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 // Copyright (C) 2003, 2008 Fernando Luis Cacciola Carballal.
 // Copyright (C) 2014 - 2021 Andrzej Krzemienski.
 //
 // Use, modification, and distribution is subject to the Boost Software
 // License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 //
 // See http://www.boost.org/libs/optional for documentation.
 //
 // You are welcome to contact the author at:
 //  fernando_cacciola@hotmail.com
 //
 // Revisions:
 // 27 Apr 2008 (improved swap) Fernando Cacciola, Niels Dekker, Thorsten Ottosen
 // 05 May 2014 (Added move semantics) Andrzej Krzemienski
 //
 #ifndef BOOST_OPTIONAL_OPTIONAL_FLC_19NOV2002_HPP
 #define BOOST_OPTIONAL_OPTIONAL_FLC_19NOV2002_HPP
 
 #include <new>
 #ifndef BOOST_NO_IOSTREAM
 #include <iosfwd>
 #endif // BOOST_NO_IOSTREAM
 
 #ifdef BOOST_OPTIONAL_DETAIL_USE_STD_TYPE_TRAITS
 #  include <type_traits>
 #endif
 
 #include <boost/assert.hpp>
 #include <boost/core/addressof.hpp>
 #include <boost/core/enable_if.hpp>
 #include <boost/core/explicit_operator_bool.hpp>
 #include <boost/core/invoke_swap.hpp>
 #include <boost/optional/bad_optional_access.hpp>
 #include <boost/static_assert.hpp>
 #include <boost/throw_exception.hpp>
 #include <boost/type.hpp>
 #include <boost/type_traits/alignment_of.hpp>
 #include <boost/type_traits/conditional.hpp>
 #include <boost/type_traits/conjunction.hpp>
 #include <boost/type_traits/disjunction.hpp>
 #include <boost/type_traits/has_nothrow_constructor.hpp>
 #include <boost/type_traits/type_with_alignment.hpp>
 #include <boost/type_traits/remove_const.hpp>
 #include <boost/type_traits/remove_reference.hpp>
 #include <boost/type_traits/decay.hpp>
 #include <boost/type_traits/is_assignable.hpp>
 #include <boost/type_traits/is_base_of.hpp>
 #include <boost/type_traits/is_const.hpp>
 #include <boost/type_traits/is_constructible.hpp>
 #include <boost/type_traits/is_convertible.hpp>
 #include <boost/type_traits/is_lvalue_reference.hpp>
 #include <boost/type_traits/is_nothrow_move_assignable.hpp>
 #include <boost/type_traits/is_nothrow_move_constructible.hpp>
 #include <boost/type_traits/is_rvalue_reference.hpp>
 #include <boost/type_traits/is_same.hpp>
 #include <boost/type_traits/is_volatile.hpp>
 #include <boost/type_traits/is_scalar.hpp>
 #include <boost/move/utility.hpp>
 #include <boost/none.hpp>
 #include <boost/utility/compare_pointees.hpp>
 #include <boost/utility/result_of.hpp>
 
 #include <boost/optional/optional_fwd.hpp>
 #include <boost/optional/detail/optional_config.hpp>
 #include <boost/optional/detail/optional_factory_support.hpp>
 #include <boost/optional/detail/optional_aligned_storage.hpp>
 #include <boost/optional/detail/optional_hash.hpp>
 
 namespace boost { namespace optional_detail {
 
 template <typename T>
 struct optional_value_type
 {
 };
 
 template <typename T>
 struct optional_value_type< ::boost::optional<T> >
 {
   typedef T type;
 };
 
 }} // namespace boost::optional_detail
 
 #ifdef BOOST_OPTIONAL_CONFIG_USE_OLD_IMPLEMENTATION_OF_OPTIONAL
 #include <boost/optional/detail/old_optional_implementation.hpp>
 #else
 namespace boost {
 
 namespace optional_ns {
 
 // a tag for in-place initialization of contained value
 struct in_place_init_t
 {
   struct init_tag{};
   explicit in_place_init_t(init_tag){}
 };
 const in_place_init_t in_place_init ((in_place_init_t::init_tag()));
 
 // a tag for conditional in-place initialization of contained value
 struct in_place_init_if_t
 {
   struct init_tag{};
   explicit in_place_init_if_t(init_tag){}
 };
 const in_place_init_if_t in_place_init_if ((in_place_init_if_t::init_tag()));
 
 } // namespace optional_ns
 
 using optional_ns::in_place_init_t;
 using optional_ns::in_place_init;
 using optional_ns::in_place_init_if_t;
 using optional_ns::in_place_init_if;
 
 namespace optional_detail {
 
 struct init_value_tag {};
 
 struct optional_tag {};
 
 
 template<class T>
 class optional_base : public optional_tag
 {
   private :
 
     typedef aligned_storage<T> storage_type ;
     typedef optional_base<T> this_type ;
 
   protected :
 
     typedef T value_type ;
     typedef typename boost::remove_const<T>::type unqualified_value_type;
 
   protected:
     typedef T &       reference_type ;
     typedef T const&  reference_const_type ;
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     typedef T &&  rval_reference_type ;
     typedef T &&  reference_type_of_temporary_wrapper ;
 #endif
     typedef T *         pointer_type ;
     typedef T const*    pointer_const_type ;
     typedef T const&    argument_type ;
 
     // Creates an optional<T> uninitialized.
     // No-throw
     optional_base()
       :
       m_initialized(false) {}
 
     // Creates an optional<T> uninitialized.
     // No-throw
     optional_base ( none_t )
       :
       m_initialized(false) {}
 
     // Creates an optional<T> initialized with 'val'.
     // Can throw if T::T(T const&) does
     optional_base ( init_value_tag, argument_type val )
       :
       m_initialized(false)
     {
         construct(val);
     }
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // move-construct an optional<T> initialized from an rvalue-ref to 'val'.
     // Can throw if T::T(T&&) does
     optional_base ( init_value_tag, rval_reference_type val )
       :
       m_initialized(false)
     {
       construct( boost::move(val) );
     }
 #endif
 
     // Creates an optional<T> initialized with 'val' IFF cond is true, otherwise creates an uninitialized optional<T>.
     // Can throw if T::T(T const&) does
     optional_base ( bool cond, argument_type val )
       :
       m_initialized(false)
     {
       if ( cond )
         construct(val);
     }
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Creates an optional<T> initialized with 'move(val)' IFF cond is true, otherwise creates an uninitialized optional<T>.
     // Can throw if T::T(T &&) does
     optional_base ( bool cond, rval_reference_type val )
       :
       m_initialized(false)
     {
       if ( cond )
         construct(boost::move(val));
     }
 #endif
 
