked at the compile time neither in the case of 
'with' aliasing neither in the classical case of today which is 
try...finally block.


- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


2. Another thing which perhaps you (as experienced Delphi developer) 
cannot see at the first glance is that


   using temp:=TStringList.Create do
   begin
	temp.Add('blah!'); //do something
//snip


gives a clear indication that the compiler will take care of the 
lifetime of temp and you should only concentrate on your coding (human) 
problem to solve.


Ok, perhaps if you'll see the code...


   temp:=TStringList.Create;
   try
     ...
//snip


....you will automatically search for a temp.Free (or similar) in a 
'finally' block. But this can be *your* habit and also can be a little 
bit tedious if the finally block is outside of screen. And also, the 
temp.free in the 'finally' block, being more manual is prone to error 
(you have to accomplish more steps to build a try/finally block). Using 
'using' is much safer.


- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


3. Also, a nested try/finally is (imho) much more less readable. Compare 
the following equivalent snippets:


MyComp := TComponent.Create(Self);
try
   MyComp.Name:='FooComponent';
   MyComp.Tag:=1; //and many other things...
   myLabel := TLabel.Create(Self);
   try
     myLabel.Parent:=Self;
     myLabel.Caption:=MyComp.Name;
     myLabel.Color:=clMagenta;
     //etc. etc.
     ShowModal;
   finally
     myLabel.Free;
   end;
finally
   MyComp.Free;
end;


---8<------8<------8<--- vs ---8<------8<------8<---




using MyComp := TComponent.Create(Self) do
begin
   MyComp.Name:='FooComponent';
   MyComp.Tag:=1; //and many other things...
   using myLabel := TLabel.Create(Self) do
   begin
     myLabel.Parent:=Self;
     myLabel.Caption:=MyComp.Name;
     myLabel.Color:=clMagenta;
     //etc. etc.
     ShowModal;
   end;
end;


....and I didn't put a 3rd level of nesting, neither a try..exception 
block there... :-)


Did I convinced you? :-)


-- 


m. Th.


On the Wings of the Wind...
http://wings-of-wind.com/