Re: [boost] [optional] problems with swap()

From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-04-12 08:31:47

Thorsten Ottosen wrote in his OP:
> I have some comments on the way optional<T>::swap()

Hmmm... I can't find optional<T>::swap in my copy of optional.hpp
(straight from the trunk)!  It looks like optional<T> only has a
non-member boost::swap function!  I think it should definitely have a
swap member function, as well!  As a guideline, whenever providing a
non-member swap function for a class type, one should provide a swap
member function as well!  (E.g., Scott Meyers, Effective C++ Third
Edition, item 25!).  Why?  Because the STL does it that way, /and/
because it allows swapping a temporary.

Of course, optional<T>::swap could be implemented very easily:

    void swap( optional & arg )
    {
      using std::swap;
      swap(*this, arg);
    }

By default, it would call the original boost::swap(optional<T>&,
optional<T>&), by appying ADL.  But it would also allow end users to
customize the behavior of optional<T>::swap for their specific types,
either by providing a template specialization of boost::swap or
std::swap, or by adding an overloaded swap function to the namespace of
T.

Thorsten Ottosen wrote:
> I can't imagine that it would result in lower performance in general ...
> have you seen many types for which default construction was more
> expensive than copy-construction?

Clearly if T has a nothrow default-constructor, it would make sense to
use it, when swapping optional<T>.  But otherwise it's hard to say "in
general" what swap implementation is best for any optional<T>.  In that
case the user could still provide her own custom swap function for
optional<T>, for her particular type T, as I just described.  Wouldn't
that solve the issue?

Kind regards,

Niels