[boost] Re: Optional, tie, and iterator_adaptor

From: David Abrahams (dave_at_[hidden])
Date: 2003-08-29 13:56:07

Mat Marcus <mmarcus-boost_at_[hidden]> writes:

> ### Question 2
>
> In another case I am trying to use optional with iterator_adaptor
> (1.30.x version). 

Whoa; don't do that ;->
It'll hurt (comparitively speaking).

> Here I would like the "base" for my iterator_adaptor to be, say,
> boost::optional<Baz>. The value_type is to be Baz. But
> iterator_adaptor chooses the signature of the eventual iterator
> class for me. 

What do you mean by "the signature of the... class"?

> As a result, clients of my iterator have to pass
> ::boost::optional<Baz>'s to the constructor. But I would also like
> them to be able to pass plain Baz's. 
> That is, I want a non-explicit constructor for optional. 

No, you want the ability to add your own constructor to adapted
iterators.  You need control over the library components *you* design,
and and are trying to remedy that by influencing the design of
optional.  Unfortunately you came to the wrong iterator_adaptor.
Please see the CVS one ;-)

> In this case I don't have the freedom to create another constructor
> to easily workaround this problem. Reading back over the formal
> review threads for optional, I noticed that the reason for the
> explicit constructor was to support the pointer-like nature of
> optional. Right now I am questioning whether the pointer-like
> semantics is worth it to me. I would probably prefer that clients
> type more to test whether a value is initialized so that they can
> type less to call a function that takes an optional as a parameter
> when they have a value to pass in. 

Well, that's a slightly different issue and you may be right.  On the
other hand, you can always define a type which implements those
implicit constructors:

template <class T>
struct implicit_optional : optional<T>
{
   implicit_optional() {}
   implicit_optional(optional<T> const& x) : optional<T>(x) {}
   implicit_optional(T const& x) : optional<T>(x) {}
};

You could use that in your interfaces.

> Sorry if this has already been covered, but here my question is:
> Have you experimented with a variant of optional that drops the
                               ^^^^^^^^^^^^^^^^^^^
Whoa; you're making me dizzy with discriminated union-ness!

> pointer-like semantics, allowing implicit construction and
> disallowing implicit safe-bool conversion, and if so what did you
> learn that made you favor the current set of tradeoffs?
>
> Thanks,
> Mat

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com