libstdc++-v3/include/bits/node_handle.h

// Node handles for containers -*- C++ -*-

// Copyright (C) 2016-2026 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 bits/node_handle.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly.
 *  @headername{map,set,unordered_map,unordered_set}
 */

#ifndef _NODE_HANDLE
#define _NODE_HANDLE 1

#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif

#include <bits/version.h>

#ifdef __glibcxx_node_extract // C++ >= 17 && HOSTED

#include <new>
#include <bits/alloc_traits.h>
#include <bits/ptr_traits.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   * @defgroup node_handles Node handles
   * @ingroup associative_containers
   * @since C++17
   *
   * The associative containers (`map`, `set`, `multimap` and `multiset`)
   * support extracting and re-inserting nodes from the container. Those
   * operations use the container's `node_handle` type, which is an alias
   * for a `_Node_handle<...>` type. You should always use the container's
   * `node_handle` type (e.g. `std::set<int>::node_handle`) to refer to
   * these types, not the non-standard internal `_Node_handle` names.
   *
   * @{
   */

  /// Base class for node handle types of maps and sets.
  template<typename _Val, typename _NodeAlloc>
    class _Node_handle_common
    {
      using _AllocTraits = allocator_traits<_NodeAlloc>;

    public:
      using allocator_type = __alloc_rebind<_NodeAlloc, _Val>;

      allocator_type
      get_allocator() const noexcept
      {
	__glibcxx_assert(!this->empty());
	return allocator_type(_M_alloc._M_alloc);
      }

      explicit operator bool() const noexcept { return _M_ptr != nullptr; }

      [[nodiscard]] bool empty() const noexcept { return _M_ptr == nullptr; }

    /// @cond undocumented
    protected:
      constexpr _Node_handle_common() noexcept : _M_ptr() { }

      ~_Node_handle_common()
      {
	if (!empty())
	  _M_reset();
      }

      _Node_handle_common(_Node_handle_common&& __nh) noexcept
      : _M_ptr(__nh._M_ptr)
      {
	if (_M_ptr)
	  _M_move(std::move(__nh));
      }

      _Node_handle_common&
      operator=(_Node_handle_common&& __nh) noexcept
      {
	if (empty())
	  {
	    if (!__nh.empty())
	      _M_move(std::move(__nh));
	  }
	else if (__nh.empty())
	  _M_reset();
	else
	  {
	    // Free the current node before replacing the allocator.
	    _AllocTraits::destroy(*_M_alloc, _M_ptr->_M_valptr());
	    _AllocTraits::deallocate(*_M_alloc, _M_ptr, 1);

	    _M_alloc = __nh._M_alloc.release(); // assigns if POCMA
	    _M_ptr = __nh._M_ptr;
	    __nh._M_ptr = nullptr;
	  }
	return *this;
      }

      _Node_handle_common(typename _AllocTraits::pointer __ptr,
			  const _NodeAlloc& __alloc)
      : _M_ptr(__ptr), _M_alloc(__alloc)
      {
	__glibcxx_assert(__ptr != nullptr);
      }

      void
      _M_swap(_Node_handle_common& __nh) noexcept
      {
	if (empty())
	  {
	    if (!__nh.empty())
	      _M_move(std::move(__nh));
	  }
	else if (__nh.empty())
	  __nh._M_move(std::move(*this));
	else
	  {
	    using std::swap;
	    swap(_M_ptr, __nh._M_ptr);
	    _M_alloc.swap(__nh._M_alloc); // swaps if POCS
	  }
      }

    private:
      // Moves the pointer and allocator from __nh to *this.
      // Precondition: empty() && !__nh.empty()
      // Postcondition: !empty() && __nh.empty()
      void
      _M_move(_Node_handle_common&& __nh) noexcept
      {
	::new (std::__addressof(_M_alloc)) _NodeAlloc(__nh._M_alloc.release());
	_M_ptr = __nh._M_ptr;
	__nh._M_ptr = nullptr;
      }

