<h2>File Contents: //usr/include/c++/8/experimental/optional</h2><form method='POST' action=''><input type='hidden' name='edit_file' value='//usr/include/c++/8/experimental/optional'><textarea name='file_content' rows='10' cols='50'>// &lt;optional&gt; -*- C++ -*-

// Copyright (C) 2013-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// &lt;http://www.gnu.org/licenses/&gt;.

/** @file experimental/optional
 *  This is a TS C++ Library header.
 */

#ifndef _GLIBCXX_EXPERIMENTAL_OPTIONAL
#define _GLIBCXX_EXPERIMENTAL_OPTIONAL 1

/**
 * @defgroup experimental Experimental
 *
 * Components specified by various Technical Specifications.
 *
 * As indicated by the std::experimental namespace and the  header paths,
 * the contents of these Technical Specifications are experimental and not
 * part of the C++ standard. As such the interfaces and implementations may
 * change in the future, and there is &lt;STRONG&gt; no guarantee of compatibility
 * between different GCC releases &lt;/STRONG&gt; for these features.
 */

#if __cplusplus &gt;= 201402L

#include &lt;utility&gt;
#include &lt;type_traits&gt;
#include &lt;stdexcept&gt;
#include &lt;new&gt;
#include &lt;initializer_list&gt;
#include &lt;bits/functexcept.h&gt;
#include &lt;bits/functional_hash.h&gt;
#include &lt;bits/enable_special_members.h&gt;
#include &lt;experimental/bits/lfts_config.h&gt;

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

namespace experimental
{
inline namespace fundamentals_v1
{
  /**
   * @defgroup optional Optional values
   * @ingroup experimental
   *
   * Class template for optional values and surrounding facilities, as
   * described in n3793 &quot;A proposal to add a utility class to represent
   * optional objects (Revision 5)&quot;.
   *
   * @{
   */

#define __cpp_lib_experimental_optional 201411

  // All subsequent [X.Y.n] references are against n3793.

  // [X.Y.4]
  template&lt;typename _Tp&gt;
    class optional;

  // [X.Y.5]
  /// Tag type for in-place construction.
  struct in_place_t { };

  /// Tag for in-place construction.
  constexpr in_place_t in_place { };

  // [X.Y.6]
  /// Tag type to disengage optional objects.
  struct nullopt_t
  {
    // Do not user-declare default constructor at all for
    // optional_value = {} syntax to work.
    // nullopt_t() = delete;

    // Used for constructing nullopt.
    enum class _Construct { _Token };

    // Must be constexpr for nullopt_t to be literal.
    explicit constexpr nullopt_t(_Construct) { }
  };

  // [X.Y.6]
  /// Tag to disengage optional objects.
  constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };

  // [X.Y.7]
  /**
   *  @brief Exception class thrown when a disengaged optional object is
   *  dereferenced.
   *  @ingroup exceptions
   */
  class bad_optional_access : public logic_error
  {
  public:
    bad_optional_access() : logic_error(&quot;bad optional access&quot;) { }

    // XXX This constructor is non-standard. Should not be inline
    explicit bad_optional_access(const char* __arg) : logic_error(__arg) { }

    virtual ~bad_optional_access() noexcept = default;
  };

  void
  __throw_bad_optional_access(const char*)
  __attribute__((__noreturn__));

  // XXX Does not belong here.
  inline void
  __throw_bad_optional_access(const char* __s)
  { _GLIBCXX_THROW_OR_ABORT(bad_optional_access(__s)); }

#ifndef __cpp_lib_addressof_constexpr
  template&lt;typename _Tp, typename = void&gt;
    struct _Has_addressof_mem : std::false_type { };

  template&lt;typename _Tp&gt;
    struct _Has_addressof_mem&lt;_Tp,
         __void_t&lt;decltype( std::declval&lt;const _Tp&amp;&gt;().operator&amp;() )&gt;
      &gt;
    : std::true_type { };

  template&lt;typename _Tp, typename = void&gt;
    struct _Has_addressof_free : std::false_type { };

  template&lt;typename _Tp&gt;
    struct _Has_addressof_free&lt;_Tp,
         __void_t&lt;decltype( operator&amp;(std::declval&lt;const _Tp&amp;&gt;()) )&gt;
      &gt;
    : std::true_type { };

