optional_ref.h

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// optional<T&> -*- C++ -*-
 
// Copyright (C) 2013-2026 Free Software Foundation, Inc.
// Copyright The GNU Toolchain Authors.
//
// 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
// <http://www.gnu.org/licenses/>.
 
/** @file bits/optional_ref.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{optional}
 */
 
#ifndef _GLIBCXX_OPTIONAL_REF
#define _GLIBCXX_OPTIONAL_REF 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#ifdef __glibcxx_optional // C++ >= 17
 
#include <type_traits>
#include <bits/move.h>
#include <bits/inplace_tags.h>
 
namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  template<typename _Iterator, typename _Container>
    class __normal_iterator;
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace __gnu_cxx
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   /**
   *  @addtogroup utilities
   *  @{
   */
 
  template<typename _Tp>
    class optional;
 
  /// 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) noexcept { }
  };
 
  /// Tag to disengage optional objects.
  inline constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };
 
  template<typename _Fn> struct _Optional_func { _Fn& _M_f; };
 
  template<typename _Tp>
    inline constexpr bool __is_valid_contained_type_for_optional =
      (
#if __glibcxx_optional >= 202506L
        is_lvalue_reference_v<_Tp> ||
#endif
        (is_object_v<_Tp> && is_destructible_v<_Tp> && !is_array_v<_Tp>)
      )
      && !is_same_v<remove_cv_t<remove_reference_t<_Tp>>, nullopt_t>
      && !is_same_v<remove_cv_t<remove_reference_t<_Tp>>, in_place_t>;
 
#if __glibcxx_optional >= 202506L // C++26
  template<typename _Tp>
    class optional<_Tp&>;
 
  template<typename _Tp>
    constexpr bool __is_optional_ref_v = false;
 
  template<typename _Tp>
    constexpr bool __is_optional_ref_v<optional<_Tp&>> = true;
 
  template<typename _Tp>
    struct __optional_ref_base
    {};
 
#ifdef __glibcxx_optional_range_support // >= C++26
  template<typename _Tp>
    struct __optional_ref_base<_Tp[]>
    {};
 
  template<typename _Tp>
    requires is_object_v<_Tp>
    struct __optional_ref_base<_Tp>
    {
      using iterator = __gnu_cxx::__normal_iterator<_Tp*, optional<_Tp&>>;
    };
#endif // __glibcxx_optional_range_support
 
  template<typename _Tp>
    class optional<_Tp&> : public __optional_ref_base<_Tp>
    {
      static_assert(__is_valid_contained_type_for_optional<_Tp&>);
 
    public:
      using value_type = _Tp;
 
      constexpr static optional
      _S_from_ptr(_Tp* __ptr)
      {
        optional __res;
        __res._M_val = __ptr;
        return __res;
      }
 
      // Constructors.
      constexpr optional() noexcept = default;
      constexpr optional(nullopt_t) noexcept : optional() {}
      constexpr optional(const optional&) noexcept = default;
 
      template<typename _Arg>
        requires is_constructible_v<_Tp&, _Arg>
          && (!reference_constructs_from_temporary_v<_Tp&, _Arg>)
        explicit constexpr
        optional(in_place_t, _Arg&& __arg)
        {
          __convert_ref_init_val(std::forward<_Arg>(__arg));
        }
 
      template<typename _Up>
        requires (!is_same_v<remove_cvref_t<_Up>, optional>)
          && (!is_same_v<remove_cvref_t<_Up>, in_place_t>)
          && is_constructible_v<_Tp&, _Up>
          && (!reference_constructs_from_temporary_v<_Tp&, _Up>)
        explicit(!is_convertible_v<_Up, _Tp&>)
        constexpr
        optional(_Up&& __u)
        noexcept(is_nothrow_constructible_v<_Tp&, _Up>)
        {
          __convert_ref_init_val(std::forward<_Up>(__u));
        }
 
      template<typename _Up>
        requires (!is_same_v<remove_cvref_t<_Up>, optional>)
          && (!is_same_v<remove_cvref_t<_Up>, in_place_t>)
          && is_constructible_v<_Tp&, _Up>
          && reference_constructs_from_temporary_v<_Tp&, _Up>
        explicit(!is_convertible_v<_Up, _Tp&>)
        constexpr
        optional(_Up&& __u) = delete;
 