     // Creates a deep copy of another optional<T>
     // Can throw if T::T(T const&) does
     optional_base ( optional_base const& rhs )
       :
       m_initialized(false)
     {
       if ( rhs.is_initialized() )
         construct(rhs.get_impl());
     }
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Creates a deep move of another optional<T>
     // Can throw if T::T(T&&) does
     optional_base ( optional_base&& rhs )
     BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value)
       :
       m_initialized(false)
     {
       if ( rhs.is_initialized() )
         construct( boost::move(rhs.get_impl()) );
     }
 #endif
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 
     template<class Expr, class PtrExpr>
     explicit optional_base ( Expr&& expr, PtrExpr const* tag )
       :
       m_initialized(false)
     {
       construct(boost::forward<Expr>(expr),tag);
     }
 
 #else
     // This is used for both converting and in-place constructions.
     // Derived classes use the 'tag' to select the appropriate
     // implementation (the correct 'construct()' overload)
     template<class Expr>
     explicit optional_base ( Expr const& expr, Expr const* tag )
       :
       m_initialized(false)
     {
       construct(expr,tag);
     }
 
 #endif
 
     optional_base& operator= ( optional_base const& rhs )
     {
       this->assign(rhs);
       return *this;
     }
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     optional_base& operator= ( optional_base && rhs )
     BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value && ::boost::is_nothrow_move_assignable<T>::value)
     {
       this->assign(static_cast<optional_base&&>(rhs));
       return *this;
     }
 #endif
 
     // No-throw (assuming T::~T() doesn't)
     ~optional_base() { destroy() ; }
 
     // Assigns from another optional<T> (deep-copies the rhs value)
     void assign ( optional_base const& rhs )
     {
       if (is_initialized())
       {
         if ( rhs.is_initialized() )
              assign_value(rhs.get_impl());
         else destroy();
       }
       else
       {
         if ( rhs.is_initialized() )
           construct(rhs.get_impl());
       }
     }
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Assigns from another optional<T> (deep-moves the rhs value)
     void assign ( optional_base&& rhs )
     {
       if (is_initialized())
       {
         if ( rhs.is_initialized() )
              assign_value( boost::move(rhs.get_impl()) );
         else destroy();
       }
       else
       {
         if ( rhs.is_initialized() )
           construct(boost::move(rhs.get_impl()));
       }
     }
 #endif
 
     // Assigns from another _convertible_ optional<U> (deep-copies the rhs value)
     template<class U>
     void assign ( optional<U> const& rhs )
     {
       if (is_initialized())
       {
         if ( rhs.is_initialized() )
 #ifndef BOOST_OPTIONAL_CONFIG_RESTORE_ASSIGNMENT_OF_NONCONVERTIBLE_TYPES
           assign_value( rhs.get() );
 #else
           assign_value( static_cast<value_type>(rhs.get()) );
 #endif
 
         else destroy();
       }
       else
       {
         if ( rhs.is_initialized() )
 #ifndef BOOST_OPTIONAL_CONFIG_RESTORE_ASSIGNMENT_OF_NONCONVERTIBLE_TYPES
           construct(rhs.get());
 #else
           construct(static_cast<value_type>(rhs.get()));
 #endif
       }
     }
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // move-assigns from another _convertible_ optional<U> (deep-moves from the rhs value)
     template<class U>
     void assign ( optional<U>&& rhs )
     {
       typedef BOOST_DEDUCED_TYPENAME optional<U>::rval_reference_type ref_type;
       if (is_initialized())
       {
         if ( rhs.is_initialized() )
              assign_value( static_cast<ref_type>(rhs.get()) );
         else destroy();
       }
       else
       {
         if ( rhs.is_initialized() )
           construct(static_cast<ref_type>(rhs.get()));
       }
     }
 #endif
 
     // Assigns from a T (deep-copies the rhs value)
     void assign ( argument_type val )
     {
       if (is_initialized())
            assign_value(val);
       else construct(val);
     }
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Assigns from a T (deep-moves the rhs value)
     void assign ( rval_reference_type val )
     {
       if (is_initialized())
            assign_value( boost::move(val) );
       else construct( boost::move(val) );
     }
 #endif
 
     // Assigns from "none", destroying the current value, if any, leaving this UNINITIALIZED
     // No-throw (assuming T::~T() doesn't)
     void assign ( none_t ) BOOST_NOEXCEPT { destroy(); }
 
 #ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     template<class Expr, class ExprPtr>
     void assign_expr ( Expr&& expr, ExprPtr const* tag )
     {
       if (is_initialized())
         assign_expr_to_initialized(boost::forward<Expr>(expr),tag);
       else construct(boost::forward<Expr>(expr),tag);
     }
 #else
     template<class Expr>
     void assign_expr ( Expr const& expr, Expr const* tag )
     {
       if (is_initialized())
         assign_expr_to_initialized(expr,tag);
       else construct(expr,tag);
     }
 #endif
 
 #endif
 
   public :
 
     // Destroys the current value, if any, leaving this UNINITIALIZED
     // No-throw (assuming T::~T() doesn't)
     void reset() BOOST_NOEXCEPT { destroy(); }
 
     // **DEPRECATED** Replaces the current value -if any- with 'val'
     void reset ( argument_type val ) { assign(val); }
 
     // Returns a pointer to the value if this is initialized, otherwise,
     // returns NULL.
     // No-throw
     pointer_const_type get_ptr() const { return m_initialized ? get_ptr_impl() : 0 ; }
     pointer_type       get_ptr()       { return m_initialized ? get_ptr_impl() : 0 ; }
 
     bool is_initialized() const BOOST_NOEXCEPT { return m_initialized ; }
 
   protected :
 
     void construct ( argument_type val )
      {
        ::new (m_storage.address()) unqualified_value_type(val) ;
        m_initialized = true ;
      }
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     void construct ( rval_reference_type val )
      {
        ::new (m_storage.address()) unqualified_value_type( boost::move(val) ) ;
        m_initialized = true ;
      }
 #endif
 