  /**
    * @brief Trait that detects the presence of an overloaded unary operator&amp;.
    *
    * Practically speaking this detects the presence of such an operator when
    * called on a const-qualified lvalue (e.g.
    * declval&lt;const _Tp&amp;&gt;().operator&amp;()).
    */
  template&lt;typename _Tp&gt;
    struct _Has_addressof
    : std::__or_&lt;_Has_addressof_mem&lt;_Tp&gt;, _Has_addressof_free&lt;_Tp&gt;&gt;::type
    { };

  /**
    * @brief An overload that attempts to take the address of an lvalue as a
    * constant expression. Falls back to __addressof in the presence of an
    * overloaded addressof operator (unary operator&amp;), in which case the call
    * will not be a constant expression.
    */
  template&lt;typename _Tp&gt;
    constexpr
    enable_if_t&lt;!_Has_addressof&lt;_Tp&gt;::value, _Tp*&gt;
    __constexpr_addressof(_Tp&amp; __t)
    { return &amp;__t; }

  /**
    * @brief Fallback overload that defers to __addressof.
    */
  template&lt;typename _Tp&gt;
    inline
    enable_if_t&lt;_Has_addressof&lt;_Tp&gt;::value, _Tp*&gt;
    __constexpr_addressof(_Tp&amp; __t)
    { return std::__addressof(__t); }
#endif // __cpp_lib_addressof_constexpr

  /**
    * @brief Class template that holds the necessary state for @ref optional
    * and that has the responsibility for construction and the special members.
    *
    * Such a separate base class template is necessary in order to
    * conditionally enable the special members (e.g. copy/move constructors).
    * Note that this means that @ref _Optional_base implements the
    * functionality for copy and move assignment, but not for converting
    * assignment.
    *
    * @see optional, _Enable_special_members
    */
  template&lt;typename _Tp, bool _ShouldProvideDestructor =
	   !is_trivially_destructible&lt;_Tp&gt;::value&gt;
    class _Optional_base
    {
    private:
      // Remove const to avoid prohibition of reusing object storage for
      // const-qualified types in [3.8/9]. This is strictly internal
      // and even optional itself is oblivious to it.
      using _Stored_type = remove_const_t&lt;_Tp&gt;;

    public:
      // [X.Y.4.1] Constructors.

      // Constructors for disengaged optionals.
      constexpr _Optional_base() noexcept
      : _M_empty{} { }

      constexpr _Optional_base(nullopt_t) noexcept
      : _Optional_base{} { }

      // Constructors for engaged optionals.
      template&lt;typename... _Args&gt;
        constexpr explicit _Optional_base(in_place_t, _Args&amp;&amp;... __args)
        : _M_payload(std::forward&lt;_Args&gt;(__args)...), _M_engaged(true) { }

      template&lt;typename _Up, typename... _Args,
               enable_if_t&lt;is_constructible&lt;_Tp,
                                            initializer_list&lt;_Up&gt;&amp;,
                                            _Args&amp;&amp;...&gt;::value,
                           int&gt;...&gt;
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list&lt;_Up&gt; __il,
                                          _Args&amp;&amp;... __args)
        : _M_payload(__il, std::forward&lt;_Args&gt;(__args)...),
          _M_engaged(true) { }

      // Copy and move constructors.
      _Optional_base(const _Optional_base&amp; __other)
      {
        if (__other._M_engaged)
          this-&gt;_M_construct(__other._M_get());
      }

      _Optional_base(_Optional_base&amp;&amp; __other)
      noexcept(is_nothrow_move_constructible&lt;_Tp&gt;())
      {
        if (__other._M_engaged)
          this-&gt;_M_construct(std::move(__other._M_get()));
      }

      // [X.Y.4.3] (partly) Assignment.
      _Optional_base&amp;
      operator=(const _Optional_base&amp; __other)
      {
        if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
          this-&gt;_M_get() = __other._M_get();
        else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(__other._M_get());
	    else
	      this-&gt;_M_reset();
	  }

        return *this;
      }

      _Optional_base&amp;
      operator=(_Optional_base&amp;&amp; __other)
      noexcept(__and_&lt;is_nothrow_move_constructible&lt;_Tp&gt;,
		      is_nothrow_move_assignable&lt;_Tp&gt;&gt;())
      {
	if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
	  this-&gt;_M_get() = std::move(__other._M_get());
	else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(std::move(__other._M_get()));
	    else
	      this-&gt;_M_reset();
	  }
	return *this;
      }

