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.

gdb/python/py-ref.h

@@ -20,140 +20,23 @@
 #ifndef GDB_PYTHON_REF_H
 #define GDB_PYTHON_REF_H
 
-/* An instance of this class either holds a reference to a PyObject,
-   or is "NULL".  If it holds a reference, then when the object is
-   destroyed, the PyObject is decref'd.
+#include "common/gdb_ref_ptr.h"
 
-   Normally an instance is constructed using a PyObject*.  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 PyObject* is disallowed to
-   avoid confusion about whether this acquires a new reference;
-   instead use the "reset" method -- which, like the PyObject*
-   constructor, transfers ownership.
-*/
-class gdbpy_ref
+/* A policy class for gdb::ref_ptr for Python reference counting.  */
+struct gdbpy_ref_policy
 {
- public:
-
-  /* Create a new NULL instance.  */
-  gdbpy_ref ()
-    : m_obj (NULL)
+  static void incref (PyObject *ptr)
   {
+    Py_INCREF (ptr);
   }
 
-  /* Create a new instance.  OBJ is a reference, management of which
-     is now transferred to this class.  */
-  explicit gdbpy_ref (PyObject *obj)
-    : m_obj (obj)
+  static void decref (PyObject *ptr)
   {
+    Py_DECREF (ptr);
   }
-
-  /* Copy another instance.  */
-  gdbpy_ref (const gdbpy_ref &other)
-    : m_obj (other.m_obj)
-  {
-    Py_XINCREF (m_obj);
-  }
-
-  /* Transfer ownership from OTHER.  */
-  gdbpy_ref (gdbpy_ref &&other)
-    : m_obj (other.m_obj)
-  {
-    other.m_obj = NULL;
-  }
-
-  /* Destroy this instance.  */
-  ~gdbpy_ref ()
-  {
-    Py_XDECREF (m_obj);
-  }
-
-  /* Copy another instance.  */
-  gdbpy_ref &operator= (const gdbpy_ref &other)
-  {
-    /* Do nothing on self-assignment.  */
-    if (this != &other)
-      {
-	reset (other.m_obj);
-	Py_XINCREF (m_obj);
-      }
-    return *this;
-  }
-
-  /* Transfer ownership from OTHER.  */
-  gdbpy_ref &operator= (gdbpy_ref &&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 (PyObject *obj)
-  {
-    Py_XDECREF (m_obj);
-    m_obj = obj;
-  }
-
-  /* Return this instance's referent.  In Python terms this is a
-     borrowed pointer.  */
-  PyObject *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.  */
-  PyObject *release ()
-  {
-    PyObject *result = m_obj;
-
-    m_obj = NULL;
-    return result;
-  }
-
- private:
-
-  PyObject *m_obj;
 };
 
-inline bool operator== (const gdbpy_ref &self, const gdbpy_ref &other)
-{
-  return self.get () == other.get ();
-}
-
-inline bool operator== (const gdbpy_ref &self, const PyObject *other)
-{
-  return self.get () == other;
-}
-
-inline bool operator== (const PyObject *self, const gdbpy_ref &other)
-{
-  return self == other.get ();
-}
-
-inline bool operator!= (const gdbpy_ref &self, const gdbpy_ref &other)
-{
-  return self.get () != other.get ();
-}
-
-inline bool operator!= (const gdbpy_ref &self, const PyObject *other)
-{
-  return self.get () != other;
-}
-
-inline bool operator!= (const PyObject *self, const gdbpy_ref &other)
-{
-  return self != other.get ();
-}
+/* A gdb::ref_ptr that has been specialized for Python objects.  */
+typedef gdb::ref_ptr<PyObject, gdbpy_ref_policy> gdbpy_ref;
 
 #endif /* GDB_PYTHON_REF_H */