[variant] Compile time checked getter

[nobody]

[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@terra.es)