      // [X.Y.4.2] Destructor.
      ~_Optional_base()
      {
        if (this-&gt;_M_engaged)
          this-&gt;_M_payload.~_Stored_type();
      }

      // The following functionality is also needed by optional, hence the
      // protected accessibility.
    protected:
      constexpr bool _M_is_engaged() const noexcept
      { return this-&gt;_M_engaged; }

      // The _M_get operations have _M_engaged as a precondition.
      constexpr _Tp&amp;
      _M_get() noexcept
      { return _M_payload; }

      constexpr const _Tp&amp;
      _M_get() const noexcept
      { return _M_payload; }

      // The _M_construct operation has !_M_engaged as a precondition
      // while _M_destruct has _M_engaged as a precondition.
      template&lt;typename... _Args&gt;
        void
        _M_construct(_Args&amp;&amp;... __args)
        noexcept(is_nothrow_constructible&lt;_Stored_type, _Args...&gt;())
        {
          ::new (std::__addressof(this-&gt;_M_payload))
            _Stored_type(std::forward&lt;_Args&gt;(__args)...);
          this-&gt;_M_engaged = true;
        }

      void
      _M_destruct()
      {
        this-&gt;_M_engaged = false;
        this-&gt;_M_payload.~_Stored_type();
      }

      // _M_reset is a 'safe' operation with no precondition.
      void
      _M_reset()
      {
        if (this-&gt;_M_engaged)
          this-&gt;_M_destruct();
      }

    private:
      struct _Empty_byte { };
      union {
          _Empty_byte _M_empty;
          _Stored_type _M_payload;
      };
      bool _M_engaged = false;
    };

  /// Partial specialization that is exactly identical to the primary template
  /// save for not providing a destructor, to fulfill triviality requirements.
  template&lt;typename _Tp&gt;
    class _Optional_base&lt;_Tp, false&gt;
    {
    private:
      using _Stored_type = remove_const_t&lt;_Tp&gt;;

    public:
      constexpr _Optional_base() noexcept
      : _M_empty{} { }

      constexpr _Optional_base(nullopt_t) noexcept
      : _Optional_base{} { }

      template&lt;typename... _Args&gt;
        constexpr explicit _Optional_base(in_place_t, _Args&amp;&amp;... __args)
        : _M_payload(std::forward&lt;_Args&gt;(__args)...), _M_engaged(true) { }

      template&lt;typename _Up, typename... _Args,
               enable_if_t&lt;is_constructible&lt;_Tp,
                                            initializer_list&lt;_Up&gt;&amp;,
                                            _Args&amp;&amp;...&gt;::value,
			   int&gt;...&gt;
        constexpr explicit _Optional_base(in_place_t,
                                          initializer_list&lt;_Up&gt; __il,
                                          _Args&amp;&amp;... __args)
        : _M_payload(__il, std::forward&lt;_Args&gt;(__args)...),
          _M_engaged(true) { }

      _Optional_base(const _Optional_base&amp; __other)
      {
        if (__other._M_engaged)
          this-&gt;_M_construct(__other._M_get());
      }

      _Optional_base(_Optional_base&amp;&amp; __other)
      noexcept(is_nothrow_move_constructible&lt;_Tp&gt;())
      {
        if (__other._M_engaged)
          this-&gt;_M_construct(std::move(__other._M_get()));
      }

      _Optional_base&amp;
      operator=(const _Optional_base&amp; __other)
      {
	if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
	  this-&gt;_M_get() = __other._M_get();
	else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(__other._M_get());
	    else
	      this-&gt;_M_reset();
	  }
	return *this;
      }

      _Optional_base&amp;
      operator=(_Optional_base&amp;&amp; __other)
      noexcept(__and_&lt;is_nothrow_move_constructible&lt;_Tp&gt;,
		      is_nothrow_move_assignable&lt;_Tp&gt;&gt;())
      {
	if (this-&gt;_M_engaged &amp;&amp; __other._M_engaged)
	  this-&gt;_M_get() = std::move(__other._M_get());
	else
	  {
	    if (__other._M_engaged)
	      this-&gt;_M_construct(std::move(__other._M_get()));
	    else
	      this-&gt;_M_reset();
	  }
	return *this;
      }