 
 #if (!defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES) && (!defined BOOST_NO_CXX11_VARIADIC_TEMPLATES)
     // Constructs in-place
     // upon exception *this is always uninitialized
     template<class... Args>
     void construct ( in_place_init_t, Args&&... args )
     {
       ::new (m_storage.address()) unqualified_value_type( boost::forward<Args>(args)... ) ;
       m_initialized = true ;
     }
 
     template<class... Args>
     void emplace_assign ( Args&&... args )
     {
       destroy();
       construct(in_place_init, boost::forward<Args>(args)...);
     }
 
     template<class... Args>
     explicit optional_base ( in_place_init_t, Args&&... args )
       :
       m_initialized(false)
     {
       construct(in_place_init, boost::forward<Args>(args)...);
     }
 
     template<class... Args>
     explicit optional_base ( in_place_init_if_t, bool cond, Args&&... args )
       :
       m_initialized(false)
     {
       if ( cond )
         construct(in_place_init, boost::forward<Args>(args)...);
     }
 #elif (!defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES)
     template<class Arg>
     void construct ( in_place_init_t, Arg&& arg )
      {
        ::new (m_storage.address()) unqualified_value_type( boost::forward<Arg>(arg) );
        m_initialized = true ;
      }
 
     void construct ( in_place_init_t )
      {
        ::new (m_storage.address()) unqualified_value_type();
        m_initialized = true ;
      }
 
     template<class Arg>
     void emplace_assign ( Arg&& arg )
      {
        destroy();
        construct(in_place_init, boost::forward<Arg>(arg)) ;
      }
 
     void emplace_assign ()
      {
        destroy();
        construct(in_place_init) ;
      }
 
     template<class Arg>
     explicit optional_base ( in_place_init_t, Arg&& arg )
       :
       m_initialized(false)
     {
       construct(in_place_init, boost::forward<Arg>(arg));
     }
 
     explicit optional_base ( in_place_init_t )
       :
       m_initialized(false)
     {
       construct(in_place_init);
     }
 
     template<class Arg>
     explicit optional_base ( in_place_init_if_t, bool cond, Arg&& arg )
       :
       m_initialized(false)
     {
       if ( cond )
         construct(in_place_init, boost::forward<Arg>(arg));
     }
 
     explicit optional_base ( in_place_init_if_t, bool cond )
       :
       m_initialized(false)
     {
       if ( cond )
         construct(in_place_init);
     }
 
 #else
 
     template<class Arg>
     void construct ( in_place_init_t, const Arg& arg )
      {
        ::new (m_storage.address()) unqualified_value_type( arg );
        m_initialized = true ;
      }
 
     template<class Arg>
     void construct ( in_place_init_t, Arg& arg )
      {
        ::new (m_storage.address()) unqualified_value_type( arg );
        m_initialized = true ;
      }
 
     void construct ( in_place_init_t )
      {
        ::new (m_storage.address()) unqualified_value_type();
        m_initialized = true ;
      }
 
     template<class Arg>
     void emplace_assign ( const Arg& arg )
     {
       destroy();
       construct(in_place_init, arg);
     }
 
     template<class Arg>
     void emplace_assign ( Arg& arg )
     {
       destroy();
       construct(in_place_init, arg);
     }
 
     void emplace_assign ()
     {
       destroy();
       construct(in_place_init);
     }
 
     template<class Arg>
     explicit optional_base ( in_place_init_t, const Arg& arg )
       : m_initialized(false)
     {
       construct(in_place_init, arg);
     }
 
     template<class Arg>
     explicit optional_base ( in_place_init_t, Arg& arg )
       : m_initialized(false)
     {
       construct(in_place_init, arg);
     }
 
     explicit optional_base ( in_place_init_t )
       : m_initialized(false)
     {
       construct(in_place_init);
     }
 
     template<class Arg>
     explicit optional_base ( in_place_init_if_t, bool cond, const Arg& arg )
       : m_initialized(false)
     {
       if ( cond )
         construct(in_place_init, arg);
     }
 
     template<class Arg>
     explicit optional_base ( in_place_init_if_t, bool cond, Arg& arg )
       : m_initialized(false)
     {
       if ( cond )
         construct(in_place_init, arg);
     }
 
     explicit optional_base ( in_place_init_if_t, bool cond )
       : m_initialized(false)
     {
       if ( cond )
         construct(in_place_init);
     }
 #endif
 
 #ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Constructs in-place using the given factory
     template<class Expr>
     void construct ( Expr&& factory, in_place_factory_base const* )
      {
        boost_optional_detail::construct<value_type>(factory, m_storage.address());
        m_initialized = true ;
      }
 
     // Constructs in-place using the given typed factory
     template<class Expr>
     void construct ( Expr&& factory, typed_in_place_factory_base const* )
      {
        factory.apply(m_storage.address()) ;
        m_initialized = true ;
      }
 
     template<class Expr>
     void assign_expr_to_initialized ( Expr&& factory, in_place_factory_base const* tag )
      {
        destroy();
        construct(factory,tag);
      }
 
     // Constructs in-place using the given typed factory
     template<class Expr>
     void assign_expr_to_initialized ( Expr&& factory, typed_in_place_factory_base const* tag )
      {
        destroy();
        construct(factory,tag);
      }
 
 #else
     // Constructs in-place using the given factory
     template<class Expr>
     void construct ( Expr const& factory, in_place_factory_base const* )
      {
        boost_optional_detail::construct<value_type>(factory, m_storage.address());
        m_initialized = true ;
      }
 
     // Constructs in-place using the given typed factory
     template<class Expr>
     void construct ( Expr const& factory, typed_in_place_factory_base const* )
      {
        factory.apply(m_storage.address()) ;
        m_initialized = true ;
      }
 
     template<class Expr>
     void assign_expr_to_initialized ( Expr const& factory, in_place_factory_base const* tag )
      {
        destroy();
        construct(factory,tag);
      }
 
     // Constructs in-place using the given typed factory
     template<class Expr>
     void assign_expr_to_initialized ( Expr const& factory, typed_in_place_factory_base const* tag )
      {
        destroy();
        construct(factory,tag);
      }
 #endif
 
 #endif
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Constructs using any expression implicitly convertible to the single argument
     // of a one-argument T constructor.
     // Converting constructions of optional<T> from optional<U> uses this function with
     // 'Expr' being of type 'U' and relying on a converting constructor of T from U.
     template<class Expr>
     void construct ( Expr&& expr, void const* )
     {
       new (m_storage.address()) unqualified_value_type(boost::forward<Expr>(expr)) ;
       m_initialized = true ;
     }
 
