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Subject: Re: [boost] [optional] Safe optional
From: Vladimir Batov (Vladimir.Batov_at_[hidden])
Date: 2014-11-17 16:12:56

On 11/17/2014 09:12 PM, Mostafa wrote:
> On Sun, 16 Nov 2014 23:04:43 -0800, Andrzej Krzemienski
> <akrzemi1_at_[hidden]> wrote:
>> Hi Everyone,
>> I would like to run an idea through everyone in this list. There is a
>> recurring complaint about Boost.Optional that it allows you to do
>> "unsafe"
>> things:
>> 1. Inadvertent mixed comparisons between T and optional<T>
>> 2. Unintended conversion from T to optional<T>
> The problem with optional is that it tries to be a drop-in replacement
> proxy for its underlying type, and, unfortunately, it can't fully do
> that. So it ends up with an identity crisis. IMO, optional should be
> treated as a first class object, not a thin wrapper around T, that
> means no implicit conversions to/from T, no implicit comparisons with
> T, etc... Last time I looked at this, that will solve the reference
> rebinding gotcha. That is, you'll have one behavior for optional<T>
> and optional<T&>....

1. With all due respect I do not feel I can agree with the above... and
I do not believe "optional" has a "problem"... especially of the
described magnitude. IMO the result of applying the conceptual/design
changes described above to the existing "optional" won't be "optional"
as we know it... not even close. And after using "optional" quite a bit
I can say I personally won't be very happy. IMO "optional" has been born
as a practical solution to a real problem and IMO it solves it quite
well. Yes, it has certain behavior that the user needs to be aware of...
but what class does not impose restrictions of that kind? Any potential
functional/behavioral change has to be looked at individually.

For example, I do agree that that there should not be implicit
optional<T> to T conversion. I was not even aware there was such.

However, implicit T to optional<T> conversion has a very practical
purpose. For example,

     int value(optional<int> =optional<int>());

allows me to shrink the API as value getter and setter are merged into
one function. Namely,

     int k = value(); // Get the value
     value(22); // Set the value. Implicit conversion of T to optional<T>

Instead, asking the user to call explicitly


is a professional suicide.

2. As for the separate/additional "safe" optional, I am personally not
that thrilled by the idea as of now. IMO that'll result in user-base
fragmentation, incompatibilities and inevitably more confusion in the end.

Just my 2c.

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