Re: [Boost-bugs] [Boost C++ Libraries] #547: [variant] Compile time checked getter

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #547: [variant] Compile time checked getter
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2014-12-10 13:44:00

#547: [variant] Compile time checked getter
-------------------------------+--------------------------
  Reporter:  nobody            |      Owner:  apolukhin
      Type:  Feature Requests  |     Status:  new
 Milestone:  Boost 1.58.0      |  Component:  variant
   Version:  None              |   Severity:  Optimization
Resolution:  None              |   Keywords:
-------------------------------+--------------------------
Changes (by apolukhin):

 * owner:  ebf => apolukhin
 * status:  assigned => new
 * milestone:   => Boost 1.58.0


Old description:

> {{{
> [Sorry if anyone feels bad about this, I already posted
> it in usera and devel lists, with no answer at all]
>

> During a redesign, I began using boost::variant, and I
> detected that trying to get a value of a type not
> included in variant type-list resulted in run-time
> error (cast or null pointer).
>
> I feel that a variant should only try to return a valid
> type from its type-list, otherwise giving a compiler
> error, and reserve runttime errors for that case when
> someone tries to get a value from a type contained in
> the typelist, but not the actual type contained in the
> actual object.
>
> In such a way, I have made a little redesign on
> boost::get and called it boost::checked_get:
>
> namespace boost {
>
> template <typename U,
> BOOST_VARIANT_ENUM_PARAMS(typename T) >
> inline
>     typename add_pointer<U>::type
> checked_get(
>       boost::variant< BOOST_VARIANT_ENUM_PARAMS(T) >*
> operand
>       BOOST_VARIANT_AUX_GET_EXPLICIT_TEMPLATE_TYPE(U)
>     )
> {
>     typedef boost::variant<
> BOOST_VARIANT_ENUM_PARAMS(T) > Variant;
>
>     /* throw a compile-time error if type is not
> contained on type
> list */
>     BOOST_STATIC_ASSERT((!is_same<
>         typename mpl::find<typename
> Variant::types,U>::type,
>         typename mpl::end<typename
> Variant::types>::type>::value));
>
>     typedef typename add_pointer<U>::type U_ptr;
>     if (!operand) return static_cast<U_ptr>(0);
>
>     detail::variant::get_visitor<U> v;
>     return operand->apply_visitor(v);
> }
>
> } // end of addition to namespace boost
>

> And, of course, the rest of the templates needed for
> const *, ref and const&..
>
> I think it would be interesting to include it in boost
> libraries. In fact, I use it intensely.
>
> Best regards,
>
> Zara (yozara_at_[hidden])
> }}}

New description:

 {{{
 [Sorry if anyone feels bad about this, I already posted
 it in usera and devel lists, with no answer at all]


 During a redesign, I began using boost::variant, and I
 detected that trying to get a value of a type not
 included in variant type-list resulted in run-time
 error (cast or null pointer).

 I feel that a variant should only try to return a valid
 type from its type-list, otherwise giving a compiler
 error, and reserve runttime errors for that case when
 someone tries to get a value from a type contained in
 the typelist, but not the actual type contained in the
 actual object.

 In such a way, I have made a little redesign on
 boost::get and called it boost::checked_get:

 namespace boost {

 template <typename U,
 BOOST_VARIANT_ENUM_PARAMS(typename T) >
 inline
     typename add_pointer<U>::type
 checked_get(
       boost::variant< BOOST_VARIANT_ENUM_PARAMS(T) >*
 operand
       BOOST_VARIANT_AUX_GET_EXPLICIT_TEMPLATE_TYPE(U)
     )
 {
     typedef boost::variant<
 BOOST_VARIANT_ENUM_PARAMS(T) > Variant;

     /* throw a compile-time error if type is not
 contained on type
 list */
     BOOST_STATIC_ASSERT((!is_same<
         typename mpl::find<typename
 Variant::types,U>::type,
         typename mpl::end<typename
 Variant::types>::type>::value));

     typedef typename add_pointer<U>::type U_ptr;
     if (!operand) return static_cast<U_ptr>(0);

     detail::variant::get_visitor<U> v;
     return operand->apply_visitor(v);
 }

 } // end of addition to namespace boost


 And, of course, the rest of the templates needed for
 const *, ref and const&..

 I think it would be interesting to include it in boost
 libraries. In fact, I use it intensely.

 Best regards,

 Zara (yozara_at_[hidden])
 }}}

--
Comment:
 Series of commits were merged to develop branch to fix this issue. At this
 moment last commit is
 [https://github.com/boostorg/variant/commit/cbf4bbf1b1df5c3bbc65fec1f2de3351c141cfd3

 cbf4bbf]
-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/547#comment:7>
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