      // Deallocates the node, destroys the allocator.
      // Precondition: !empty()
      // Postcondition: empty()
      void
      _M_reset() noexcept
      {
	_NodeAlloc __alloc = _M_alloc.release();
	_AllocTraits::destroy(__alloc, _M_ptr->_M_valptr());
	_AllocTraits::deallocate(__alloc, _M_ptr, 1);
	_M_ptr = nullptr;
      }

      // Destroys the allocator. Does not deallocate or destroy the node.
      // Precondition: !empty()
      // Postcondition: empty()
      void
      release() noexcept
      {
	_M_alloc.release();
	_M_ptr = nullptr;
      }

    protected:
      typename _AllocTraits::pointer _M_ptr;

    private:
      // A simplified, non-copyable std::optional<_NodeAlloc>.
      // Call release() before destruction iff the allocator member is active.
      union _Optional_alloc
      {
	_Optional_alloc() { }
	~_Optional_alloc() { }

	_Optional_alloc(_Optional_alloc&&) = delete;
	_Optional_alloc& operator=(_Optional_alloc&&) = delete;

	_Optional_alloc(const _NodeAlloc& __alloc) noexcept
	: _M_alloc(__alloc)
	{ }

	// Precondition: _M_alloc is the active member of the union.
	void
	operator=(_NodeAlloc&& __alloc) noexcept
	{
	  using _ATr = _AllocTraits;
	  if constexpr (_ATr::propagate_on_container_move_assignment::value)
	    _M_alloc = std::move(__alloc);
	  else if constexpr (!_AllocTraits::is_always_equal::value)
	    __glibcxx_assert(_M_alloc == __alloc);
	}

	// Precondition: _M_alloc is the active member of both unions.
	void
	swap(_Optional_alloc& __other) noexcept
	{
	  using std::swap;
	  if constexpr (_AllocTraits::propagate_on_container_swap::value)
	    swap(_M_alloc, __other._M_alloc);
	  else if constexpr (!_AllocTraits::is_always_equal::value)
	    __glibcxx_assert(_M_alloc == __other._M_alloc);
	}

	// Precondition: _M_alloc is the active member of the union.
	_NodeAlloc& operator*() noexcept { return _M_alloc; }

	// Precondition: _M_alloc is the active member of the union.
	_NodeAlloc release() noexcept
	{
	  _NodeAlloc __tmp = std::move(_M_alloc);
	  _M_alloc.~_NodeAlloc();
	  return __tmp;
	}

	[[__no_unique_address__]] _NodeAlloc _M_alloc;
      };

      [[__no_unique_address__]] _Optional_alloc _M_alloc;

      template<typename _Key2, typename _Value2, typename _KeyOfValue,
	       typename _Compare, typename _ValueAlloc>
	friend class _Rb_tree;

      template<typename _Key2, typename _Value2, typename _ValueAlloc,
	       typename _ExtractKey, typename _Equal,
	       typename _Hash, typename _RangeHash, typename _Unused,
	       typename _RehashPolicy, typename _Traits>
	friend class _Hashtable;

      /// @endcond
    };

  /// Node handle type for maps.
  template<typename _Key, typename _Value, typename _NodeAlloc>
    class _Node_handle : public _Node_handle_common<_Value, _NodeAlloc>
    {
    public:
      constexpr _Node_handle() noexcept = default;
      ~_Node_handle() = default;
      _Node_handle(_Node_handle&&) noexcept = default;

      _Node_handle&
      operator=(_Node_handle&&) noexcept = default;

      using key_type = _Key;
      using mapped_type = typename _Value::second_type;

      key_type&
      key() const noexcept
      {
	__glibcxx_assert(!this->empty());
	return *_M_pkey;
      }

      mapped_type&
      mapped() const noexcept
      {
	__glibcxx_assert(!this->empty());
	return *_M_pmapped;
      }

      void
      swap(_Node_handle& __nh) noexcept
      {
	this->_M_swap(__nh);
	using std::swap;
	swap(_M_pkey, __nh._M_pkey);
	swap(_M_pmapped, __nh._M_pmapped);
      }

      friend void
      swap(_Node_handle& __x, _Node_handle& __y)
      noexcept(noexcept(__x.swap(__y)))
      { __x.swap(__y); }

    private:
      using _AllocTraits = allocator_traits<_NodeAlloc>;