      // optional<U> &
      template<typename _Up>
        requires (!is_same_v<remove_cv_t<_Tp>, optional<_Up>>)
          && (!is_same_v<_Tp&, _Up>)
          && is_constructible_v<_Tp&, _Up&>
          && (!reference_constructs_from_temporary_v<_Tp&, _Up&>)
        explicit(!is_convertible_v<_Up&, _Tp&>)
        constexpr
        optional(optional<_Up>& __rhs)
        noexcept(is_nothrow_constructible_v<_Tp&, _Up&>)
        {
          if (__rhs)
            __convert_ref_init_val(__rhs._M_fwd());
        }
 
      template<typename _Up>
        requires (!is_same_v<remove_cv_t<_Tp>, optional<_Up>>)
          && (!is_same_v<_Tp&, _Up>)
          && is_constructible_v<_Tp&, _Up&>
          && reference_constructs_from_temporary_v<_Tp&, _Up&>
        explicit(!is_convertible_v<_Up&, _Tp&>)
        constexpr
        optional(optional<_Up>& __rhs) = delete;
 
      // const optional<U>&
      template<typename _Up>
        requires (!is_same_v<remove_cv_t<_Tp>, optional<_Up>>)
          && (!is_same_v<_Tp&, _Up>)
          && is_constructible_v<_Tp&, const _Up&>
          && (!reference_constructs_from_temporary_v<_Tp&, const _Up&>)
        explicit(!is_convertible_v<const _Up&, _Tp&>)
        constexpr
        optional(const optional<_Up>& __rhs)
        noexcept(is_nothrow_constructible_v<_Tp&, _Up&>)
        {
          if (__rhs)
            __convert_ref_init_val(__rhs._M_fwd());
        }
 
      template<typename _Up>
        requires (!is_same_v<remove_cv_t<_Tp>, optional<_Up>>)
          && (!is_same_v<_Tp&, _Up>)
          && is_constructible_v<_Tp&, const _Up&>
          && reference_constructs_from_temporary_v<_Tp&, const _Up&>
        explicit(!is_convertible_v<const _Up&, _Tp&>)
        constexpr
        optional(const optional<_Up>& __rhs) = delete;
 
      // optional<U>&&
      template<typename _Up>
        requires (!is_same_v<remove_cv_t<_Tp>, optional<_Up>>)
          && (!is_same_v<_Tp&, _Up>)
          && is_constructible_v<_Tp&, _Up>
          && (!reference_constructs_from_temporary_v<_Tp&, _Up>)
        explicit(!is_convertible_v<_Up, _Tp&>)
        constexpr
        optional(optional<_Up>&& __rhs)
        noexcept(is_nothrow_constructible_v<_Tp&, _Up>)
        {
          if (__rhs)
            __convert_ref_init_val(std::move(__rhs)._M_fwd());
        }
 
      template<typename _Up>
        requires (!is_same_v<remove_cv_t<_Tp>, optional<_Up>>)
          && (!is_same_v<_Tp&, _Up>)
          && is_constructible_v<_Tp&, _Up>
          && reference_constructs_from_temporary_v<_Tp&, _Up>
        explicit(!is_convertible_v<_Up, _Tp&>)
        constexpr
        optional(optional<_Up>&& __rhs) = delete;
 
