Hi all !

-
Hi all !

I can't find an answer to the following question :
Is there an interest to use var in an object declaration as parameter :

Procedure GetResults(pID : Integer ; var pResults : TList);

vs

Procedure GetResults(pID : Integer ; pResults : TList);

as objects are passed by ref anyway ? and does const has a meaning in object declarations ?

Comments

  1. Dorin DuminicaApril 25, 2013 at 1:04 AM

    if you had const pResults:... then you can only call methods on pResults, not assign to it(i.e. pResults := TList.Create).
    In your case, using or not using var shouldn't be any difference since it's a pointer behind the scenes.

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  2. Olivier SCHWABApril 25, 2013 at 1:07 AM

    OK, so IIUC then putting const in all objects declaration as parameter is a
    good practice, as assigning a parameter object inside a function is not
    recommended ?

    ReplyDelete
  3. Dorin DuminicaApril 25, 2013 at 1:11 AM

    usually it is, because the compiler can do much more aggresive optimisations when it knows that the caller won't be able to do nasty stuff.

    as for assigning an instance to a parameter... this is very debatable at religious scale depending on who you ask, personally, I go for what works for me, but usually, it's a good practice to create and free objects in the same method if possible.

    ReplyDelete
  4. Martin WienoldApril 25, 2013 at 9:36 AM

    const can be funny:

    Using const for managed arguments is ok.
    Using const for Objects is ok (at least pre ARC)
    Using const for Interfaces can be tricky.
    Example:
    procedure DoSomething(const AIntf: IInterface);
    begin
      if Assigned(AIntf) then;
    end;

    If you just pass a variable to DoSomething, everything is fine. If you do something like this: DoSomething(TInterfacedObject.Create()) you are facing a memleak.

    Post ARC it should be the same for Objects.

    ReplyDelete
  5. François PietteApril 27, 2013 at 3:25 AM

    Olivier SCHWAB Use var for an object parameter when the method actually create an object and has to return that object reference.

    ReplyDelete

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