[boost] Re: [optional] new assignmet semantic and references

From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2005-03-01 22:37:03

Joao Abecasis wrote:
> Fernando Cacciola wrote:
>>> Maybe boost::reference_wrapper can be used internally to wrap
>>> reference types and solve this issue.
>>>
>>
>> How?
>
> I was suggesting the use of reference_wrapper to avoid singling out
> reference types everywhere in the code. The new assignment operators
> might just work.
>
Ha, well, optional<> uses make_reference_content<> (from variant<>) so I 
don't even need to distinguish between reference and non-reference types 
except were explicitely needed.
In fact, the new assignment operators "just work" in this case with no need 
of special dispatching code.

> Anyway, I haven't tested any of this so...
>
> I suggest you document how users may workaround this issue in their
> code using boost::reference_wrapper/ref/cref. Here's a small snippet
> you can use.
>      #include <boost/optional.hpp>
>      #include <boost/none.hpp>
>      #include <boost/ref.hpp>
>
>      int main()
>      {
>          int a = 32;
>          int b = 1279;
>
>          boost::reference_wrapper<int> ra(a);
>
>          boost::optional<boost::reference_wrapper<int> >
>              orw1,                           // default constructor
>              orw2(boost::none),              // none_t constructor
>              orw3(ra),                       // value constructor
>              orw4(boost::ref(a));            // make reference
>
>          boost::optional<int &> ori1(orw1),
>              ori2(orw2),
>              ori3(orw3.get());
>
>          orw1 = boost::ref(b);
>      }
>
> Best regards,
>
Nice :-)
I can at least tell users how to get the "old" semantics if they really need 
it.

Thank you!

Fernando Cacciola