variant.hpp

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    typedef typename mpl::eval_if<
          is_recursive_
        , T0_
        , mpl::identity< T0_ >
        >::type unwrapped_T0_;

    struct is_sequence_based_
        : detail::variant::is_over_sequence<unwrapped_T0_>
    {
    };

#if !defined(BOOST_VARIANT_NO_TYPE_SEQUENCE_SUPPORT)

private: // helpers, for typedefs (below)

    typedef typename mpl::eval_if<
          is_sequence_based_
        , unwrapped_T0_ // over_sequence<...>::type
        , detail::variant::make_variant_list<
              unwrapped_T0_
            , BOOST_VARIANT_ENUM_SHIFTED_PARAMS(T)
            >
        >::type specified_types;

    BOOST_STATIC_ASSERT((
          ::boost::mpl::not_< mpl::empty<specified_types> >::value
        ));

    typedef typename mpl::eval_if<
          is_recursive_
        , mpl::transform<
              specified_types
            , mpl::protect<
                  detail::variant::quoted_enable_recursive<wknd_self_t>
                >
            >
        , mpl::identity< specified_types >
        >::type recursive_enabled_types;

public: // public typedefs

    typedef typename mpl::transform<
          recursive_enabled_types
        , unwrap_recursive<mpl::_1>
        >::type types;

private: // internal typedefs

    typedef typename mpl::transform<
          recursive_enabled_types
        , mpl::protect< detail::make_reference_content<> >
        >::type internal_types;

    typedef typename mpl::front<
          internal_types
        >::type internal_T0;

#else // defined(BOOST_VARIANT_NO_TYPE_SEQUENCE_SUPPORT)

private: // helpers, for typedefs (below)

    typedef unwrapped_T0_ T0;

    #define BOOST_VARIANT_AUX_ENABLE_RECURSIVE_TYPEDEFS(z,N,_) \
        typedef typename mpl::eval_if< \
              is_recursive_ \
            , detail::variant::enable_recursive< \
                  BOOST_PP_CAT(T,N) \
                , wknd_self_t \
                > \
            , mpl::identity< BOOST_PP_CAT(T,N) > \
            >::type BOOST_PP_CAT(recursive_enabled_T,N); \
        /**/

    BOOST_PP_REPEAT(
          BOOST_VARIANT_LIMIT_TYPES
        , BOOST_VARIANT_AUX_ENABLE_RECURSIVE_TYPEDEFS
        , _
        )

    #undef BOOST_VARIANT_AUX_ENABLE_RECURSIVE_TYPEDEFS

    #define BOOST_VARIANT_AUX_UNWRAP_RECURSIVE_TYPEDEFS(z,N,_) \
        typedef typename unwrap_recursive< \
              BOOST_PP_CAT(recursive_enabled_T,N) \
            >::type BOOST_PP_CAT(public_T,N); \
        /**/

    BOOST_PP_REPEAT(
          BOOST_VARIANT_LIMIT_TYPES
        , BOOST_VARIANT_AUX_UNWRAP_RECURSIVE_TYPEDEFS
        , _
        )

    #undef BOOST_VARIANT_AUX_UNWRAP_RECURSIVE_TYPEDEFS

public: // public typedefs

    typedef typename detail::variant::make_variant_list<
          BOOST_VARIANT_ENUM_PARAMS(public_T)
        >::type types;

private: // helpers, for internal typedefs (below)

    #define BOOST_VARIANT_AUX_MAKE_REFERENCE_CONTENT_TYPEDEFS(z,N,_) \
        typedef detail::make_reference_content< \
              BOOST_PP_CAT(recursive_enabled_T,N) \
            >::type BOOST_PP_CAT(internal_T,N); \
        /**/

    BOOST_PP_REPEAT(
          BOOST_VARIANT_LIMIT_TYPES
        , BOOST_VARIANT_AUX_MAKE_REFERENCE_CONTENT_TYPEDEFS
        , _
        )