     // Assigns using a form any expression implicitly convertible to the single argument
     // of a T's assignment operator.
     // Converting assignments of optional<T> from optional<U> uses this function with
     // 'Expr' being of type 'U' and relying on a converting assignment of T from U.
     template<class Expr>
     void assign_expr_to_initialized ( Expr&& expr, void const* )
     {
       assign_value( boost::forward<Expr>(expr) );
     }
 #else
     // Constructs using any expression implicitly convertible to the single argument
     // of a one-argument T constructor.
     // Converting constructions of optional<T> from optional<U> uses this function with
     // 'Expr' being of type 'U' and relying on a converting constructor of T from U.
     template<class Expr>
     void construct ( Expr const& expr, void const* )
      {
        new (m_storage.address()) unqualified_value_type(expr) ;
        m_initialized = true ;
      }
 
     // Assigns using a form any expression implicitly convertible to the single argument
     // of a T's assignment operator.
     // Converting assignments of optional<T> from optional<U> uses this function with
     // 'Expr' being of type 'U' and relying on a converting assignment of T from U.
     template<class Expr>
     void assign_expr_to_initialized ( Expr const& expr, void const* )
      {
        assign_value(expr);
      }
 
 #endif
 
 #ifdef BOOST_OPTIONAL_WEAK_OVERLOAD_RESOLUTION
     // BCB5.64 (and probably lower versions) workaround.
     //   The in-place factories are supported by means of catch-all constructors
     //   and assignment operators (the functions are parameterized in terms of
     //   an arbitrary 'Expr' type)
     //   This compiler incorrectly resolves the overload set and sinks optional<T> and optional<U>
     //   to the 'Expr'-taking functions even though explicit overloads are present for them.
     //   Thus, the following overload is needed to properly handle the case when the 'lhs'
     //   is another optional.
     //
     // For VC<=70 compilers this workaround doesn't work because the compiler issues and error
     // instead of choosing the wrong overload
     //
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Notice that 'Expr' will be optional<T> or optional<U> (but not optional_base<..>)
     template<class Expr>
     void construct ( Expr&& expr, optional_tag const* )
      {
        if ( expr.is_initialized() )
        {
          // An exception can be thrown here.
          // It it happens, THIS will be left uninitialized.
          new (m_storage.address()) unqualified_value_type(boost::move(expr.get())) ;
          m_initialized = true ;
        }
      }
 #else
     // Notice that 'Expr' will be optional<T> or optional<U> (but not optional_base<..>)
     template<class Expr>
     void construct ( Expr const& expr, optional_tag const* )
      {
        if ( expr.is_initialized() )
        {
          // An exception can be thrown here.
          // It it happens, THIS will be left uninitialized.
          new (m_storage.address()) unqualified_value_type(expr.get()) ;
          m_initialized = true ;
        }
      }
 #endif
 #endif // defined BOOST_OPTIONAL_WEAK_OVERLOAD_RESOLUTION
 
     void assign_value ( argument_type val ) { get_impl() = val; }
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     void assign_value ( rval_reference_type val ) { get_impl() = static_cast<rval_reference_type>(val); }
 #endif
 
     void destroy()
     {
       if ( m_initialized )
         destroy_impl() ;
     }
 
     reference_const_type get_impl() const { return m_storage.ref() ; }
     reference_type       get_impl()       { return m_storage.ref() ; }
 
     pointer_const_type get_ptr_impl() const { return m_storage.ptr_ref(); }
     pointer_type       get_ptr_impl()       { return m_storage.ptr_ref(); }
 
   private :
 
 #if BOOST_WORKAROUND(BOOST_MSVC, BOOST_TESTED_AT(1900))
     void destroy_impl ( ) { m_storage.ptr_ref()->~T() ; m_initialized = false ; }
 #else
     void destroy_impl ( ) { m_storage.ref().T::~T() ; m_initialized = false ; }
 #endif
 
     bool m_initialized ;
     storage_type m_storage ;
 } ;
 
 #include <boost/optional/detail/optional_trivially_copyable_base.hpp>
 
 // definition of metafunction is_optional_val_init_candidate
 template <typename U>
 struct is_optional_or_tag
   : boost::conditional< boost::is_base_of<optional_detail::optional_tag, BOOST_DEDUCED_TYPENAME boost::decay<U>::type>::value
                      || boost::is_same<BOOST_DEDUCED_TYPENAME boost::decay<U>::type, none_t>::value
                      || boost::is_same<BOOST_DEDUCED_TYPENAME boost::decay<U>::type, in_place_init_t>::value
                      || boost::is_same<BOOST_DEDUCED_TYPENAME boost::decay<U>::type, in_place_init_if_t>::value,
     boost::true_type, boost::false_type>::type
 {};
 
 template <typename T, typename U>
 struct has_dedicated_constructor
   : boost::disjunction<is_optional_or_tag<U>, boost::is_same<T, BOOST_DEDUCED_TYPENAME boost::decay<U>::type> >
 {};
 
 template <typename U>
 struct is_in_place_factory
   : boost::disjunction< boost::is_base_of<boost::in_place_factory_base, BOOST_DEDUCED_TYPENAME boost::decay<U>::type>,
                         boost::is_base_of<boost::typed_in_place_factory_base, BOOST_DEDUCED_TYPENAME boost::decay<U>::type> >
 {};
 
 #if !defined(BOOST_OPTIONAL_DETAIL_NO_IS_CONSTRUCTIBLE_TRAIT)
 
 template <typename T, typename U>
 struct is_factory_or_constructible_to_T
   : boost::disjunction< is_in_place_factory<U>, boost::is_constructible<T, U&&> >
 {};
 
 template <typename T, typename U>
 struct is_optional_constructible : boost::is_constructible<T, U>
 {};
 
 #else
 
 template <typename, typename>
 struct is_factory_or_constructible_to_T : boost::true_type
 {};
 
 template <typename T, typename U>
 struct is_optional_constructible : boost::true_type
 {};
 
 #endif // is_convertible condition
 
 #if !defined(BOOST_NO_CXX11_DECLTYPE) && !BOOST_WORKAROUND(BOOST_MSVC, < 1800)
 // for is_assignable
 
 #if (!defined BOOST_NO_CXX11_RVALUE_REFERENCES)
 // On some initial rvalue reference implementations GCC does it in a strange way,
 // preferring perfect-forwarding constructor to implicit copy constructor.
 
