Create a special "destroyed" type rather than using null

When a QPDF is destroyed, changing indirect objects to direct nulls
makes them effectively disappear silently when they sneak into other
places. Instead, we should treat this as an error. Adding a destroyed
object type makes this possible.

1 files changed, 7 insertions, 5 deletions

diff --git a/manual/release-notes.rst b/manual/release-notes.rst
index 9cf09840..bf6a8301 100644
--- a/manual/release-notes.rst
+++ b/manual/release-notes.rst
@@ -202,11 +202,13 @@ For a detailed list of changes, please see the file
       ``replaceKeyAndGetOld``, a ``null`` object if the object was not
       previously there.
 
-    - The method ``QPDFObjectHandle::getOwningQPDF`` now returns a
-      null pointer (rather than an invalid pointer) if the owning
-      ``QPDF`` object has been destroyed. This situation should
-      generally not happen for correct code, but at least the
-      situation is detectible now.
+    - It is now possible to detect when a ``QPDFObjectHandle`` is an
+      indirect object that belongs to a ``QPDF`` that has been
+      destroyed. Any attempt to unparse this type of
+      ``QPDFObjectHandle`` will throw a logic error. You can detect
+      this by calling the new ``QPDFObjectHandle::isDestroyed``
+      method. Also the ``QPDFObjectHandle::getOwningQPDF`` method now
+      returns a null pointer rather than an invalid pointer.
 
     - The method ``QPDFObjectHandle::getQPDF`` returns a ``QPDF&``
       (rather than a ``QPDF*``) and is an alternative to