variant.hpp

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        // Hint: Are any of the bounded types const-qualified or references?
        //
        lhs_content = *static_cast< const T* >(rhs_storage_);
        BOOST_VARIANT_AUX_RETURN_VOID;
    }

};

///////////////////////////////////////////////////////////////////////////////
// (detail) class direct_assigner
//
// Generic static visitor that: if and only if the visited value is of the
// specified type, assigns the given value to the visited value and returns
// true; else returns false.
//
template <typename T>
class direct_assigner
    : public static_visitor<bool>
{
private: // representation

    T& rhs_;

public: // structors

    explicit direct_assigner(T& rhs)
        : rhs_(rhs)
    {
    }

#if !BOOST_WORKAROUND(BOOST_MSVC, <= 1200)

public: // visitor interface

    bool operator()(T& lhs)
    {
        lhs = rhs_;
        return true;
    }

    template <typename U>
    bool operator()(U&)
    {
        return false;
    }

#else // MSVC6

private: // helpers, for visitor interface (below)

    bool execute(T& lhs, mpl::true_)
    {
        lhs = rhs_;
        return true;
    }

    template <typename U>
    bool execute(U&, mpl::false_)
    {
        return false;
    }

public: // visitor interface

    template <typename U>
    bool operator()(U& lhs)
    {
        typedef typename is_same<U,T>::type U_is_T;
        return execute(lhs, U_is_T());
    }

#endif // MSVC6 workaround

};

///////////////////////////////////////////////////////////////////////////////
// (detail) class backup_assigner
//
// Internal visitor that "assigns" the given value to the visited value,
// using backup to recover if the destroy-copy sequence fails.
//
// NOTE: This needs to be a friend of variant, as it needs access to
// indicate_which, indicate_backup_which, etc.
//
template <typename Variant, typename RhsT>
class backup_assigner
    : public static_visitor<>
{
private: // representation

    Variant& lhs_;
    int rhs_which_;
    const RhsT& rhs_content_;

public: // structors

    backup_assigner(Variant& lhs, int rhs_which, const RhsT& rhs_content)
        : lhs_(lhs)
        , rhs_which_(rhs_which)
        , rhs_content_(rhs_content)
    {
    }

private: // helpers, for visitor interface (below)

    template <typename LhsT>
    void backup_assign_impl(
          LhsT& lhs_content
        , mpl::true_// has_nothrow_move
        )
    {
        // Move lhs content to backup...
        LhsT backup_lhs_content(
              ::boost::detail::variant::move(lhs_content)
            ); // nothrow

        // ...destroy lhs content...
        lhs_content.~LhsT(); // nothrow

        try
        {
            // ...and attempt to copy rhs content into lhs storage:
            new(lhs_.storage_.address()) RhsT(rhs_content_);
        }
        catch (...)
        {
            // In case of failure, restore backup content to lhs storage...
            new(lhs_.storage_.address())
                LhsT(
                      ::boost::detail::variant::move(backup_lhs_content)
                    ); // nothrow

            // ...and rethrow:
            throw;
        }

        // In case of success, indicate new content type:
        lhs_.indicate_which(rhs_which_); // nothrow
    }

    template <typename LhsT>
    void backup_assign_impl(
          LhsT& lhs_content
        , mpl::false_// has_nothrow_move
        )
    {
        // Backup lhs content...
        LhsT* backup_lhs_ptr = new LhsT(lhs_content);

        // ...destroy lhs content...
        lhs_content.~LhsT(); // nothrow

        try
        {
            // ...and attempt to copy rhs content into lhs storage:
            new(lhs_.storage_.address()) RhsT(rhs_content_);
        }
        catch (...)
        {
            // In case of failure, copy backup pointer to lhs storage...
            new(lhs_.storage_.address())
                backup_holder<LhsT>( backup_lhs_ptr ); // nothrow

            // ...indicate now using backup...
            lhs_.indicate_backup_which( lhs_.which() ); // nothrow

            // ...and rethrow:
            throw;
        }

        // In case of success, indicate new content type...
        lhs_.indicate_which(rhs_which_); // nothrow

        // ...and delete backup:
        delete backup_lhs_ptr; // nothrow
    }

public: // visitor interface

    template <typename LhsT>
        BOOST_VARIANT_AUX_RETURN_VOID_TYPE
    internal_visit(LhsT& lhs_content, int)
    {
        typedef typename has_nothrow_move_constructor<LhsT>::type
            nothrow_move;

        backup_assign_impl( lhs_content, nothrow_move() );

        BOOST_VARIANT_AUX_RETURN_VOID;
    }

};