 template <typename T, typename U>
 struct is_opt_assignable
   : boost::conjunction<boost::is_convertible<U&&, T>, boost::is_assignable<T&, U&&> >
 {};
 
 #else
 
 template <typename T, typename U>
 struct is_opt_assignable
   : boost::conjunction<boost::is_convertible<U, T>, boost::is_assignable<T&, U> >
 {};
 
 #endif
 
 #else
 
 template <typename T, typename U>
 struct is_opt_assignable : boost::is_convertible<U, T>
 {};
 
 #endif
 
 template <typename T, typename U>
 struct is_factory_or_opt_assignable_to_T
   : boost::disjunction< is_in_place_factory<U>, is_opt_assignable<T, U> >
 {};
 
 template <typename T, typename U, bool = has_dedicated_constructor<T, U>::value>
 struct is_optional_val_init_candidate
   : boost::false_type
 {};
 
 template <typename T, typename U>
 struct is_optional_val_init_candidate<T, U, false>
   : is_factory_or_constructible_to_T<T, U>
 {};
 
 template <typename T, typename U, bool = has_dedicated_constructor<T, U>::value>
 struct is_optional_val_assign_candidate
   : boost::false_type
 {};
 
 template <typename T, typename U>
 struct is_optional_val_assign_candidate<T, U, false>
   : is_factory_or_opt_assignable_to_T<T, U>
 {};
 
 } // namespace optional_detail
 
 namespace optional_config {
 
 template <typename T>
 struct optional_uses_direct_storage_for
   : boost::conditional<(boost::is_scalar<T>::value && !boost::is_const<T>::value && !boost::is_volatile<T>::value)
                       , boost::true_type, boost::false_type>::type
 {};
 
 } // namespace optional_config
 
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_DIRECT_STORAGE_SPEC
 #  define BOOST_OPTIONAL_BASE_TYPE(T) boost::conditional< optional_config::optional_uses_direct_storage_for<T>::value, \
                                       optional_detail::tc_optional_base<T>, \
                                       optional_detail::optional_base<T> \
                                       >::type
 #else
 #  define BOOST_OPTIONAL_BASE_TYPE(T) optional_detail::optional_base<T>
 #endif
 
 template<class T>
 class optional
   : public BOOST_OPTIONAL_BASE_TYPE(T)
 {
     typedef typename BOOST_OPTIONAL_BASE_TYPE(T) base ;
 
   public :
 
     typedef optional<T> this_type ;
 
     typedef BOOST_DEDUCED_TYPENAME base::value_type           value_type ;
     typedef BOOST_DEDUCED_TYPENAME base::reference_type       reference_type ;
     typedef BOOST_DEDUCED_TYPENAME base::reference_const_type reference_const_type ;
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     typedef BOOST_DEDUCED_TYPENAME base::rval_reference_type  rval_reference_type ;
     typedef BOOST_DEDUCED_TYPENAME base::reference_type_of_temporary_wrapper reference_type_of_temporary_wrapper ;
 #endif
     typedef BOOST_DEDUCED_TYPENAME base::pointer_type         pointer_type ;
     typedef BOOST_DEDUCED_TYPENAME base::pointer_const_type   pointer_const_type ;
     typedef BOOST_DEDUCED_TYPENAME base::argument_type        argument_type ;
 
     // Creates an optional<T> uninitialized.
     // No-throw
     optional() BOOST_NOEXCEPT : base() {}
 
     // Creates an optional<T> uninitialized.
     // No-throw
     optional( none_t none_ ) BOOST_NOEXCEPT : base(none_) {}
 
     // Creates an optional<T> initialized with 'val'.
     // Can throw if T::T(T const&) does
     optional ( argument_type val ) : base(optional_detail::init_value_tag(), val) {}
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Creates an optional<T> initialized with 'move(val)'.
     // Can throw if T::T(T &&) does
     optional ( rval_reference_type val ) : base(optional_detail::init_value_tag(), boost::forward<T>(val))
       {}
 #endif
 
     // Creates an optional<T> initialized with 'val' IFF cond is true, otherwise creates an uninitialized optional.
     // Can throw if T::T(T const&) does
     optional ( bool cond, argument_type val ) : base(cond,val) {}
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     /// Creates an optional<T> initialized with 'val' IFF cond is true, otherwise creates an uninitialized optional.
     // Can throw if T::T(T &&) does
     optional ( bool cond, rval_reference_type val ) : base( cond, boost::forward<T>(val) )
       {}
 #endif
 
     // NOTE: MSVC needs templated versions first
 
     // Creates a deep copy of another convertible optional<U>
     // Requires a valid conversion from U to T.
     // Can throw if T::T(U const&) does
     template<class U>
     explicit optional ( optional<U> const& rhs
 #ifndef BOOST_OPTIONAL_DETAIL_NO_SFINAE_FRIENDLY_CONSTRUCTORS
                         ,BOOST_DEDUCED_TYPENAME boost::enable_if< optional_detail::is_optional_constructible<T, U const&>, bool>::type = true
 #endif
                       )
       :
       base()
     {
       if ( rhs.is_initialized() )
         this->construct(rhs.get());
     }
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Creates a deep move of another convertible optional<U>
     // Requires a valid conversion from U to T.
     // Can throw if T::T(U&&) does
     template<class U>
     explicit optional ( optional<U> && rhs
 #ifndef BOOST_OPTIONAL_DETAIL_NO_SFINAE_FRIENDLY_CONSTRUCTORS
                         ,BOOST_DEDUCED_TYPENAME boost::enable_if< optional_detail::is_optional_constructible<T, U>, bool>::type = true
 #endif
                       )
       :
       base()
     {
       if ( rhs.is_initialized() )
         this->construct( boost::move(rhs.get()) );
     }
 #endif
 
 #ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
     // Creates an optional<T> with an expression which can be either
     //  (a) An instance of InPlaceFactory (i.e. in_place(a,b,...,n);
     //  (b) An instance of TypedInPlaceFactory ( i.e. in_place<T>(a,b,...,n);
     //  (c) Any expression implicitly convertible to the single type
     //      of a one-argument T's constructor.
     //  (d*) Weak compilers (BCB) might also resolved Expr as optional<T> and optional<U>
     //       even though explicit overloads are present for these.
     // Depending on the above some T ctor is called.
     // Can throw if the resolved T ctor throws.
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 
 
   template<class Expr>
   explicit optional ( Expr&& expr,
                       BOOST_DEDUCED_TYPENAME boost::enable_if< optional_detail::is_optional_val_init_candidate<T, Expr>, bool>::type = true
   )
     : base(boost::forward<Expr>(expr),boost::addressof(expr))
     {}
 
