[Trilinos-Users] Teuchos::RCP Call by reference

[Hoemmen, Mark]

On Aug 8, 2013, at 12:00 PM, <trilinos-users-request at software.sandia.gov>
 <trilinos-users-request at software.sandia.gov> wrote:
> Message: 1
> Date: Fri, 9 Aug 2013 00:38:34 +0900
> From: "Sunghwan Choi" <iomn159753 at kaist.ac.kr>
> Subject: [Trilinos-Users] Teucho::RCP Call by reference
> To: trilinos-users at software.sandia.gov
> Message-ID: <1b9a01ce944d$58b45910$0a1d0b30$@kaist.ac.kr>
> Content-Type: text/plain; charset="us-ascii"
> 
> Dear all,
> 
> Hi I met a weird situation. I computed scaling of Epetra_MultiVector in a
> function and I got it through 'call by reference' I check the Scale does do
> its work but outside of function the value was not changed. I wrote pseudo
> code 
> 
> 
> 
> int func(Teuchos::RCP<Epetra_MultiVector>
> before_values,Teuchos::RCP<Epetra_MultiVector> after_values){
> 
> after_values = Teuchos::rcp(new Epetra_MultiVector(*before_values) );
> 
>            double scaling = 0.5;
> 
> cout <<"before values" <<endl;
> 
>            cout << *after_values <<endl;
> 
>            after_values->Scale(1.0/scaling) ;
> 
>            cout <<"after values" <<endl;
> 
>            cout << *after_values<<endl;
> 
>            return;
> 
> }
> 
> 
> 
> This is super simple code but I don't know why it is not working. In other
> part of my code, I used call-by-reference using Teuchos::RCP but this is not
> working. Do you have any idea on this situation? 

This is not "call by reference"; this is "call by pointer."  Imagine replacing the RCP with a raw pointer, like this:

int 
func (Epetra_MultiVector* before_values, 
      Epetra_MultiVector* after_values)
{
  after_values = new Epetra_MultiVector (*before_values);
  double scaling = 0.5;
  after_values->Scale (1.0 / scaling);
  return 0; // your function returns int
}

This function would have the same problem as your function above (in addition to the problem of leaking memory).  (The only difference between an RCP and a raw pointer is that RCP handles deallocation safely.)  If you want to change the _pointer_, you either need to pass in the RCP _itself_ by nonconst reference ("RCP<Epetra_MultiVector>&"), or you need to return the RCP.  I recommend the latter.  I also recommend passing before_values by const reference, since the function doesn't need to keep it:

RCP<Epetra_MultiVector>
func (const Epetra_MultiVector& before_values)
{
  RCP<Epetra_MultiVector> after_values = rcp (new Epetra_MultiVector (before_values));
  double scaling = 0.5;
  after_values->Scale (1.0 / scaling);
  return after_values;
}

mfh