[RFC 1/4] Add gdbpy_borrowed_ref

[Tom Tromey <tom@tromey.com>]

This adds a new gdbpy_borrowed_ref class.  This class is primarily for
code "documentation" purposes -- it makes it clear to the reader that
a given reference is borrowed.  However, it also adds a tiny bit of
safety, in that conversion to gdbpy_ref<> will acquire a new
reference.
---
 gdb/python/py-ref.h | 47 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/gdb/python/py-ref.h b/gdb/python/py-ref.h
index 0a56436634d..56508996eaa 100644
--- a/gdb/python/py-ref.h
+++ b/gdb/python/py-ref.h
@@ -41,6 +41,53 @@ struct gdbpy_ref_policy
 template<typename T = PyObject> using gdbpy_ref
   = gdb::ref_ptr<T, gdbpy_ref_policy>;
 
+/* A class representing a borrowed reference.
+
+   This is a simple wrapper for a PyObject*.  Aside from documenting
+   what the code does, the main advantage of using this is that
+   conversion to a gdbpy_ref<> is guaranteed to make a new
+   reference.  */
+class gdbpy_borrowed_ref
+{
+public:
+
+  gdbpy_borrowed_ref (PyObject *obj)
+    : m_obj (obj)
+  {
+  }
+
+  template<typename T>
+  gdbpy_borrowed_ref (const gdbpy_ref<T> &ref)
+    : m_obj (ref.get ())
+  {
+  }
+
+  /* Allow a (checked) conversion to any subclass of PyObject.  */
+  template<typename T,
+	   typename = std::is_convertible<T *, PyObject *>>
+  operator T * ()
+  {
+    gdb_assert (PyObject_TypeCheck (m_obj, T::corresponding_object_type));
+    return static_cast<T *> (m_obj);
+  }
+
+  operator PyObject * ()
+  {
+    return m_obj;
+  }
+
+  /* When converting a borrowed reference to a gdbpy_ref<>, a new
+     reference is acquired.  */
+  operator gdbpy_ref<> ()
+  {
+    gdb_assert (m_obj != nullptr);
+    return gdbpy_ref<>::new_reference (m_obj);
+  }
+
+private:
+  PyObject *m_obj;
+};
+
 /* A wrapper class for Python extension objects that have a __dict__ attribute.
 
    Any Python C object extension needing __dict__ should inherit from this
-- 
2.49.0