    #undef BOOST_VARIANT_AUX_MAKE_REFERENCE_CONTENT_TYPEDEFS

private: // internal typedefs

    typedef typename detail::variant::make_variant_list<
          BOOST_VARIANT_ENUM_PARAMS(internal_T)
        >::type internal_types;

private: // static precondition assertions

    // NOTE TO USER :
    // variant< type-sequence > syntax is not supported on this compiler!
    //
    BOOST_MPL_ASSERT_NOT(( is_sequence_based_ ));

#endif // BOOST_VARIANT_NO_TYPE_SEQUENCE_SUPPORT workaround

private: // helpers, for representation (below)

    typedef typename detail::variant::find_fallback_type<
          internal_types
        >::type fallback_type_result_;

    typedef typename fallback_type_result_::first
        fallback_type_index_;
    typedef typename fallback_type_result_::second
        fallback_type_;

    struct has_fallback_type_
        : mpl::not_<
              is_same< fallback_type_, detail::variant::no_fallback_type >
            >
    {
    };

    typedef has_fallback_type_
        never_uses_backup_flag;

    typedef typename detail::variant::make_storage<
          internal_types, never_uses_backup_flag
        >::type storage_t;

private: // helpers, for representation (below)

    // which_ on:
    // * [0,  size<internal_types>) indicates stack content
    // * [-size<internal_types>, 0) indicates pointer to heap backup
    // if which_ >= 0:
    // * then which() -> which_
    // * else which() -> -(which_ + 1)

#if !defined(BOOST_VARIANT_MINIMIZE_SIZE)

    typedef int which_t;

#else // defined(BOOST_VARIANT_MINIMIZE_SIZE)

    // [if O1_size available, then attempt which_t size optimization...]
    // [select signed char if fewer than SCHAR_MAX types, else signed int:]
    typedef typename mpl::eval_if<
          mpl::equal_to< mpl::O1_size<internal_types>, mpl::long_<-1> >
        , mpl::identity< int >
        , mpl::if_<
              mpl::less< mpl::O1_size<internal_types>, mpl::int_<SCHAR_MAX> >
            , signed char
            , int
            >
        >::type which_t;

#endif // BOOST_VARIANT_MINIMIZE_SIZE switch

// representation -- private when possible
#if !defined(BOOST_NO_MEMBER_TEMPLATE_FRIENDS)
    private:
#else
    public:
#endif

    which_t which_;
    storage_t storage_;

    void indicate_which(int which)
    {
        which_ = static_cast<which_t>( which );
    }

    void indicate_backup_which(int which)
    {
        which_ = static_cast<which_t>( -(which + 1) );
    }

private: // helpers, for queries (below)

    bool using_backup() const
    {
        return which_ < 0;
    }

public: // queries

    int which() const
    {
        // If using heap backup...
        if (using_backup())
            // ...then return adjusted which_:
            return -(which_ + 1);

        // Otherwise, return which_ directly:
        return which_;
    }

private: // helpers, for structors (below)

    struct initializer
        : BOOST_VARIANT_AUX_INITIALIZER_T(
              recursive_enabled_types, recursive_enabled_T
            )
    {
    };

    void destroy_content()
    {
        detail::variant::destroyer visitor;
        this->internal_apply_visitor(visitor);
    }

public: // structors

    ~variant()
    {
        destroy_content();
    }

    variant()
    {
        // NOTE TO USER :
        // Compile error from here indicates that the first bound
        // type is not default-constructible, and so variant cannot
        // support its own default-construction.
        //
        new( storage_.address() ) internal_T0();
        indicate_which(0); // zero is the index of the first bounded type
    }

private: // helpers, for structors, cont. (below)

    class convert_copy_into
        : public static_visitor<int>
    {
    private: // representation

        void* storage_;

    public: // structors

        explicit convert_copy_into(void* storage)
            : storage_(storage)
        {
        }

    public: // internal visitor interfaces (below)

        template <typename T>
        int internal_visit(T& operand, int) const
        {
            // NOTE TO USER :
            // Compile error here indicates one of the source variant's types 
            // cannot be unambiguously converted to the destination variant's
            // types (or that no conversion exists).
            //
            return initializer::initialize(storage_, operand);
        }

