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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

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,


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