propagate_const

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// <experimental/propagate_const> -*- C++ -*-

// Copyright (C) 2015-2026 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
// <http://www.gnu.org/licenses/>.

/** @file experimental/propagate_const
 *  This is a TS C++ Library header.
 *  @ingroup libfund-ts
 */

#ifndef _GLIBCXX_EXPERIMENTAL_PROPAGATE_CONST
#define _GLIBCXX_EXPERIMENTAL_PROPAGATE_CONST 1

#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif

#include <bits/requires_hosted.h> // experimental is currently omitted

#if __cplusplus >= 201402L

#include <type_traits>
#include <bits/functional_hash.h>
#include <bits/move.h>
#include <bits/stl_function.h>
#include <experimental/bits/lfts_config.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

namespace experimental
{
inline namespace fundamentals_v2
{
  template<typename _Tp>
    using __propagate_const_elem_type
      = remove_reference_t<decltype(*std::declval<_Tp&>())>;

  template<typename _Tp,
           typename _Elem = __propagate_const_elem_type<_Tp>,
           bool = is_convertible<const _Tp, const _Elem*>::value>
    struct __propagate_const_conversion_c
    { };

  template<typename _Tp, typename _Elem>
    struct __propagate_const_conversion_c<_Tp, _Elem, true>
    {
      constexpr operator const _Elem*() const;
    };

  template<typename _Tp,
           typename _Elem = __propagate_const_elem_type<_Tp>,
           bool = is_convertible<_Tp, _Elem*>::value>
    struct __propagate_const_conversion_nc
    { };

  template<typename _Tp, typename _Elem>
    struct __propagate_const_conversion_nc<_Tp, _Elem, true>
    {
      constexpr operator _Elem*();
    };

  // Base class of propagate_const<T> when T is a class type.
  template <typename _Tp>
    struct __propagate_const_conversions
    : __propagate_const_conversion_c<_Tp>, __propagate_const_conversion_nc<_Tp>
    { };

  // Base class of propagate_const<T> when T is a pointer type.
  template<typename _Tp>
    struct __propagate_const_conversions<_Tp*>
    {
      constexpr operator const _Tp*() const noexcept;
      constexpr operator _Tp*() noexcept;
    };

  /**
   * @defgroup propagate_const Const-propagating wrapper
   * @ingroup libfund-ts
   *
   * A const-propagating wrapper that propagates const to pointer-like members,
   * as described in n4388 "A Proposal to Add a Const-Propagating Wrapper
   * to the Standard Library".
   *
   * @{
   */

  /// Const-propagating wrapper.
  template <typename _Tp>
    class propagate_const : public __propagate_const_conversions<_Tp>
    {
    public:
      using element_type = __propagate_const_elem_type<_Tp>;

    private:
      template <typename _Up>
        struct __is_propagate_const : false_type
        { };

      template <typename _Up>
        struct __is_propagate_const<propagate_const<_Up>> : true_type
        { };

      template <typename _Up>
        friend constexpr const _Up&
        get_underlying(const propagate_const<_Up>& __pt) noexcept;
      template <typename _Up>
        friend constexpr _Up&
        get_underlying(propagate_const<_Up>& __pt) noexcept;

      template <typename _Up>
        static constexpr element_type*
        __to_raw_pointer(_Up* __u)
        { return __u; }

      template <typename _Up>
        static constexpr element_type*
        __to_raw_pointer(_Up& __u)
        { return __u.get(); }

      template <typename _Up>
        static constexpr const element_type*
        __to_raw_pointer(const _Up* __u)
        { return __u; }

      template <typename _Up>
        static constexpr const element_type*
        __to_raw_pointer(const _Up& __u)
        { return __u.get(); }

    public:
      static_assert(__and_<is_object<typename remove_pointer<_Tp>::type>,
                           __not_<is_array<_Tp>>,
                           __or_<is_class<_Tp>, is_pointer<_Tp>>>::value,
                    "propagate_const requires a class or a pointer to an"
                    " object type");