///////////////////////////////////////////////////////////////////////////////
// (detail) class swap_with
//
// Visitor that swaps visited value with content of given variant.
//
// Precondition: Given variant MUST have same logical type as visited value.
//
template <typename Variant>
struct swap_with
    : public static_visitor<>
{
private: // representation

    Variant& toswap_;

public: // structors

    explicit swap_with(Variant& toswap)
        : toswap_(toswap)
    {
    }

public: // internal visitor interfaces

    template <typename T>
    void operator()(T& operand) const
    {
        // Since the precondition ensures types are same, get T...
        known_get<T> getter;
        T& other = toswap_.apply_visitor(getter);

        // ...and swap:
        ::boost::detail::variant::move_swap( operand, other );
    }

};

///////////////////////////////////////////////////////////////////////////////
// (detail) class reflect
//
// Generic static visitor that performs a typeid on the value it visits.
//
class reflect
    : public static_visitor<const std::type_info&>
{
public: // visitor interfaces

    template <typename T>
    const std::type_info& operator()(const T&) const
    {
        return typeid(T);
    }

};

///////////////////////////////////////////////////////////////////////////////
// (detail) class comparer
//
// Generic static visitor that compares the content of the given lhs variant
// with the visited rhs content using Comp.
//
// Precondition: lhs.which() == rhs.which()
//
template <typename Variant, typename Comp>
class comparer
    : public static_visitor<bool>
{
private: // representation

    const Variant& lhs_;

public: // structors

    explicit comparer(const Variant& lhs)
        : lhs_(lhs)
    {
    }

public: // visitor interfaces

    template <typename T>
    bool operator()(const T& rhs_content) const
    {
        // Since the precondition ensures lhs and rhs types are same, get T...
        known_get<const T> getter;
        const T& lhs_content = lhs_.apply_visitor(getter);

        // ...and compare lhs and rhs contents:
        return Comp()(lhs_content, rhs_content);
    }

};

///////////////////////////////////////////////////////////////////////////////
// (detail) class equal_comp
//
// Generic function object compares lhs with rhs using operator==.
//
struct equal_comp
{
    template <typename T>
    bool operator()(const T& lhs, const T& rhs) const
    {
        return lhs == rhs;
    }
};

///////////////////////////////////////////////////////////////////////////////
// (detail) class less_comp
//
// Generic function object compares lhs with rhs using operator<.
//
struct less_comp
{
    template <typename T>
    bool operator()(const T& lhs, const T& rhs) const
    {
        return lhs < rhs;
    }
};

///////////////////////////////////////////////////////////////////////////////
// (detail) class template invoke_visitor
//
// Internal visitor that invokes the given visitor using:
//  * for wrappers (e.g., recursive_wrapper), the wrapper's held value.
//  * for all other values, the value itself.
//
template <typename Visitor>
class invoke_visitor
{
private: // representation

    Visitor& visitor_;

public: // visitor typedefs

    typedef typename Visitor::result_type
        result_type;

public: // structors

    explicit invoke_visitor(Visitor& visitor)
        : visitor_(visitor)
    {
    }

#if !defined(BOOST_NO_VOID_RETURNS)

public: // internal visitor interfaces

    template <typename T>
    result_type internal_visit(T& operand, int)
    {
        return visitor_(operand);
    }

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

#else // defined(BOOST_NO_VOID_RETURNS)

private: // helpers, for internal visitor interfaces (below)

    template <typename T>
        BOOST_VARIANT_AUX_GENERIC_RESULT_TYPE(result_type)
    visit_impl(T& operand, mpl::false_)
    {
        return visitor_(operand);
    }

    template <typename T>
        BOOST_VARIANT_AUX_RETURN_VOID_TYPE
    visit_impl(T& operand, mpl::true_)
    {
        visitor_(operand);
        BOOST_VARIANT_AUX_RETURN_VOID;
    }

public: // internal visitor interfaces

    template <typename T>
        BOOST_VARIANT_AUX_GENERIC_RESULT_TYPE(result_type)
    internal_visit(T& operand, int)
    {
        typedef typename is_same<result_type, void>::type
            has_void_result_type;

        return visit_impl(operand, has_void_result_type());
    }

#endif // BOOST_NO_VOID_RETURNS) workaround

public: // internal visitor interfaces, cont.

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

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

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

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

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

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

};

}} // namespace detail::variant

///////////////////////////////////////////////////////////////////////////////
// class template variant (concept inspired by Andrei Alexandrescu)
//
// See docs and boost/variant/variant_fwd.hpp for more information.
//
template <
      typename T0_
    , BOOST_VARIANT_ENUM_SHIFTED_PARAMS(typename T)
    >
class variant
{
private: // helpers, for typedefs (below)

    typedef variant wknd_self_t;

    struct is_recursive_
        : detail::variant::is_recursive_flag<T0_>
    {
    };