 #else
     template<class Expr>
     explicit optional ( Expr const& expr ) : base(expr,boost::addressof(expr)) {}
 #endif // !defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 #endif // !defined BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
 
     // Creates a deep copy of another optional<T>
     // Can throw if T::T(T const&) does
 #ifndef BOOST_OPTIONAL_DETAIL_NO_DEFAULTED_MOVE_FUNCTIONS
     optional ( optional const& ) = default;
 #else
     optional ( optional const& rhs ) : base( static_cast<base const&>(rhs) ) {}
 #endif
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Creates a deep move of another optional<T>
     // Can throw if T::T(T&&) does
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_DEFAULTED_MOVE_FUNCTIONS
     optional ( optional && ) = default;
 #else
     optional ( optional && rhs )
       BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value)
       : base( boost::move(rhs) )
     {}
 #endif
 
 #endif
 
 #if BOOST_WORKAROUND(_MSC_VER, <= 1600)
     //  On old MSVC compilers the implicitly declared dtor is not called
     ~optional() {}
 #endif
 
 
 #if !defined(BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT) && !defined(BOOST_OPTIONAL_WEAK_OVERLOAD_RESOLUTION)
     // Assigns from an expression. See corresponding constructor.
     // Basic Guarantee: If the resolved T ctor throws, this is left UNINITIALIZED
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 
     template<class Expr>
     BOOST_DEDUCED_TYPENAME boost::enable_if<optional_detail::is_optional_val_assign_candidate<T, Expr>, optional&>::type
     operator= ( Expr&& expr )
       {
         this->assign_expr(boost::forward<Expr>(expr),boost::addressof(expr));
         return *this ;
       }
 
 #else
     template<class Expr>
     optional& operator= ( Expr const& expr )
       {
         this->assign_expr(expr,boost::addressof(expr));
         return *this ;
       }
 #endif // !defined  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 #endif // !defined(BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT) && !defined(BOOST_OPTIONAL_WEAK_OVERLOAD_RESOLUTION)
 
     // Copy-assigns from another convertible optional<U> (converts && deep-copies the rhs value)
     // Requires a valid conversion from U to T.
     // Basic Guarantee: If T::T( U const& ) throws, this is left UNINITIALIZED
     template<class U>
     optional& operator= ( optional<U> const& rhs )
       {
         this->assign(rhs);
         return *this ;
       }
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Move-assigns from another convertible optional<U> (converts && deep-moves the rhs value)
     // Requires a valid conversion from U to T.
     // Basic Guarantee: If T::T( U && ) throws, this is left UNINITIALIZED
     template<class U>
     optional& operator= ( optional<U> && rhs )
       {
         this->assign(boost::move(rhs));
         return *this ;
       }
 #endif
 
     // Assigns from another optional<T> (deep-copies the rhs value)
     // Basic Guarantee: If T::T( T const& ) throws, this is left UNINITIALIZED
     //  (NOTE: On BCB, this operator is not actually called and left is left UNMODIFIED in case of a throw)
 #ifndef BOOST_OPTIONAL_DETAIL_NO_DEFAULTED_MOVE_FUNCTIONS
     optional& operator= ( optional const& rhs ) = default;
 #else
     optional& operator= ( optional const& rhs )
       {
         this->assign( static_cast<base const&>(rhs) ) ;
         return *this ;
       }
 #endif
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Assigns from another optional<T> (deep-moves the rhs value)
 #ifndef BOOST_OPTIONAL_DETAIL_NO_DEFAULTED_MOVE_FUNCTIONS
     optional& operator= ( optional && ) = default;
 #else
     optional& operator= ( optional && rhs )
       BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value && ::boost::is_nothrow_move_assignable<T>::value)
       {
         this->assign( static_cast<base &&>(rhs) ) ;
         return *this ;
       }
 #endif
 
 #endif // BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 
 #ifndef BOOST_NO_CXX11_UNIFIED_INITIALIZATION_SYNTAX
 
     // Assigns from a T (deep-moves/copies the rhs value)
     template <typename T_>
     BOOST_DEDUCED_TYPENAME boost::enable_if<boost::is_same<T, BOOST_DEDUCED_TYPENAME boost::decay<T_>::type>, optional&>::type
     operator= ( T_&& val )
       {
         this->assign( boost::forward<T_>(val) ) ;
         return *this ;
       }
 
 #else
 
     // Assigns from a T (deep-copies the rhs value)
     // Basic Guarantee: If T::( T const& ) throws, this is left UNINITIALIZED
     optional& operator= ( argument_type val )
       {
         this->assign( val ) ;
         return *this ;
       }
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     // Assigns from a T (deep-moves the rhs value)
     optional& operator= ( rval_reference_type val )
       {
         this->assign( boost::move(val) ) ;
         return *this ;
       }
 #endif
 
 #endif // BOOST_NO_CXX11_UNIFIED_INITIALIZATION_SYNTAX
 
     // Assigns from a "none"
     // Which destroys the current value, if any, leaving this UNINITIALIZED
     // No-throw (assuming T::~T() doesn't)
     optional& operator= ( none_t none_ ) BOOST_NOEXCEPT
       {
         this->assign( none_ ) ;
         return *this ;
       }
 
 #if (!defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES) && (!defined BOOST_NO_CXX11_VARIADIC_TEMPLATES)
     // Constructs in-place
     // upon exception *this is always uninitialized
     template<class... Args>
     void emplace ( Args&&... args )
     {
       this->emplace_assign( boost::forward<Args>(args)... );
     }
 
     template<class... Args>
     explicit optional ( in_place_init_t, Args&&... args )
     : base( in_place_init, boost::forward<Args>(args)... )
     {}
 
     template<class... Args>
     explicit optional ( in_place_init_if_t, bool cond, Args&&... args )
     : base( in_place_init_if, cond, boost::forward<Args>(args)... )
     {}
 
