create a new field inside a tmp<class> member function
 

Hello,
I want to create a new field inside tmp<volVectorField> class::func()
assign it to existing vector field, do some operations on it end return the result.

So far, I was unable to do this, or figure out how to do it. I can create a new field, and assign a value. And after that I can only return it, and not access it again. example:
tmp<class> func() const {
return (
new class("Name", a + b)
);
}

Where I would like to do some other operations on Name before I return it.
Is that a silly thing to do?
Thanks for reading