      _Node_handle(typename _AllocTraits::pointer __ptr,
		   const _NodeAlloc& __alloc)
      : _Node_handle_common<_Value, _NodeAlloc>(__ptr, __alloc)
      {
	if (__ptr)
	  {
	    auto& __key = const_cast<_Key&>(__ptr->_M_valptr()->first);
	    _M_pkey = _S_pointer_to(__key);
	    _M_pmapped = _S_pointer_to(__ptr->_M_valptr()->second);
	  }
	else
	  {
	    _M_pkey = nullptr;
	    _M_pmapped = nullptr;
	  }
      }

      template<typename _Tp>
	using __pointer
	  = __ptr_rebind<typename _AllocTraits::pointer,
			 remove_reference_t<_Tp>>;

      __pointer<_Key>				_M_pkey = nullptr;
      __pointer<typename _Value::second_type>	_M_pmapped = nullptr;

      template<typename _Tp>
	__pointer<_Tp>
	_S_pointer_to(_Tp& __obj)
	{ return pointer_traits<__pointer<_Tp>>::pointer_to(__obj); }

      const key_type&
      _M_key() const noexcept { return key(); }

      template<typename _Key2, typename _Value2, typename _KeyOfValue,
	       typename _Compare, typename _ValueAlloc>
	friend class _Rb_tree;

      template<typename _Key2, typename _Value2, typename _ValueAlloc,
	       typename _ExtractKey, typename _Equal,
	       typename _Hash, typename _RangeHash, typename _Unused,
	       typename _RehashPolicy, typename _Traits>
	friend class _Hashtable;
    };

  /// Node handle type for sets.
  template<typename _Value, typename _NodeAlloc>
    class _Node_handle<_Value, _Value, _NodeAlloc>
    : public _Node_handle_common<_Value, _NodeAlloc>
    {
    public:
      constexpr _Node_handle() noexcept = default;
      ~_Node_handle() = default;
      _Node_handle(_Node_handle&&) noexcept = default;

      _Node_handle&
      operator=(_Node_handle&&) noexcept = default;

      using value_type = _Value;

      value_type&
      value() const noexcept
      {
	__glibcxx_assert(!this->empty());
	return *this->_M_ptr->_M_valptr();
      }

      void
      swap(_Node_handle& __nh) noexcept
      { this->_M_swap(__nh); }

      friend void
      swap(_Node_handle& __x, _Node_handle& __y)
      noexcept(noexcept(__x.swap(__y)))
      { __x.swap(__y); }

    private:
      using _AllocTraits = allocator_traits<_NodeAlloc>;

      _Node_handle(typename _AllocTraits::pointer __ptr,
		   const _NodeAlloc& __alloc)
      : _Node_handle_common<_Value, _NodeAlloc>(__ptr, __alloc) { }

      const value_type&
      _M_key() const noexcept { return value(); }

      template<typename _Key, typename _Val, typename _KeyOfValue,
	       typename _Compare, typename _Alloc>
	friend class _Rb_tree;

      template<typename _Key2, typename _Value2, typename _ValueAlloc,
	       typename _ExtractKey, typename _Equal,
	       typename _Hash, typename _RangeHash, typename _Unused,
	       typename _RehashPolicy, typename _Traits>
	friend class _Hashtable;
    };

  /// Return type of insert(node_handle&&) on unique maps/sets.
  template<typename _Iterator, typename _NodeHandle>
    struct _Node_insert_return
    {
      _Iterator		position = _Iterator();
      bool		inserted = false;
      _NodeHandle	node;
    };

  /// @}

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // __glibcxx_node_extract
#endif