Re: [boost] [optional] Safe optional

Subject: Re: [boost] [optional] Safe optional
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2014-11-24 12:53:14

On Tue, Nov 18, 2014 at 4:23 AM, Andrzej Krzemienski <akrzemi1_at_[hidden]> wrote:
> 2014-11-18 8:00 GMT+01:00 Andrey Semashev <andrey.semashev_at_[hidden]>:
>
>> On Mon, Nov 17, 2014 at 12:24 PM, Andrzej Krzemienski
>> <akrzemi1_at_[hidden]> wrote:
>> >
>> > The solution is based on the observation that there is a limited number
>> of
>> > things you can do with optional<T>
>> >
>> > optional<int> oi = ...;
>> >
>> > 1:
>> > if (oi) doSomething(*oi);
>> > else doSomethingElse(); // or nothing
>> >
>> > 2:
>> > if (oi) doSomething(*oi);
>> > else doSomething(-1); // use default value
>> >
>> > 3:
>> > if (!oi) oi = (int)getIntByOtherMeans();
>> > doSomething(*oi); // now it is safe
>> >
>> > Now we simply provide a dedicated interface for each of these three
>> usages:
>> >
>> > 1:
>> > oi.use(&doSomething, &doSomethingElse); // or use lambdas
>>
>> I'd call it visit(). Also, a one argument visit() could be useful (the
>> visitor would be passed boost::none if the value is absent).
>>
>> > 2:
>> > doSomething(oi.value_or(-1));
>>
>> We already have that.
>>
>> > 3:
>> > doSomething(oi.value_or_eval(getIntByOtherMeans));
>>
>> So, basically, the proposal is to add visitation API, am I correct?
>
>
> And to remove operator* and get() and value() and get_pointer() - anything
> that could cause UB. And remove implicit conversion from T
>

value() throws, not UB.  (At least in std::experimental)

I could see a "safer" or at least "more explicit" optional that didn't
use operator*, but still allowed access via named functions.
And maybe did less implicit conversion / comparison.

Not exactly safer, but more explicit, which helps safety.

Tony