 #elif (!defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES)
     template<class Arg>
     void emplace ( Arg&& arg )
      {
        this->emplace_assign( boost::forward<Arg>(arg) );
      }
 
     void emplace ()
      {
        this->emplace_assign();
      }
 
     template<class Args>
     explicit optional ( in_place_init_t, Args&& args )
     : base( in_place_init, boost::forward<Args>(args) )
     {}
 
     explicit optional ( in_place_init_t )
     : base( in_place_init )
     {}
 
     template<class Args>
     explicit optional ( in_place_init_if_t, bool cond, Args&& args )
     : base( in_place_init_if, cond, boost::forward<Args>(args) )
     {}
 
     explicit optional ( in_place_init_if_t, bool cond )
     : base( in_place_init_if, cond )
     {}
 #else
     template<class Arg>
     void emplace ( const Arg& arg )
      {
        this->emplace_assign( arg );
      }
 
     template<class Arg>
     void emplace ( Arg& arg )
      {
        this->emplace_assign( arg );
      }
 
     void emplace ()
      {
        this->emplace_assign();
      }
 
     template<class Arg>
     explicit optional ( in_place_init_t, const Arg& arg )
     : base( in_place_init, arg )
     {}
 
     template<class Arg>
     explicit optional ( in_place_init_t, Arg& arg )
     : base( in_place_init, arg )
     {}
 
     explicit optional ( in_place_init_t )
     : base( in_place_init )
     {}
 
     template<class Arg>
     explicit optional ( in_place_init_if_t, bool cond, const Arg& arg )
     : base( in_place_init_if, cond, arg )
     {}
 
     template<class Arg>
     explicit optional ( in_place_init_if_t, bool cond, Arg& arg )
     : base( in_place_init_if, cond, arg )
     {}
 
     explicit optional ( in_place_init_if_t, bool cond )
     : base( in_place_init_if, cond )
     {}
 #endif
 
     void swap( optional & arg )
       BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value && ::boost::is_nothrow_move_assignable<T>::value)
       {
         // allow for Koenig lookup
         boost::core::invoke_swap(*this, arg);
       }
 
 
     // Returns a reference to the value if this is initialized, otherwise,
     // the behaviour is UNDEFINED
     // No-throw
     reference_const_type get() const { BOOST_ASSERT(this->is_initialized()) ; return this->get_impl(); }
     reference_type       get()       { BOOST_ASSERT(this->is_initialized()) ; return this->get_impl(); }
 
     // Returns a copy of the value if this is initialized, 'v' otherwise
     reference_const_type get_value_or ( reference_const_type v ) const { return this->is_initialized() ? get() : v ; }
     reference_type       get_value_or ( reference_type       v )       { return this->is_initialized() ? get() : v ; }
 
     // Returns a pointer to the value if this is initialized, otherwise,
     // the behaviour is UNDEFINED
     // No-throw
     pointer_const_type operator->() const { BOOST_ASSERT(this->is_initialized()) ; return this->get_ptr_impl() ; }
     pointer_type       operator->()       { BOOST_ASSERT(this->is_initialized()) ; return this->get_ptr_impl() ; }
 
     // Returns a reference to the value if this is initialized, otherwise,
     // the behaviour is UNDEFINED
     // No-throw
 #if (!defined BOOST_NO_CXX11_REF_QUALIFIERS) && (!defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES)
     reference_const_type operator *() const& { return this->get() ; }
     reference_type       operator *() &      { return this->get() ; }
     reference_type_of_temporary_wrapper operator *() && { return boost::move(this->get()) ; }
 #else
     reference_const_type operator *() const { return this->get() ; }
     reference_type       operator *()       { return this->get() ; }
 #endif // !defined BOOST_NO_CXX11_REF_QUALIFIERS
 
 #if (!defined BOOST_NO_CXX11_REF_QUALIFIERS) && (!defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES)
     reference_const_type value() const&
       {
         if (this->is_initialized())
           return this->get() ;
         else
           throw_exception(bad_optional_access());
       }
 
     reference_type value() &
       {
         if (this->is_initialized())
           return this->get() ;
         else
           throw_exception(bad_optional_access());
       }
 
     reference_type_of_temporary_wrapper value() &&
       {
         if (this->is_initialized())
           return boost::move(this->get()) ;
         else
           throw_exception(bad_optional_access());
       }
 
 #else
     reference_const_type value() const
       {
         if (this->is_initialized())
           return this->get() ;
         else
           throw_exception(bad_optional_access());
       }
 
     reference_type value()
       {
         if (this->is_initialized())
           return this->get() ;
         else
           throw_exception(bad_optional_access());
       }
 #endif
 
 
 #ifndef BOOST_NO_CXX11_REF_QUALIFIERS
     template <class U>
     value_type value_or ( U&& v ) const&
       {
         if (this->is_initialized())
           return get();
         else
           return boost::forward<U>(v);
       }
 
     template <class U>
     value_type value_or ( U&& v ) &&
       {
         if (this->is_initialized())
           return boost::move(get());
         else
           return boost::forward<U>(v);
       }
 #elif !defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
     template <class U>
     value_type value_or ( U&& v ) const
       {
         if (this->is_initialized())
           return get();
         else
           return boost::forward<U>(v);
       }
 #else
     template <class U>
     value_type value_or ( U const& v ) const
       {
         if (this->is_initialized())
           return get();
         else
           return v;
       }
 
     template <class U>
     value_type value_or ( U& v ) const
       {
         if (this->is_initialized())
           return get();
         else
           return v;
       }
 #endif
 
 
 #if (!defined BOOST_NO_CXX11_REF_QUALIFIERS) && (!defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES)
     template <typename F>
     value_type value_or_eval ( F f ) const&
       {
         if (this->is_initialized())
           return get();
         else
           return f();
       }
 
     template <typename F>
     value_type value_or_eval ( F f ) &&
       {
         if (this->is_initialized())
           return boost::move(get());
         else
           return f();
       }
 
     template <typename F>
     optional<typename boost::result_of<F(reference_type)>::type> map(F f) &
       {
         if (this->has_value())
           return f(get());
         else
           return none;
       }
 
     template <typename F>
     optional<typename boost::result_of<F(reference_const_type)>::type> map(F f) const&
       {
         if (this->has_value())
           return f(get());
         else
           return none;
       }
 
     template <typename F>
     optional<typename boost::result_of<F(reference_type_of_temporary_wrapper)>::type> map(F f) &&
       {
         if (this->has_value())
           return f(boost::move(this->get()));
         else
           return none;
       }
 
     template <typename F>
     optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_type)>::type>::type> flat_map(F f) &
       {
         if (this->has_value())
           return f(get());
         else
           return none;
       }
 
     template <typename F>
     optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_const_type)>::type>::type> flat_map(F f) const&
       {
         if (this->has_value())
           return f(get());
         else
           return none;
       }
 
     template <typename F>
     optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_type_of_temporary_wrapper)>::type>::type> flat_map(F f) &&
       {
         if (this->has_value())
           return f(boost::move(get()));
         else
           return none;
       }
 
