Add gdb_ref_ptr.h

This adds a new gdb_ref_ptr.h, that implements a reference-counting
smart pointer class, where the user of the class supplies a
reference-counting policy object.

This class will be used in the next patch, which changes most explicit
BFD reference counts to use this new type.  Meanwhile, this patch
changes gdbpy_ref to be a specialization of this new class.

This change required adding new nullptr_t overloads some operators in
gdb_ref_ptr.h.  I suspect this was needed because some Python header
redefines NULL, but I'm not certain.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* common/gdb_ref_ptr.h: New file.
	* python/py-ref.h (struct gdbpy_ref_policy): New.
	(gdbpy_ref): Now a typedef.

1 files changed, 215 insertions, 0 deletions

diff --git a/gdb/common/gdb_ref_ptr.h b/gdb/common/gdb_ref_ptr.h
new file mode 100644
index 0000000..fbf4688
--- /dev/null
+++ b/gdb/common/gdb_ref_ptr.h
@@ -0,0 +1,215 @@
+/* Reference-counted smart pointer class
+
+   Copyright (C) 2016 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program 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 of the License, or
+   (at your option) any later version.
+
+   This program 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.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_REF_PTR_H
+#define GDB_REF_PTR_H
+
+#include <cstddef>
+
+namespace gdb
+{
+
+/* An instance of this class either holds a reference to a
+   reference-counted object or is "NULL".  Reference counting is
+   handled externally by a policy class.  If the object holds a
+   reference, then when the object is destroyed, the reference is
+   decref'd.
+
+   Normally an instance is constructed using a pointer.  This sort of
+   initialization lets this class manage the lifetime of that
+   reference.
+
+   Assignment and copy construction will make a new reference as
+   appropriate.  Assignment from a plain pointer is disallowed to
+   avoid confusion about whether this acquires a new reference;
+   instead use the "reset" method -- which, like the pointer
+   constructor, transfers ownership.
+
+   The policy class must provide two static methods:
+   void incref (T *);
+   void decref (T *);
+*/
+template<typename T, typename Policy>
+class ref_ptr
+{
+ public:
+
+  /* Create a new NULL instance.  */
+  ref_ptr ()
+    : m_obj (NULL)
+  {
+  }
+
+  /* Create a new NULL instance.  Note that this is not explicit.  */
+  ref_ptr (const std::nullptr_t)
+    : m_obj (NULL)
+  {
+  }
+
+  /* Create a new instance.  OBJ is a reference, management of which
+     is now transferred to this class.  */
+  explicit ref_ptr (T *obj)
+    : m_obj (obj)
+  {
+  }
+
+  /* Copy another instance.  */
+  ref_ptr (const ref_ptr &other)
+    : m_obj (other.m_obj)
+  {
+    if (m_obj != NULL)
+      Policy::incref (m_obj);
+  }
+
+  /* Transfer ownership from OTHER.  */
+  ref_ptr (ref_ptr &&other)
+    : m_obj (other.m_obj)
+  {
+    other.m_obj = NULL;
+  }
+
+  /* Destroy this instance.  */
+  ~ref_ptr ()
+  {
+    if (m_obj != NULL)
+      Policy::decref (m_obj);
+  }
+
+  /* Copy another instance.  */
+  ref_ptr &operator= (const ref_ptr &other)
+  {
+    /* Do nothing on self-assignment.  */
+    if (this != &other)
+      {
+	reset (other.m_obj);
+	if (m_obj != NULL)
+	  Policy::incref (m_obj);
+      }
+    return *this;
+  }
+
+  /* Transfer ownership from OTHER.  */
+  ref_ptr &operator= (ref_ptr &&other)
+  {
+    /* Do nothing on self-assignment.  */
+    if (this != &other)
+      {
+	reset (other.m_obj);
+	other.m_obj = NULL;
+      }
+    return *this;
+  }
+
+  /* Change this instance's referent.  OBJ is a reference, management
+     of which is now transferred to this class.  */
+  void reset (T *obj)
+  {
+    if (m_obj != NULL)
+      Policy::decref (m_obj);
+    m_obj = obj;
+  }
+
+  /* Return this instance's referent without changing the state of
+     this class.  */
+  T *get () const
+  {
+    return m_obj;
+  }
+
+  /* Return this instance's referent, and stop managing this
+     reference.  The caller is now responsible for the ownership of
+     the reference.  */
+  T *release ()
+  {
+    T *result = m_obj;
+
+    m_obj = NULL;
+    return result;
+  }
+
+ private:
+
+  T *m_obj;
+};
+
+template<typename T, typename Policy>
+inline bool operator== (const ref_ptr<T, Policy> &lhs,
+			const ref_ptr<T, Policy> &rhs)
+{
+  return lhs.get () == rhs.get ();
+}
+
+template<typename T, typename Policy>
+inline bool operator== (const ref_ptr<T, Policy> &lhs, const T *rhs)
+{
+  return lhs.get () == rhs;
+}
+
+template<typename T, typename Policy>
+inline bool operator== (const ref_ptr<T, Policy> &lhs, const std::nullptr_t)
+{
+  return lhs.get () == nullptr;
+}
+
+template<typename T, typename Policy>
+inline bool operator== (const T *lhs, const ref_ptr<T, Policy> &rhs)
+{
+  return lhs == rhs.get ();
+}
+
+template<typename T, typename Policy>
+inline bool operator== (const std::nullptr_t, const ref_ptr<T, Policy> &rhs)
+{
+  return nullptr == rhs.get ();
+}
+
+template<typename T, typename Policy>
+inline bool operator!= (const ref_ptr<T, Policy> &lhs,
+			const ref_ptr<T, Policy> &rhs)
+{
+  return lhs.get () != rhs.get ();
+}
+
+template<typename T, typename Policy>
+inline bool operator!= (const ref_ptr<T, Policy> &lhs, const T *rhs)
+{
+  return lhs.get () != rhs;
+}
+
+template<typename T, typename Policy>
+inline bool operator!= (const ref_ptr<T, Policy> &lhs, const std::nullptr_t)
+{
+  return lhs.get () != nullptr;
+}
+
+template<typename T, typename Policy>
+inline bool operator!= (const T *lhs, const ref_ptr<T, Policy> &rhs)
+{
+  return lhs != rhs.get ();
+}
+
+template<typename T, typename Policy>
+inline bool operator!= (const std::nullptr_t, const ref_ptr<T, Policy> &rhs)
+{
+  return nullptr != rhs.get ();
+}
+
+}
+
+#endif /* GDB_REF_PTR_H */