      // [propagate_const.ctor], constructors
      constexpr propagate_const() = default;
      propagate_const(const propagate_const& __p) = delete;
      constexpr propagate_const(propagate_const&& __p) = default;

      template <typename _Up, typename
                enable_if<__and_<is_constructible<_Tp, _Up&&>,
                                 is_convertible<_Up&&, _Tp>>::value, bool
                          >::type=true>
      constexpr propagate_const(propagate_const<_Up>&& __pu)
        : _M_t(std::move(get_underlying(__pu)))
      {}

      template <typename _Up, typename
                enable_if<__and_<is_constructible<_Tp, _Up&&>,
                                 __not_<is_convertible<_Up&&, _Tp>>>::value,
                          bool>::type=false>
      constexpr explicit propagate_const(propagate_const<_Up>&& __pu)
        : _M_t(std::move(get_underlying(__pu)))
      {}

      template <typename _Up, typename
                enable_if<__and_<is_constructible<_Tp, _Up&&>,
                                 is_convertible<_Up&&, _Tp>,
                                 __not_<__is_propagate_const<
                                          typename decay<_Up>::type>>
                                 >::value, bool>::type=true>
      constexpr propagate_const(_Up&& __u)
        : _M_t(std::forward<_Up>(__u))
      {}

      template <typename _Up, typename
                enable_if<__and_<is_constructible<_Tp, _Up&&>,
                                 __not_<is_convertible<_Up&&, _Tp>>,
                                 __not_<__is_propagate_const<
                                          typename decay<_Up>::type>>
                                 >::value, bool>::type=false>
      constexpr explicit propagate_const(_Up&& __u)
        : _M_t(std::forward<_Up>(__u))
      {}

      // [propagate_const.assignment], assignment
      propagate_const& operator=(const propagate_const& __p) = delete;
      constexpr propagate_const& operator=(propagate_const&& __p) = default;

      template <typename _Up, typename =
                typename enable_if<is_convertible<_Up&&, _Tp>::value>::type>
      constexpr propagate_const& operator=(propagate_const<_Up>&& __pu)
      {
        _M_t = std::move(get_underlying(__pu));
        return *this;
      }

      template <typename _Up, typename =
                typename enable_if<__and_<is_convertible<_Up&&, _Tp>,
                                          __not_<__is_propagate_const<
                                                   typename decay<_Up>::type>>
                                          >::value>::type>
      constexpr propagate_const& operator=(_Up&& __u)
      {
        _M_t = std::forward<_Up>(__u);
        return *this;
      }

      // [propagate_const.const_observers], const observers
      explicit constexpr operator bool() const
      {
        return bool(_M_t);
      }

      constexpr const element_type* operator->() const
      {
        return get();
      }

      constexpr const element_type& operator*() const
      {
        return *get();
      }

      constexpr const element_type* get() const
      {
        return __to_raw_pointer(_M_t);
      }

      // [propagate_const.non_const_observers], non-const observers
      constexpr element_type* operator->()
      {
        return get();
      }

      constexpr element_type& operator*()
      {
        return *get();
      }

      constexpr element_type* get()
      {
        return __to_raw_pointer(_M_t);
      }

      // [propagate_const.modifiers], modifiers
      constexpr void
      swap(propagate_const& __pt) noexcept(__is_nothrow_swappable<_Tp>::value)
      {
        using std::swap;
        swap(_M_t, get_underlying(__pt));
      }

    private:
      _Tp _M_t;
    };

  // [propagate_const.relational], relational operators
  template <typename _Tp>
    constexpr bool
    operator==(const propagate_const<_Tp>& __pt, nullptr_t)
    {
      return get_underlying(__pt) == nullptr;
    }

  template <typename _Tp>
    constexpr bool
    operator==(nullptr_t, const propagate_const<_Tp>& __pu)
    {
      return nullptr == get_underlying(__pu);
    }

  template <typename _Tp>
    constexpr bool
    operator!=(const propagate_const<_Tp>& __pt, nullptr_t)
    {
      return get_underlying(__pt) != nullptr;
    }

