allocated_ptr.h

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// Guarded Allocation -*- C++ -*-
 
// Copyright (C) 2014-2025 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/allocated_ptr.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{memory}
 */
 
#ifndef _ALLOCATED_PTR_H
#define _ALLOCATED_PTR_H 1
 
#if __cplusplus < 201103L
# include <bits/c++0xwarning.h>
#else
# include <type_traits>
# include <bits/ptr_traits.h>
# include <bits/alloc_traits.h>
# include <bits/utility.h>
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
/// @cond undocumented
 
  /// Non-standard RAII type for managing pointers obtained from allocators.
  template<typename _Alloc>
    struct __allocated_ptr
    {
      using pointer = typename allocator_traits<_Alloc>::pointer;
      using value_type = typename allocator_traits<_Alloc>::value_type;
 
      /// Take ownership of __ptr
      __allocated_ptr(_Alloc& __a, pointer __ptr) noexcept
      : _M_alloc(std::__addressof(__a)), _M_ptr(__ptr)
      { }
 
      /// Convert __ptr to allocator's pointer type and take ownership of it
      template<typename _Ptr,
               typename _Req = _Require<is_same<_Ptr, value_type*>>>
      __allocated_ptr(_Alloc& __a, _Ptr __ptr)
      : _M_alloc(std::__addressof(__a)),
        _M_ptr(pointer_traits<pointer>::pointer_to(*__ptr))
      { }
 
      /// Transfer ownership of the owned pointer
      __allocated_ptr(__allocated_ptr&& __gd) noexcept
      : _M_alloc(__gd._M_alloc), _M_ptr(__gd._M_ptr)
      { __gd._M_ptr = nullptr; }
 
      /// Deallocate the owned pointer
      ~__allocated_ptr()
      {
        if (_M_ptr != nullptr)
          std::allocator_traits<_Alloc>::deallocate(*_M_alloc, _M_ptr, 1);
      }
 
      /// Release ownership of the owned pointer
      __allocated_ptr&
      operator=(std::nullptr_t) noexcept
      {
        _M_ptr = nullptr;
        return *this;
      }
 
      explicit operator bool() const noexcept { return (bool)_M_ptr; }
 
      /// Get the address that the owned pointer refers to.
      value_type* get() const { return std::__to_address(_M_ptr); }
 
      pointer release() { return std::__exchange(_M_ptr, nullptr); }
 
    private:
      _Alloc* _M_alloc;
      pointer _M_ptr;
    };
 
  /// Allocate space for a single object using __a.
  template<typename _Alloc>
    inline __allocated_ptr<_Alloc>
    __allocate_guarded(_Alloc& __a)
    {
      return { __a, std::allocator_traits<_Alloc>::allocate(__a, 1) };
    }
 
  /// RAII type for constructing/destroying an object with an allocated pointer
  template<typename _Alloc>
    struct __allocated_obj : __allocated_ptr<_Alloc>
    {
      using value_type = typename __allocated_ptr<_Alloc>::value_type;
 
      __allocated_obj(__allocated_obj<_Alloc>&&) = default;
 
      // Default-initialize a value_type at *__ptr
      __allocated_obj(__allocated_ptr<_Alloc>&& __ptr)
      : __allocated_ptr<_Alloc>(std::move(__ptr))
      { ::new ((void*)this->get()) value_type; }
 
      // Call the destructor if an object is owned.
      ~__allocated_obj()
      {
        if (static_cast<bool>(*this))
          this->get()->~value_type();
      }
 
      using __allocated_ptr<_Alloc>::operator=;
 
      value_type& operator*() const { return *this->get(); }
      value_type* operator->() const { return this->get(); }
    };
 
  /// Construct an object in storage allocated using __a.
  template<typename _Alloc>
    inline __allocated_obj<_Alloc>
    __allocate_guarded_obj(_Alloc& __a)
    {
      return { std::__allocate_guarded(__a) };
    }
 
  // An RAII type that acquires memory from an allocator.
  // N.B.  'scoped' here in in the RAII sense, not the scoped allocator model,
  // so this has nothing to do with `std::scoped_allocator_adaptor`.
  // This class can be used to simplify the common pattern:
  //
  //     auto ptr = alloc.allocate(1);
  //     try {
  //       std::construct_at(std::to_address(ptr), args);
  //       m_ptr = ptr;
  //     } catch (...) {
  //       alloc.deallocate(ptr, 1);
  //       throw;
  //     }
  //
  // Instead you can do:
  //
  //     _Scoped_allocation sa(alloc);
  //     m_ptr = std::construct_at(sa.get(), args);
  //     (void) sa.release();
  //
  // Or even simpler:
  //
  //     _Scoped_allocation sa(alloc, std::in_place, args);
  //     m_ptr = sa.release();
  //
  template<typename _Alloc>
    struct _Scoped_allocation
    {
      using value_type = typename allocator_traits<_Alloc>::value_type;
      using pointer = typename allocator_traits<_Alloc>::pointer;
 
      // Use `a` to allocate memory for `n` objects.
      constexpr explicit
      _Scoped_allocation(const _Alloc& __a, size_t __n = 1)
      : _M_a(__a), _M_n(__n), _M_p(_M_a.allocate(__n))
      { }
 
#if __glibcxx_optional >= 201606L
      // Allocate memory for a single object and if that succeeds,
      // construct an object using args.
      //
      // Does not do uses-allocator construction; don't use if you need that.
      //
      // CAUTION: the destructor will *not* destroy this object, it will only
      // free the memory. That means the following pattern is unsafe:
      //
      //     _Scoped_allocation  sa(alloc, in_place, args);
      //     potentially_throwing_operations();
      //     return sa.release();
      //
      // If the middle operation throws, the object will not be destroyed.
      template<typename... _Args>
        constexpr explicit
        _Scoped_allocation(const _Alloc& __a, in_place_t, _Args&&... __args)
        : _Scoped_allocation(__a, 1)
        {
          // The target constructor has completed, so if the next line throws,
          // the destructor will deallocate the memory.
          allocator_traits<_Alloc>::construct(_M_a, get(),
                                              std::forward<_Args>(__args)...);
        }
#endif
 
      _GLIBCXX20_CONSTEXPR
      ~_Scoped_allocation()
      {
        if (_M_p) [[__unlikely__]]
          _M_a.deallocate(_M_p, _M_n);
      }
 
      _Scoped_allocation(_Scoped_allocation&&) = delete;
 
      constexpr _Alloc
      get_allocator() const noexcept { return _M_a; }
 
      constexpr value_type*
      get() const noexcept
      { return std::__to_address(_M_p); }
 
      [[__nodiscard__]]
      constexpr pointer
      release() noexcept { return std::__exchange(_M_p, nullptr); }
 
    private:
      [[__no_unique_address__]] _Alloc _M_a;
      size_t _M_n;
      pointer _M_p;
    };
 
#if __glibcxx_optional >= 201606L && __cpp_deduction_guides >= 201606L
  template<typename _Alloc, typename... _Args>
    _Scoped_allocation(_Alloc, in_place_t, _Args...)
      -> _Scoped_allocation<_Alloc>;
#endif
 
/// @endcond
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#endif
#endif