From: Dave Gomboc (dave_at_[hidden])
Date: 2003-03-25 22:01:23
> It would be nice if boost::optional<T> had operator< defined whenever
> operator< was defined for T. This would allow us to use optional<T>
> as the key of an associative container. I suggest the following
> semantics:
>
> bool operator<(optional<T> const &x, optional<T> const &y);
>
> Returns: If y is uninitialized, false. If y is initialized and x is
> uninitialized, true. If x and y are both initialized, (*x < *y).
>
> This results in a strict weak ordering with uninitialized optional<T>
> objects being sorted first.
Yes, I previously implemented this functionality -- exactly in the manner
you described -- in my nilable<T> implementaton. It's quite useful to
have.
(If anyone is curious, nilable<T> was inspired by optional<T>, though the
implementation is not derived from optional's. The main differences are
that nilable<T> doesn't use pointer deferencing for access to the internal
value. Instead, it provides a direct, checked conversion to T (using
boost::throw_exception with a bad cast error when the object obj is nil),
obj.nil() returning bool, and obj.unsafe_reference() and
obj.unsafe_value() for when one has previously ensured obj.nil() is false
and want to make further accesses without that being tested repeatedly.)
Dave