 #else
     template <typename F>
     value_type value_or_eval ( F f ) const
       {
         if (this->is_initialized())
           return get();
         else
           return f();
       }
 
     template <typename F>
     optional<typename boost::result_of<F(reference_type)>::type> map(F f)
       {
         if (this->has_value())
           return f(get());
         else
           return none;
       }
 
     template <typename F>
     optional<typename boost::result_of<F(reference_const_type)>::type> map(F f) const
       {
         if (this->has_value())
           return f(get());
         else
           return none;
       }
 
     template <typename F>
     optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_type)>::type>::type> flat_map(F f)
       {
         if (this->has_value())
           return f(get());
         else
           return none;
       }
 
     template <typename F>
     optional<typename optional_detail::optional_value_type<typename boost::result_of<F(reference_const_type)>::type>::type> flat_map(F f) const
       {
         if (this->has_value())
           return f(get());
         else
           return none;
       }
 
 #endif
 
     bool has_value() const BOOST_NOEXCEPT { return this->is_initialized() ; }
 
     bool operator!() const BOOST_NOEXCEPT { return !this->is_initialized() ; }
 
     BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT()
 } ;
 
 } // namespace boost
 
 #endif // BOOST_OPTIONAL_CONFIG_USE_OLD_IMPLEMENTATION_OF_OPTIONAL
 
 namespace boost {
 
 #ifndef  BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 template<class T>
 class optional<T&&>
 {
   BOOST_STATIC_ASSERT_MSG(sizeof(T) == 0, "Optional rvalue references are illegal.");
 } ;
 #endif
 
 } // namespace boost
 
 #ifndef BOOST_OPTIONAL_CONFIG_DONT_SPECIALIZE_OPTIONAL_REFS
 # include <boost/optional/detail/optional_reference_spec.hpp>
 #endif
 
 namespace boost {
 
 #ifndef BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 
 template<class T>
 inline
 optional<BOOST_DEDUCED_TYPENAME boost::decay<T>::type> make_optional ( T && v  )
 {
   return optional<BOOST_DEDUCED_TYPENAME boost::decay<T>::type>(boost::forward<T>(v));
 }
 
 // Returns optional<T>(cond,v)
 template<class T>
 inline
 optional<BOOST_DEDUCED_TYPENAME boost::decay<T>::type> make_optional ( bool cond, T && v )
 {
   return optional<BOOST_DEDUCED_TYPENAME boost::decay<T>::type>(cond,boost::forward<T>(v));
 }
 
 #else
 
 // Returns optional<T>(v)
 template<class T>
 inline
 optional<T> make_optional ( T const& v  )
 {
   return optional<T>(v);
 }
 
 // Returns optional<T>(cond,v)
 template<class T>
 inline
 optional<T> make_optional ( bool cond, T const& v )
 {
   return optional<T>(cond,v);
 }
 
 #endif // BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES
 
 // Returns a reference to the value if this is initialized, otherwise, the behaviour is UNDEFINED.
 // No-throw
 template<class T>
 inline
 BOOST_DEDUCED_TYPENAME optional<T>::reference_const_type
 get ( optional<T> const& opt )
 {
   return opt.get() ;
 }
 
 template<class T>
 inline
 BOOST_DEDUCED_TYPENAME optional<T>::reference_type
 get ( optional<T>& opt )
 {
   return opt.get() ;
 }
 
 // Returns a pointer to the value if this is initialized, otherwise, returns NULL.
 // No-throw
 template<class T>
 inline
 BOOST_DEDUCED_TYPENAME optional<T>::pointer_const_type
 get ( optional<T> const* opt )
 {
   return opt->get_ptr() ;
 }
 
 template<class T>
 inline
 BOOST_DEDUCED_TYPENAME optional<T>::pointer_type
 get ( optional<T>* opt )
 {
   return opt->get_ptr() ;
 }
 
 // Returns a reference to the value if this is initialized, otherwise, the behaviour is UNDEFINED.
 // No-throw
 template<class T>
 inline
 BOOST_DEDUCED_TYPENAME optional<T>::reference_const_type
 get_optional_value_or ( optional<T> const& opt, BOOST_DEDUCED_TYPENAME optional<T>::reference_const_type v )
 {
   return opt.get_value_or(v) ;
 }
 
 template<class T>
 inline
 BOOST_DEDUCED_TYPENAME optional<T>::reference_type
 get_optional_value_or ( optional<T>& opt, BOOST_DEDUCED_TYPENAME optional<T>::reference_type v )
 {
   return opt.get_value_or(v) ;
 }
 
 // Returns a pointer to the value if this is initialized, otherwise, returns NULL.
 // No-throw
 template<class T>
 inline
 BOOST_DEDUCED_TYPENAME optional<T>::pointer_const_type
 get_pointer ( optional<T> const& opt )
 {
   return opt.get_ptr() ;
 }
 
 template<class T>
 inline
 BOOST_DEDUCED_TYPENAME optional<T>::pointer_type
 get_pointer ( optional<T>& opt )
 {
   return opt.get_ptr() ;
 }
 
 } // namespace boost
 
 #ifndef BOOST_NO_IOSTREAM
 namespace boost {
 
 // The following declaration prevents a bug where operator safe-bool is used upon streaming optional object if you forget the IO header.
 template<class CharType, class CharTrait>
 std::basic_ostream<CharType, CharTrait>&
 operator<<(std::basic_ostream<CharType, CharTrait>& os, optional_detail::optional_tag const&)
 {
   BOOST_STATIC_ASSERT_MSG(sizeof(CharType) == 0, "If you want to output boost::optional, include header <boost/optional/optional_io.hpp>");
   return os;
 }
 
 } // namespace boost
 #endif // BOOST_NO_IOSTREAM
 
 #include <boost/optional/detail/optional_relops.hpp>
 #include <boost/optional/detail/optional_swap.hpp>
 
 #endif // header guard

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