  template <typename _Tp>
    constexpr bool operator!=(nullptr_t, const propagate_const<_Tp>& __pu)
    {
      return nullptr != get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator==(const propagate_const<_Tp>& __pt,
               const propagate_const<_Up>& __pu)
    {
      return get_underlying(__pt) == get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator!=(const propagate_const<_Tp>& __pt,
               const propagate_const<_Up>& __pu)
    {
      return get_underlying(__pt) != get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator<(const propagate_const<_Tp>& __pt,
              const propagate_const<_Up>& __pu)
    {
      return get_underlying(__pt) < get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator>(const propagate_const<_Tp>& __pt,
              const propagate_const<_Up>& __pu)
    {
      return get_underlying(__pt) > get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator<=(const propagate_const<_Tp>& __pt,
               const propagate_const<_Up>& __pu)
    {
      return get_underlying(__pt) <= get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator>=(const propagate_const<_Tp>& __pt,
               const propagate_const<_Up>& __pu)
    {
      return get_underlying(__pt) >= get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator==(const propagate_const<_Tp>& __pt, const _Up& __u)
    {
      return get_underlying(__pt) == __u;
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator!=(const propagate_const<_Tp>& __pt, const _Up& __u)
    {
      return get_underlying(__pt) != __u;
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator<(const propagate_const<_Tp>& __pt, const _Up& __u)
    {
      return get_underlying(__pt) < __u;
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator>(const propagate_const<_Tp>& __pt, const _Up& __u)
    {
      return get_underlying(__pt) > __u;
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator<=(const propagate_const<_Tp>& __pt, const _Up& __u)
    {
      return get_underlying(__pt) <= __u;
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator>=(const propagate_const<_Tp>& __pt, const _Up& __u)
    {
      return get_underlying(__pt) >= __u;
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator==(const _Tp& __t, const propagate_const<_Up>& __pu)
    {
      return __t == get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator!=(const _Tp& __t, const propagate_const<_Up>& __pu)
    {
      return __t != get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator<(const _Tp& __t, const propagate_const<_Up>& __pu)
    {
      return __t < get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator>(const _Tp& __t, const propagate_const<_Up>& __pu)
    {
      return __t > get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator<=(const _Tp& __t, const propagate_const<_Up>& __pu)
    {
      return __t <= get_underlying(__pu);
    }

  template <typename _Tp, typename _Up>
    constexpr bool
    operator>=(const _Tp& __t, const propagate_const<_Up>& __pu)
    {
      return __t >= get_underlying(__pu);
    }

  // [propagate_const.algorithms], specialized algorithms
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 3413. propagate_const's swap [...] needs to be constrained and use a trait
  template <typename _Tp>
    constexpr enable_if_t<__is_swappable<_Tp>::value, void>
    swap(propagate_const<_Tp>& __pt, propagate_const<_Tp>& __pt2)
      noexcept(__is_nothrow_swappable<_Tp>::value)
    {
      __pt.swap(__pt2);
    }

  // [propagate_const.underlying], underlying pointer access
  template <typename _Tp>
    constexpr const _Tp&
    get_underlying(const propagate_const<_Tp>& __pt) noexcept
    {
      return __pt._M_t;
    }

  template <typename _Tp>
    constexpr _Tp&
    get_underlying(propagate_const<_Tp>& __pt) noexcept
    {
      return __pt._M_t;
    }

  template<typename _Tp>
    constexpr
    __propagate_const_conversions<_Tp*>::operator const _Tp*() const noexcept
    { return static_cast<const propagate_const<_Tp*>*>(this)->get(); }

  template<typename _Tp>
    constexpr
    __propagate_const_conversions<_Tp*>::operator _Tp*() noexcept
    { return static_cast<propagate_const<_Tp*>*>(this)->get(); }

  template<typename _Tp, typename _Elem>
    constexpr
    __propagate_const_conversion_c<_Tp, _Elem, true>::
    operator const _Elem*() const
    { return static_cast<const propagate_const<_Tp>*>(this)->get(); }

  template<typename _Tp, typename _Elem>
    constexpr
    __propagate_const_conversion_nc<_Tp, _Elem, true>::
    operator _Elem*()
    { return static_cast<propagate_const<_Tp>*>(this)->get(); }