#   if BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x0564))
        template <typename T>
        result_type internal_visit(const T& operand, int) const
        {
            return initializer::initialize(storage_, operand);
        }
#   endif

        template <typename T>
        int internal_visit(boost::detail::reference_content<T>& operand, long) const
        {
            return internal_visit( operand.get(), 1L );
        }

        template <typename T>
        int internal_visit(const boost::detail::reference_content<T>& operand, long) const
        {
            return internal_visit( operand.get(), 1L );
        }

        template <typename T>
        int internal_visit(boost::detail::variant::backup_holder<T>& operand, long) const
        {
            return internal_visit( operand.get(), 1L );
        }

        template <typename T>
        int internal_visit(const boost::detail::variant::backup_holder<T>& operand, long) const
        {
            return internal_visit( operand.get(), 1L );
        }

        template <typename T>
        int internal_visit(boost::recursive_wrapper<T>& operand, long) const
        {
            return internal_visit( operand.get(), 1L );
        }

        template <typename T>
        int internal_visit(const boost::recursive_wrapper<T>& operand, long) const
        {
            return internal_visit( operand.get(), 1L );
        }

    };

    friend class convert_copy_into;

private: // helpers, for structors, below 

    template <typename T>
    void convert_construct(
          T& operand
        , int
        , mpl::false_ = mpl::false_() // is_foreign_variant
        )
    {
        // NOTE TO USER :
        // Compile error here indicates that the given type is not 
        // unambiguously convertible to one of the variant's types
        // (or that no conversion exists).
        //
        indicate_which(
              initializer::initialize(
                  storage_.address()
                , operand
                )
            );
    }

    template <typename Variant>
    void convert_construct(
          Variant& operand
        , long
        , mpl::true_// is_foreign_variant
        )
    {
        convert_copy_into visitor(storage_.address());
        indicate_which(
              operand.internal_apply_visitor(visitor)
            );
    }

    template <typename Variant>
    void convert_construct_variant(Variant& operand)
    {
        // [Determine if the given variant is itself a bounded type, or if its
        //  content needs to be converted (i.e., it is a 'foreign' variant):]
        //

        typedef typename mpl::find_if<
              types
            , is_same<
                  add_const<mpl::_1>
                , const Variant
                >
            >::type found_it;

        typedef typename mpl::end<types>::type not_found;
        typedef typename is_same<
              found_it, not_found
            >::type is_foreign_variant;

        // Convert construct from operand:
        convert_construct(
              operand, 1L
            , is_foreign_variant()
            );
    }

    template <BOOST_VARIANT_ENUM_PARAMS(typename U)>
    void convert_construct(
          boost::variant<BOOST_VARIANT_ENUM_PARAMS(U)>& operand
        , long
        )
    {
        convert_construct_variant(operand);
    }

    template <BOOST_VARIANT_ENUM_PARAMS(typename U)>
    void convert_construct(
          const boost::variant<BOOST_VARIANT_ENUM_PARAMS(U)>& operand
        , long
        )
    {
        convert_construct_variant(operand);    
    }

public: // structors, cont.

#if !defined(BOOST_VARIANT_AUX_BROKEN_CONSTRUCTOR_TEMPLATE_ORDERING)

    template <typename T>
    variant(const T& operand)
    {
        convert_construct(operand, 1L);
    }

    template <typename T>
    variant(T& operand)
    {
        convert_construct(operand, 1L);
    }

#elif defined(BOOST_VARIANT_AUX_HAS_CONSTRUCTOR_TEMPLATE_ORDERING_SFINAE_WKND)

    // For compilers that cannot distinguish between T& and const T& in
    // template constructors, but do fully support SFINAE, we can workaround:

    template <typename T>
    variant(const T& operand)
    {
        convert_construct(operand, 1L);
    }

    template <typename T>
    variant(
          T& operand
        , typename enable_if<
              mpl::not_< is_const<T> >
            , void
            >::type* = 0
        )
    {
        convert_construct(operand, 1L);
    }

#else // !defined(BOOST_VARIANT_AUX_HAS_CONSTRUCTOR_TEMPLATE_ORDERING_SFINAE_WKND)