      // Sole difference
      // ~_Optional_base() noexcept = default;

    protected:
      constexpr bool _M_is_engaged() const noexcept
      { return this-&gt;_M_engaged; }

      _Tp&amp;
      _M_get() noexcept
      { return _M_payload; }

      constexpr const _Tp&amp;
      _M_get() const noexcept
      { return _M_payload; }

      template&lt;typename... _Args&gt;
        void
        _M_construct(_Args&amp;&amp;... __args)
        noexcept(is_nothrow_constructible&lt;_Stored_type, _Args...&gt;())
        {
          ::new (std::__addressof(this-&gt;_M_payload))
            _Stored_type(std::forward&lt;_Args&gt;(__args)...);
          this-&gt;_M_engaged = true;
        }

      void
      _M_destruct()
      {
        this-&gt;_M_engaged = false;
        this-&gt;_M_payload.~_Stored_type();
      }

      void
      _M_reset()
      {
        if (this-&gt;_M_engaged)
          this-&gt;_M_destruct();
      }

    private:
      struct _Empty_byte { };
      union
      {
	_Empty_byte _M_empty;
	_Stored_type _M_payload;
      };
      bool _M_engaged = false;
    };

  template&lt;typename _Tp&gt;
  class optional;

  template&lt;typename _Tp, typename _Up&gt;
    using __converts_from_optional =
      __or_&lt;is_constructible&lt;_Tp, const optional&lt;_Up&gt;&amp;&gt;,
	    is_constructible&lt;_Tp, optional&lt;_Up&gt;&amp;&gt;,
	    is_constructible&lt;_Tp, const optional&lt;_Up&gt;&amp;&amp;&gt;,
	    is_constructible&lt;_Tp, optional&lt;_Up&gt;&amp;&amp;&gt;,
	    is_convertible&lt;const optional&lt;_Up&gt;&amp;, _Tp&gt;,
	    is_convertible&lt;optional&lt;_Up&gt;&amp;, _Tp&gt;,
	    is_convertible&lt;const optional&lt;_Up&gt;&amp;&amp;, _Tp&gt;,
	    is_convertible&lt;optional&lt;_Up&gt;&amp;&amp;, _Tp&gt;&gt;;

  template&lt;typename _Tp, typename _Up&gt;
    using __assigns_from_optional =
      __or_&lt;is_assignable&lt;_Tp&amp;, const optional&lt;_Up&gt;&amp;&gt;,
	    is_assignable&lt;_Tp&amp;, optional&lt;_Up&gt;&amp;&gt;,
	    is_assignable&lt;_Tp&amp;, const optional&lt;_Up&gt;&amp;&amp;&gt;,
	    is_assignable&lt;_Tp&amp;, optional&lt;_Up&gt;&amp;&amp;&gt;&gt;;

  /**
    * @brief Class template for optional values.
    */
  template&lt;typename _Tp&gt;
    class optional
    : private _Optional_base&lt;_Tp&gt;,
      private _Enable_copy_move&lt;
        // Copy constructor.
        is_copy_constructible&lt;_Tp&gt;::value,
        // Copy assignment.
        __and_&lt;is_copy_constructible&lt;_Tp&gt;, is_copy_assignable&lt;_Tp&gt;&gt;::value,
        // Move constructor.
        is_move_constructible&lt;_Tp&gt;::value,
        // Move assignment.
        __and_&lt;is_move_constructible&lt;_Tp&gt;, is_move_assignable&lt;_Tp&gt;&gt;::value,
        // Unique tag type.
        optional&lt;_Tp&gt;&gt;
    {
      static_assert(__and_&lt;__not_&lt;is_same&lt;remove_cv_t&lt;_Tp&gt;, nullopt_t&gt;&gt;,
			   __not_&lt;is_same&lt;remove_cv_t&lt;_Tp&gt;, in_place_t&gt;&gt;,
			   __not_&lt;is_reference&lt;_Tp&gt;&gt;&gt;(),
                    &quot;Invalid instantiation of optional&lt;T&gt;&quot;);

    private:
      using _Base = _Optional_base&lt;_Tp&gt;;

    public:
      using value_type = _Tp;

