[boost] Re: boost::optional feature request.

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