  /// @} group propagate_const
} // namespace fundamentals_v2
} // namespace experimental

// [propagate_const.hash], hash support
 template <typename _Tp>
   struct hash<experimental::propagate_const<_Tp>>
   {
     using result_type = size_t;
     using argument_type = experimental::propagate_const<_Tp>;

     size_t
     operator()(const experimental::propagate_const<_Tp>& __t) const
     noexcept(noexcept(hash<_Tp>{}(get_underlying(__t))))
     {
       return hash<_Tp>{}(get_underlying(__t));
     }
   };

 // [propagate_const.comparison_function_objects], comparison function objects
 template <typename _Tp>
   struct equal_to<experimental::propagate_const<_Tp>>
   {
     constexpr bool
     operator()(const experimental::propagate_const<_Tp>& __x,
                const experimental::propagate_const<_Tp>& __y) const
     {
       return equal_to<_Tp>{}(get_underlying(__x), get_underlying(__y));
     }

     typedef experimental::propagate_const<_Tp> first_argument_type;
     typedef experimental::propagate_const<_Tp> second_argument_type;
     typedef bool result_type;
   };

 template <typename _Tp>
   struct not_equal_to<experimental::propagate_const<_Tp>>
   {
     constexpr bool
     operator()(const experimental::propagate_const<_Tp>& __x,
                const experimental::propagate_const<_Tp>& __y) const
     {
       return not_equal_to<_Tp>{}(get_underlying(__x), get_underlying(__y));
     }

     typedef experimental::propagate_const<_Tp> first_argument_type;
     typedef experimental::propagate_const<_Tp> second_argument_type;
     typedef bool result_type;
   };

 template <typename _Tp>
   struct less<experimental::propagate_const<_Tp>>
   {
     constexpr bool
     operator()(const experimental::propagate_const<_Tp>& __x,
                const experimental::propagate_const<_Tp>& __y) const
     {
       return less<_Tp>{}(get_underlying(__x), get_underlying(__y));
     }

     typedef experimental::propagate_const<_Tp> first_argument_type;
     typedef experimental::propagate_const<_Tp> second_argument_type;
     typedef bool result_type;
   };

 template <typename _Tp>
   struct greater<experimental::propagate_const<_Tp>>
   {
     constexpr bool
     operator()(const experimental::propagate_const<_Tp>& __x,
                const experimental::propagate_const<_Tp>& __y) const
     {
       return greater<_Tp>{}(get_underlying(__x), get_underlying(__y));
     }

     typedef experimental::propagate_const<_Tp> first_argument_type;
     typedef experimental::propagate_const<_Tp> second_argument_type;
     typedef bool result_type;
   };

 template <typename _Tp>
   struct less_equal<experimental::propagate_const<_Tp>>
   {
     constexpr bool
     operator()(const experimental::propagate_const<_Tp>& __x,
                const experimental::propagate_const<_Tp>& __y) const
     {
       return less_equal<_Tp>{}(get_underlying(__x), get_underlying(__y));
     }

     typedef experimental::propagate_const<_Tp> first_argument_type;
     typedef experimental::propagate_const<_Tp> second_argument_type;
     typedef bool result_type;
   };

 template <typename _Tp>
   struct greater_equal<experimental::propagate_const<_Tp>>
   {
     constexpr bool
     operator()(const experimental::propagate_const<_Tp>& __x,
                const experimental::propagate_const<_Tp>& __y) const
     {
       return greater_equal<_Tp>{}(get_underlying(__x), get_underlying(__y));
     }

     typedef experimental::propagate_const<_Tp> first_argument_type;
     typedef experimental::propagate_const<_Tp> second_argument_type;
     typedef bool result_type;
   };

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++14

#endif // _GLIBCXX_EXPERIMENTAL_PROPAGATE_CONST