gdbsupport/gdb_ref_ptr.h

   1 /* Reference-counted smart pointer class
   2
   3    Copyright (C) 2016-2021 Free Software Foundation, Inc.
   4
   5    This file is part of GDB.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 #ifndef COMMON_GDB_REF_PTR_H
  21 #define COMMON_GDB_REF_PTR_H
  22
  23 #include <cstddef>
  24
  25 namespace gdb
  26 {
  27
  28 /* An instance of this class either holds a reference to a
  29    reference-counted object or is "NULL".  Reference counting is
  30    handled externally by a policy class.  If the object holds a
  31    reference, then when the object is destroyed, the reference is
  32    decref'd.
  33
  34    Normally an instance is constructed using a pointer.  This sort of
  35    initialization lets this class manage the lifetime of that
  36    reference.
  37
  38    Assignment and copy construction will make a new reference as
  39    appropriate.  Assignment from a plain pointer is disallowed to
  40    avoid confusion about whether this acquires a new reference;
  41    instead use the "reset" method -- which, like the pointer
  42    constructor, transfers ownership.
  43
  44    The policy class must provide two static methods:
  45    void incref (T *);
  46    void decref (T *);
  47 */
  48 template<typename T, typename Policy>
  49 class ref_ptr
  50 {
  51  public:
  52
  53   /* Create a new NULL instance.  */
  54   ref_ptr ()
  55     : m_obj (NULL)
  56   {
  57   }
  58
  59   /* Create a new NULL instance.  Note that this is not explicit.  */
  60   ref_ptr (const std::nullptr_t)
  61     : m_obj (NULL)
  62   {
  63   }
  64
  65   /* Create a new instance.  OBJ is a reference, management of which
  66      is now transferred to this class.  */
  67   explicit ref_ptr (T *obj)
  68     : m_obj (obj)
  69   {
  70   }
  71
  72   /* Copy another instance.  */
  73   ref_ptr (const ref_ptr &other)
  74     : m_obj (other.m_obj)
  75   {
  76     if (m_obj != NULL)
  77       Policy::incref (m_obj);
  78   }
  79
  80   /* Transfer ownership from OTHER.  */
  81   ref_ptr (ref_ptr &&other) noexcept
  82     : m_obj (other.m_obj)
  83   {
  84     other.m_obj = NULL;
  85   }
  86
  87   /* Destroy this instance.  */
  88   ~ref_ptr ()
  89   {
  90     if (m_obj != NULL)
  91       Policy::decref (m_obj);
  92   }
  93
  94   /* Copy another instance.  */
  95   ref_ptr &operator= (const ref_ptr &other)
  96   {
  97     /* Do nothing on self-assignment.  */
  98     if (this != &other)
  99       {
 100         reset (other.m_obj);
 101         if (m_obj != NULL)
 102           Policy::incref (m_obj);
 103       }
 104     return *this;
 105   }
 106
 107   /* Transfer ownership from OTHER.  */
 108   ref_ptr &operator= (ref_ptr &&other)
 109   {
 110     /* Do nothing on self-assignment.  */
 111     if (this != &other)
 112       {
 113         reset (other.m_obj);
 114         other.m_obj = NULL;
 115       }
 116     return *this;
 117   }
 118
 119   /* Change this instance's referent.  OBJ is a reference, management
 120      of which is now transferred to this class.  */
 121   void reset (T *obj)
 122   {
 123     if (m_obj != NULL)
 124       Policy::decref (m_obj);
 125     m_obj = obj;
 126   }
 127
 128   /* Return this instance's referent without changing the state of
 129      this class.  */
 130   T *get () const
 131   {
 132     return m_obj;
 133   }
 134
 135   /* Return this instance's referent, and stop managing this
 136      reference.  The caller is now responsible for the ownership of
 137      the reference.  */
 138   ATTRIBUTE_UNUSED_RESULT T *release ()
 139   {
 140     T *result = m_obj;
 141
 142     m_obj = NULL;
 143     return result;
 144   }
 145
 146   /* Let users refer to members of the underlying pointer.  */
 147   T *operator-> () const
 148   {
 149     return m_obj;
 150   }
 151
 152   /* Acquire a new reference and return a ref_ptr that owns it.  */
 153   static ref_ptr<T, Policy> new_reference (T *obj)
 154   {
 155     Policy::incref (obj);
 156     return ref_ptr<T, Policy> (obj);
 157   }
 158
 159  private:
 160
 161   T *m_obj;
 162 };
 163
 164 template<typename T, typename Policy>
 165 inline bool operator== (const ref_ptr<T, Policy> &lhs,
 166                         const ref_ptr<T, Policy> &rhs)
 167 {
 168   return lhs.get () == rhs.get ();
 169 }
 170
 171 template<typename T, typename Policy>
 172 inline bool operator== (const ref_ptr<T, Policy> &lhs, const T *rhs)
 173 {
 174   return lhs.get () == rhs;
 175 }
 176
 177 template<typename T, typename Policy>
 178 inline bool operator== (const ref_ptr<T, Policy> &lhs, const std::nullptr_t)
 179 {
 180   return lhs.get () == nullptr;
 181 }
 182
 183 template<typename T, typename Policy>
 184 inline bool operator== (const T *lhs, const ref_ptr<T, Policy> &rhs)
 185 {
 186   return lhs == rhs.get ();
 187 }
 188
 189 template<typename T, typename Policy>
 190 inline bool operator== (const std::nullptr_t, const ref_ptr<T, Policy> &rhs)
 191 {
 192   return nullptr == rhs.get ();
 193 }
 194
 195 template<typename T, typename Policy>
 196 inline bool operator!= (const ref_ptr<T, Policy> &lhs,
 197                         const ref_ptr<T, Policy> &rhs)
 198 {
 199   return lhs.get () != rhs.get ();
 200 }
 201
 202 template<typename T, typename Policy>
 203 inline bool operator!= (const ref_ptr<T, Policy> &lhs, const T *rhs)
 204 {
 205   return lhs.get () != rhs;
 206 }
 207
 208 template<typename T, typename Policy>
 209 inline bool operator!= (const ref_ptr<T, Policy> &lhs, const std::nullptr_t)
 210 {
 211   return lhs.get () != nullptr;
 212 }
 213
 214 template<typename T, typename Policy>
 215 inline bool operator!= (const T *lhs, const ref_ptr<T, Policy> &rhs)
 216 {
 217   return lhs != rhs.get ();
 218 }
 219
 220 template<typename T, typename Policy>
 221 inline bool operator!= (const std::nullptr_t, const ref_ptr<T, Policy> &rhs)
 222 {
 223   return nullptr != rhs.get ();
 224 }
 225
 226 }
 227
 228 #endif /* COMMON_GDB_REF_PTR_H */