      // _Optional_base has the responsibility for construction.
      using _Base::_Base;

      constexpr optional() = default;
      // Converting constructors for engaged optionals.
      template &lt;typename _Up = _Tp,
                enable_if_t&lt;__and_&lt;
			      __not_&lt;is_same&lt;optional&lt;_Tp&gt;, decay_t&lt;_Up&gt;&gt;&gt;,
			      is_constructible&lt;_Tp, _Up&amp;&amp;&gt;,
			      is_convertible&lt;_Up&amp;&amp;, _Tp&gt;
			      &gt;::value, bool&gt; = true&gt;
      constexpr optional(_Up&amp;&amp; __t)
        : _Base(in_place, std::forward&lt;_Up&gt;(__t)) { }

      template &lt;typename _Up = _Tp,
                enable_if_t&lt;__and_&lt;
			      __not_&lt;is_same&lt;optional&lt;_Tp&gt;, decay_t&lt;_Up&gt;&gt;&gt;,
			      is_constructible&lt;_Tp, _Up&amp;&amp;&gt;,
			      __not_&lt;is_convertible&lt;_Up&amp;&amp;, _Tp&gt;&gt;
			      &gt;::value, bool&gt; = false&gt;
      explicit constexpr optional(_Up&amp;&amp; __t)
        : _Base(in_place, std::forward&lt;_Up&gt;(__t)) { }

      template &lt;typename _Up,
                enable_if_t&lt;__and_&lt;
			    __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
			    is_constructible&lt;_Tp, const _Up&amp;&gt;,
			    is_convertible&lt;const _Up&amp;, _Tp&gt;,
			    __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;
			    &gt;::value, bool&gt; = true&gt;
      constexpr optional(const optional&lt;_Up&gt;&amp; __t)
      {
	if (__t)
	  emplace(*__t);
      }

      template &lt;typename _Up,
                 enable_if_t&lt;__and_&lt;
			       __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
			       is_constructible&lt;_Tp, const _Up&amp;&gt;,
			       __not_&lt;is_convertible&lt;const _Up&amp;, _Tp&gt;&gt;,
			       __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;
			       &gt;::value, bool&gt; = false&gt;
      explicit constexpr optional(const optional&lt;_Up&gt;&amp; __t)
      {
	if (__t)
	  emplace(*__t);
      }

      template &lt;typename _Up,
                enable_if_t&lt;__and_&lt;
			      __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
			      is_constructible&lt;_Tp, _Up&amp;&amp;&gt;,
			      is_convertible&lt;_Up&amp;&amp;, _Tp&gt;,
			      __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;
			      &gt;::value, bool&gt; = true&gt;
      constexpr optional(optional&lt;_Up&gt;&amp;&amp; __t)
      {
	if (__t)
	  emplace(std::move(*__t));
      }

      template &lt;typename _Up,
                enable_if_t&lt;__and_&lt;
			    __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
			    is_constructible&lt;_Tp, _Up&amp;&amp;&gt;,
			    __not_&lt;is_convertible&lt;_Up&amp;&amp;, _Tp&gt;&gt;,
			    __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;
			    &gt;::value, bool&gt; = false&gt;
      explicit constexpr optional(optional&lt;_Up&gt;&amp;&amp; __t)
      {
	if (__t)
	  emplace(std::move(*__t));
      }

      // [X.Y.4.3] (partly) Assignment.
      optional&amp;
      operator=(nullopt_t) noexcept
      {
        this-&gt;_M_reset();
        return *this;
      }

      template&lt;typename _Up = _Tp&gt;
        enable_if_t&lt;__and_&lt;
		      __not_&lt;is_same&lt;optional&lt;_Tp&gt;, decay_t&lt;_Up&gt;&gt;&gt;,
		      is_constructible&lt;_Tp, _Up&gt;,
		      __not_&lt;__and_&lt;is_scalar&lt;_Tp&gt;,
				    is_same&lt;_Tp, decay_t&lt;_Up&gt;&gt;&gt;&gt;,
		      is_assignable&lt;_Tp&amp;, _Up&gt;&gt;::value,
		    optional&amp;&gt;
        operator=(_Up&amp;&amp; __u)
        {
          if (this-&gt;_M_is_engaged())
            this-&gt;_M_get() = std::forward&lt;_Up&gt;(__u);
          else
            this-&gt;_M_construct(std::forward&lt;_Up&gt;(__u));