      // const optional<U>&&
      template<typename _Up>
        requires (!is_same_v<remove_cv_t<_Tp>, optional<_Up>>)
          && (!is_same_v<_Tp&, _Up>)
          && is_constructible_v<_Tp&, const _Up>
          && (!reference_constructs_from_temporary_v<_Tp&, _Up>)
        explicit(!is_convertible_v<const _Up, _Tp&>)
        constexpr
        optional(const optional<_Up>&& __rhs)
        noexcept(is_nothrow_constructible_v<_Tp&, const _Up>)
        {
          if (__rhs)
            __convert_ref_init_val(std::move(__rhs)._M_fwd());
        }
 
      template<typename _Up>
        requires (!is_same_v<remove_cv_t<_Tp>, optional<_Up>>)
          && (!is_same_v<_Tp&, _Up>)
          && is_constructible_v<_Tp&, const _Up>
          && reference_constructs_from_temporary_v<_Tp&, const _Up>
        explicit(!is_convertible_v<const _Up, _Tp&>)
        constexpr
        optional(const optional<_Up>&& __rhs) = delete;
 
      constexpr ~optional() = default;
 
      // Assignment.
      constexpr optional& operator=(nullopt_t) noexcept
      {
        _M_val = nullptr;
        return *this;
      }
 
      constexpr optional& operator=(const optional&) noexcept = default;
 
      template<typename _Up>
        requires is_constructible_v<_Tp&, _Up>
          && (!reference_constructs_from_temporary_v<_Tp&, _Up>)
        constexpr _Tp&
        emplace(_Up&& __u)
        noexcept(is_nothrow_constructible_v<_Tp&, _Up>)
        {
          __convert_ref_init_val(std::forward<_Up>(__u));
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 4300. Missing Returns: element in optional<T&>::emplace
          return *_M_val;
        }
 
      // Swap.
      constexpr void swap(optional& __rhs) noexcept
      { std::swap(_M_val, __rhs._M_val); }
 
#ifdef __glibcxx_optional_range_support // >= C++26
      // Iterator support.
      constexpr auto begin() const noexcept
        requires is_object_v<_Tp> && (!is_unbounded_array_v<_Tp>);
 
      constexpr auto end() const noexcept
        requires is_object_v<_Tp> && (!is_unbounded_array_v<_Tp>);
#endif // __glibcxx_optional_range_support
 
      // Observers.
      constexpr _Tp* operator->() const noexcept
      {
        __glibcxx_assert(_M_val); // hardened precondition
        return _M_val;
      }
 
      constexpr _Tp& operator*() const noexcept
      {
        __glibcxx_assert(_M_val); // hardened precondition
        return *_M_val;
      }
 
      constexpr explicit operator bool() const noexcept
      {
        return _M_val;
      }
 
      constexpr bool has_value() const noexcept
      {
        return _M_val;
      }
 
      constexpr _Tp& value() const;
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 4304. std::optional<NonReturnable&> is ill-formed due to value_or
      template<typename _Up = remove_cv_t<_Tp>>
        requires is_object_v<_Tp> && (!is_array_v<_Tp>)
        constexpr decay_t<_Tp>
        value_or(_Up&& __u) const;
 
      // Monadic operations.
      template<typename _Fn>
        constexpr auto
        and_then(_Fn&& __f) const;
 
      template<typename _Fn>
        constexpr
        optional<remove_cv_t<invoke_result_t<_Fn, _Tp&>>>
        transform(_Fn&& __f) const;
 
      template<typename _Fn>
        requires is_invocable_v<_Fn>
        constexpr
        optional
        or_else(_Fn&& __f) const;
 
      // Modifiers.
      constexpr void reset() noexcept
      {
        _M_val = nullptr;
      }
 
    private:
      _Tp *_M_val = nullptr;
 
      [[__gnu__::__always_inline__]]
      constexpr _Tp&
      _M_fwd() const noexcept
      { return *_M_val; }
 
      template<typename _Up> friend class optional;
 
      template<typename _Up>
        constexpr
        void
        __convert_ref_init_val(_Up&& __u)
        noexcept
        {
          _Tp& __r(std::forward<_Up>(__u));
          _M_val = std::addressof(__r);
        }
 
      template<typename _Fn, typename _Value>
        explicit constexpr
        optional(_Optional_func<_Fn> __f, _Value&& __v);
    };
#endif // __glibcxx_optional >= 202506L
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#endif // __glibcxx_optional
 
#endif // _GLIBCXX_OPTIONAL_REF