Brian McNamara <[EMAIL PROTECTED]> wrote:
> So I completely disagree that optionals should mix the interfaces of
> optional and the wrapped object into one. I think there should be an
> explicit unwrapping operation. But this is just my opinion, based on no
> practical evidence/experience, and I'm sure there are trade-offs either
> way. I'll explain the crux of my own reasoning below.
One can think of an optional<T> as conceptually a specialized but
nevertheless, *IS-A* T, with the added specialization that it can
be in a dead-uninitialized state. Maybe we'll call it a zombie object,
undead object, you name it ;-)
Point well taken though.
> On Sun, Aug 31, 2003 at 11:59:42PM +0800, Joel de Guzman wrote:
>> It's really strange (and hard to explain) that you have to dereference
>> optional<B>. Example:
>>
>> F(tuple<A, optional<B>, C> args)
>> {
>> A a = get<1>(args);
>> B b = *get<2>(args); // strange!
>> C c = get<3>(args);
>> }
>
> I don't think it's strange at all; an optional<B> is not a B!
Even if I agree with you that an optional<T> should not be a T,
an optional<T> is definitely not a pointer to T.
>> Not only is it strange, it complexifies the code a lot when the code is
>> automatically generated by some meta-program. Example:
>>
>> template <class A, class B, class C>
>> F(tuple<A, B, C> args)
>> {
>> A a = get<1>(args);
>> B b = get<2>(args);
>> C c = get<3>(args);
>> }
>>
>> Whoops, have to do a special case IFF B is an optional!
>
> Well, no. B==optional<Bprime> via template unification, right?
> If you want to do something with a Bprime, then yes, you have to
> special-case it, as you should.
You are right. That was some sloppy code above. Thanks for spotting that,
but I guess you got the idea.
> Here's a (contrived) example of how the implicit conversion breaks
> generic code:
>
> template <class Container>
> void contrived( Container& c ) {
> c.begin();
> }
> ...
> vector<int> v;
> contrived(v); // fine
> optional<vector<int> > ov( v );
> contrived(ov); // fails
Right.
> The point is that optional<T> is not a T, and most notably, a template
Your point.
> function will never perform the coercion. Replace the lines like
> B b = get<2>(args);
> in your example with real calls to, e.g.
> do_something( get<2>(args) )
> and do_something() is likely to fail if it's a template function
> (expecting a T and not an optional<T>).
Right.
> As another example, FC++ lists support conversion-to-bool as a test for
> NIL:
> fcpp::list<int> l = NIL;
> if( l ) // if l not empty
> cout << head(l); // print first element
> Clearly if 'l' were an optional, the "if" test would have a different
> meaning. I know you've been supporting a separate way to do the test
> (with "==none"), but I think that's completely arbitrary. The two
> issues are both really the same issue, in my mind.
>
>
>> Of all the types involved in the passing of arguments to the semantic
>> actions, the optional doesn't fit nicely because it is the only one
>> that has mixed value/pointer semantics. I am tempted to not use
>> optional<T> because of this and instead use variant<T, none>,
>> but I hate to miss the performance advantage of optional vs.
>> variant<T, none>.
>
> I have not used variant and know little about it, but I imagine you have
> to explicitly tell a variant "which type" you expect to get out of it.
> I think the same is true of optional; the call to operator*() (or
> whatever) says "I want the T type" (rather than the 'none').
Right. In fact, looking at it more closely, I *could* almost agree with you
that an optional<T> is not a T. There is in fact a getter function (get).
In fact all three (tuple, optional and variant) have a get function. That's
fine, yet, here again, the optional does not jive well because it returns
access *by pointer* whereas both tuple and variant return access by
reference.
--
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net
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