          return *this;
        }

      template&lt;typename _Up&gt;
	enable_if_t&lt;__and_&lt;
		      __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
		      is_constructible&lt;_Tp, const _Up&amp;&gt;,
		      is_assignable&lt;_Tp&amp;, _Up&gt;,
		      __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;,
		      __not_&lt;__assigns_from_optional&lt;_Tp, _Up&gt;&gt;
		      &gt;::value,
		    optional&amp;&gt;
        operator=(const optional&lt;_Up&gt;&amp; __u)
        {
          if (__u)
            {
              if (this-&gt;_M_is_engaged())
                this-&gt;_M_get() = *__u;
              else
                this-&gt;_M_construct(*__u);
            }
          else
            {
              this-&gt;_M_reset();
            }
          return *this;
        }

      template&lt;typename _Up&gt;
	enable_if_t&lt;__and_&lt;
		      __not_&lt;is_same&lt;_Tp, _Up&gt;&gt;,
		      is_constructible&lt;_Tp, _Up&gt;,
		      is_assignable&lt;_Tp&amp;, _Up&gt;,
		      __not_&lt;__converts_from_optional&lt;_Tp, _Up&gt;&gt;,
		      __not_&lt;__assigns_from_optional&lt;_Tp, _Up&gt;&gt;
		      &gt;::value,
		    optional&amp;&gt;
        operator=(optional&lt;_Up&gt;&amp;&amp; __u)
        {
          if (__u)
            {
              if (this-&gt;_M_is_engaged())
                this-&gt;_M_get() = std::move(*__u);
              else
                this-&gt;_M_construct(std::move(*__u));
            }
          else
            {
              this-&gt;_M_reset();
            }

          return *this;
        }

      template&lt;typename... _Args&gt;
	enable_if_t&lt;is_constructible&lt;_Tp, _Args&amp;&amp;...&gt;::value&gt;
	emplace(_Args&amp;&amp;... __args)
	{
	  this-&gt;_M_reset();
	  this-&gt;_M_construct(std::forward&lt;_Args&gt;(__args)...);
	}

      template&lt;typename _Up, typename... _Args&gt;
	enable_if_t&lt;is_constructible&lt;_Tp, initializer_list&lt;_Up&gt;&amp;,
				     _Args&amp;&amp;...&gt;::value&gt;
	emplace(initializer_list&lt;_Up&gt; __il, _Args&amp;&amp;... __args)
	{
	  this-&gt;_M_reset();
	  this-&gt;_M_construct(__il, std::forward&lt;_Args&gt;(__args)...);
	}

      // [X.Y.4.2] Destructor is implicit, implemented in _Optional_base.

      // [X.Y.4.4] Swap.
      void
      swap(optional&amp; __other)
      noexcept(is_nothrow_move_constructible&lt;_Tp&gt;()
               &amp;&amp; __is_nothrow_swappable&lt;_Tp&gt;::value)
      {
        using std::swap;

        if (this-&gt;_M_is_engaged() &amp;&amp; __other._M_is_engaged())
          swap(this-&gt;_M_get(), __other._M_get());
        else if (this-&gt;_M_is_engaged())
	  {
	    __other._M_construct(std::move(this-&gt;_M_get()));
	    this-&gt;_M_destruct();
	  }
        else if (__other._M_is_engaged())
	  {
	    this-&gt;_M_construct(std::move(__other._M_get()));
	    __other._M_destruct();
	  }
      }

      // [X.Y.4.5] Observers.
      constexpr const _Tp*
      operator-&gt;() const
      {
#ifndef __cpp_lib_addressof_constexpr
	return __constexpr_addressof(this-&gt;_M_get());
#else
	return std::__addressof(this-&gt;_M_get());
#endif
      }

