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Subject: Re: [boost] [optional] Safe optional
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2014-11-19 03:37:33

2014-11-18 22:35 GMT+01:00 Vladimir Batov <Vladimir.Batov_at_[hidden]>

> In
>>> that case why not add it to the regular optional?
>>> IMHO, in order to introduce an alternative component, there should be
>>> significant and incompatible design and interface differences between
>>> the two. So far I don't see the need for such differences.
>>> As explained above: serious backwards incompatibility. Unacceptable by
>> many, including myself.
> I have to admit I find it somewhat of a concern hearing from the
> "optional" maintainer and someone leading "optional" for standardization
> that he does not seem to share/agree with the current "optional" design to
> the point where you are keen on forking a "seriously incompatible" variant.
> Going ahead with the latter will be a lot of effort implementing,
> promoting, educating, defending... IMO that'll be the effort 1) not
> necessarily successful; 2) fragmenting the user-base; 3) most importantly,
> it'll be the effort taken *away* from the current "optional"... which seems
> so close (hopefully) to standardization. If you have concerns (you would
> not initiate the conversation if you did not), let's discuss them within
> the "optional" boundaries and document/justify the decisions made. Do I
> worry too much?

Ok, this concern deserves a clarification from my part. I am more than
satisfied with the current design of both boost::optional and
std::optional. i find the decisions right. The direct (unchecked) access to
the contained value is essential. The conversion from T may not be
critically essential but is very useful. My motivation is NOT the
dissatisfaction with the current interface.

You are right that by starting this thread I might have been sending the
opposite message. Therefore I am declaring my position and motivation here

It is simply that I observe and acknowledge the expectations of a portion
of the community to have a different interface. I also acknowledge that
having two interfaces for nearly the same thing may be a bad idea in
itself. This is why I am simply discussing (talking) and not taking any
actions. I want to hear others' input. And I find yours particularly

Note that the interface I am proposing is not just fool-proof. It is
designed in a way that forces you to express your intentions rather that
just giving orders. The safety is not achieved by just prohibiting things
but by forcing you to rethink what you are doing.

To some extent, I consider Boost an incubator of ideas rather than "the
standard way of doing things". I would agree that having two optionals in
STD would be a bad message. This is less so for Boost. We would have people
experiment with an alternative and decide which they find better, and they
could deliver their opinion to the Standardization committee.

> On 11/18/2014 08:23 PM, Andrzej Krzemienski wrote:
>> ...
>> And to remove operator* and get() and value() and get_pointer() -
>> anything that could cause UB. And remove implicit conversion from T
> 1. I might be open to the idea of unifying/cleaning the access interface
> and settling on one... but taking them *all* out?.. Or you want to keep
> operator->()? Then, I still can do
> optional<T> ot;
> T* t = ot.operator->();
> You might say, it's crazy. My point is that if you want to save me (the
> user) from myself with "safe optional", you are most likely wasting your
> effort -- an uneducated idiot will always find a way to hurt himself.

Note that my intention is not to remove these from boost::optional, but to
provide a second type that does not have them. This is not affecting you:
you will stick to boost::optional interface which remains unchanged.

Yes, in the new hypothetical I would be also removing operator-> No
unchecked access. Yes, I do acknowledge that direct unchecked access is
necessary (e.g., for performance reasons). It is only for the people that
are determined to use the alternate interface and are willing to pay the

> 2. I find it troubling that you keep bringing "remove implicit conversion
> from T " up. That indicates that the property is close to the top of your
> "grievances" list. On my side, I find that implicit conversion essential
> for sane interactions of users with "optional". Back a while, when we were
> discussing "optional" (was it ISO forum?), I remember Fernando Cacciola
> expressing the same opinion (and as strongly). I'd greatly appreciate if
> you could please provide practical usage/deployment examples where implicit
> conversion from T causes a problem.

optional<double> Aircraft::weight()
  if (Impl* impl = get_impl())
    return impl->compute_weight();
    return none;

double Aircraft::max_weight()
  return 10000.0;

bool too_heavy(Aircraft& ac)
  return ac.weight() > ac.max_weight();
  // in "safe" optional compiler would spot the bug

This example only tells me that there exist some use cases where conversion
from T is dangerous/unintended/surprising. I DO NOT conclude that it should
have never been added.


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