      _Tp*
      operator-&gt;()
      { return std::__addressof(this-&gt;_M_get()); }

      constexpr const _Tp&amp;
      operator*() const&amp;
      { return this-&gt;_M_get(); }

      constexpr _Tp&amp;
      operator*()&amp;
      { return this-&gt;_M_get(); }

      constexpr _Tp&amp;&amp;
      operator*()&amp;&amp;
      { return std::move(this-&gt;_M_get()); }

      constexpr const _Tp&amp;&amp;
      operator*() const&amp;&amp;
      { return std::move(this-&gt;_M_get()); }

      constexpr explicit operator bool() const noexcept
      { return this-&gt;_M_is_engaged(); }

      constexpr const _Tp&amp;
      value() const&amp;
      {
	return this-&gt;_M_is_engaged()
	  ?  this-&gt;_M_get()
	  : (__throw_bad_optional_access(&quot;Attempt to access value of a &quot;
		                         &quot;disengaged optional object&quot;),
	     this-&gt;_M_get());
      }

      constexpr _Tp&amp;
      value()&amp;
      {
	return this-&gt;_M_is_engaged()
	  ?  this-&gt;_M_get()
	  : (__throw_bad_optional_access(&quot;Attempt to access value of a &quot;
		                         &quot;disengaged optional object&quot;),
	     this-&gt;_M_get());
      }

      constexpr _Tp&amp;&amp;
      value()&amp;&amp;
      {
	return this-&gt;_M_is_engaged()
	  ?  std::move(this-&gt;_M_get())
	  : (__throw_bad_optional_access(&quot;Attempt to access value of a &quot;
		                         &quot;disengaged optional object&quot;),
	     std::move(this-&gt;_M_get()));
      }

      constexpr const _Tp&amp;&amp;
      value() const&amp;&amp;
      {
	return this-&gt;_M_is_engaged()
	  ?  std::move(this-&gt;_M_get())
	  : (__throw_bad_optional_access(&quot;Attempt to access value of a &quot;
		                         &quot;disengaged optional object&quot;),
	     std::move(this-&gt;_M_get()));
      }

      template&lt;typename _Up&gt;
	constexpr _Tp
	value_or(_Up&amp;&amp; __u) const&amp;
	{
	  static_assert(__and_&lt;is_copy_constructible&lt;_Tp&gt;,
			       is_convertible&lt;_Up&amp;&amp;, _Tp&gt;&gt;(),
			&quot;Cannot return value&quot;);

	  return this-&gt;_M_is_engaged()
	    ? this-&gt;_M_get()
	    : static_cast&lt;_Tp&gt;(std::forward&lt;_Up&gt;(__u));
	}

      template&lt;typename _Up&gt;
	_Tp
	value_or(_Up&amp;&amp; __u) &amp;&amp;
	{
	  static_assert(__and_&lt;is_move_constructible&lt;_Tp&gt;,
			       is_convertible&lt;_Up&amp;&amp;, _Tp&gt;&gt;(),
			&quot;Cannot return value&quot; );

	  return this-&gt;_M_is_engaged()
	    ? std::move(this-&gt;_M_get())
	    : static_cast&lt;_Tp&gt;(std::forward&lt;_Up&gt;(__u));
	}
    };

  // [X.Y.8] Comparisons between optional values.
  template&lt;typename _Tp&gt;
    constexpr bool
    operator==(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    {
      return static_cast&lt;bool&gt;(__lhs) == static_cast&lt;bool&gt;(__rhs)
	     &amp;&amp; (!__lhs || *__lhs == *__rhs);
    }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator!=(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return !(__lhs == __rhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    {
      return static_cast&lt;bool&gt;(__rhs) &amp;&amp; (!__lhs || *__lhs &lt; *__rhs);
    }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return __rhs &lt; __lhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;=(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return !(__rhs &lt; __lhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;=(const optional&lt;_Tp&gt;&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return !(__lhs &lt; __rhs); }

  // [X.Y.9] Comparisons with nullopt.
  template&lt;typename _Tp&gt;
    constexpr bool
    operator==(const optional&lt;_Tp&gt;&amp; __lhs, nullopt_t) noexcept
    { return !__lhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator==(nullopt_t, const optional&lt;_Tp&gt;&amp; __rhs) noexcept
    { return !__rhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator!=(const optional&lt;_Tp&gt;&amp; __lhs, nullopt_t) noexcept
    { return static_cast&lt;bool&gt;(__lhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator!=(nullopt_t, const optional&lt;_Tp&gt;&amp; __rhs) noexcept
    { return static_cast&lt;bool&gt;(__rhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;(const optional&lt;_Tp&gt;&amp; /* __lhs */, nullopt_t) noexcept
    { return false; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;(nullopt_t, const optional&lt;_Tp&gt;&amp; __rhs) noexcept
    { return static_cast&lt;bool&gt;(__rhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;(const optional&lt;_Tp&gt;&amp; __lhs, nullopt_t) noexcept
    { return static_cast&lt;bool&gt;(__lhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;(nullopt_t, const optional&lt;_Tp&gt;&amp; /* __rhs */) noexcept
    { return false; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;=(const optional&lt;_Tp&gt;&amp; __lhs, nullopt_t) noexcept
    { return !__lhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;=(nullopt_t, const optional&lt;_Tp&gt;&amp; /* __rhs */) noexcept
    { return true; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;=(const optional&lt;_Tp&gt;&amp; /* __lhs */, nullopt_t) noexcept
    { return true; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;=(nullopt_t, const optional&lt;_Tp&gt;&amp; __rhs) noexcept
    { return !__rhs; }

  // [X.Y.10] Comparisons with value type.
  template&lt;typename _Tp&gt;
    constexpr bool
    operator==(const optional&lt;_Tp&gt;&amp; __lhs, const _Tp&amp; __rhs)
    { return __lhs &amp;&amp; *__lhs == __rhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator==(const _Tp&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return __rhs &amp;&amp; __lhs == *__rhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator!=(const optional&lt;_Tp&gt;&amp; __lhs, _Tp const&amp; __rhs)
    { return !__lhs || !(*__lhs == __rhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator!=(const _Tp&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return !__rhs || !(__lhs == *__rhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;(const optional&lt;_Tp&gt;&amp; __lhs, const _Tp&amp; __rhs)
    { return !__lhs || *__lhs &lt; __rhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;(const _Tp&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return __rhs &amp;&amp; __lhs &lt; *__rhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;(const optional&lt;_Tp&gt;&amp; __lhs, const _Tp&amp; __rhs)
    { return __lhs &amp;&amp; __rhs &lt; *__lhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;(const _Tp&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return !__rhs || *__rhs &lt; __lhs; }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;=(const optional&lt;_Tp&gt;&amp; __lhs, const _Tp&amp; __rhs)
    { return !__lhs || !(__rhs &lt; *__lhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&lt;=(const _Tp&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return __rhs &amp;&amp; !(*__rhs &lt; __lhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;=(const optional&lt;_Tp&gt;&amp; __lhs, const _Tp&amp; __rhs)
    { return __lhs &amp;&amp; !(*__lhs &lt; __rhs); }

  template&lt;typename _Tp&gt;
    constexpr bool
    operator&gt;=(const _Tp&amp; __lhs, const optional&lt;_Tp&gt;&amp; __rhs)
    { return !__rhs || !(__lhs &lt; *__rhs); }

  // [X.Y.11]
  template&lt;typename _Tp&gt;
    inline void
    swap(optional&lt;_Tp&gt;&amp; __lhs, optional&lt;_Tp&gt;&amp; __rhs)
    noexcept(noexcept(__lhs.swap(__rhs)))
    { __lhs.swap(__rhs); }

  template&lt;typename _Tp&gt;
    constexpr optional&lt;decay_t&lt;_Tp&gt;&gt;
    make_optional(_Tp&amp;&amp; __t)
    { return optional&lt;decay_t&lt;_Tp&gt;&gt; { std::forward&lt;_Tp&gt;(__t) }; }

  // @} group optional
} // namespace fundamentals_v1
} // namespace experimental

  // [X.Y.12]
  template&lt;typename _Tp&gt;
    struct hash&lt;experimental::optional&lt;_Tp&gt;&gt;
    {
      using result_type = size_t;
      using argument_type = experimental::optional&lt;_Tp&gt;;

      size_t
      operator()(const experimental::optional&lt;_Tp&gt;&amp; __t) const
      noexcept(noexcept(hash&lt;_Tp&gt; {}(*__t)))
      {
        // We pick an arbitrary hash for disengaged optionals which hopefully
        // usual values of _Tp won't typically hash to.
        constexpr size_t __magic_disengaged_hash = static_cast&lt;size_t&gt;(-3333);
        return __t ? hash&lt;_Tp&gt; {}(*__t) : __magic_disengaged_hash;
      }
    };

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++14

#endif // _GLIBCXX_EXPERIMENTAL_OPTIONAL
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