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, 8 insertions, 6 deletions

diff --git a/TODO b/TODO
index cdec24b7..6f491dff 100644
--- a/TODO
+++ b/TODO
@@ -812,9 +812,11 @@ Rejected Ideas
   QPDFObjectHandle because of indirect objects. This only pertains to
   direct objects, which are always "resolved" in QPDFObjectHandle.
 
-  If this is addressed, read comments in QPDFWriter.cc::enqueueObject
-  near the call to getOwningQPDF, comments in QPDFValueProxy::reset,
-  and comments in QPDF::~QPDF() near the line that assigns to null.
-  This will also affect test 92 in test_driver.cc. All these
-  references were from the release of qpdf 11 (in case they have moved
-  by such time as this might be resurrected).
+  If this is addressed, read comments in the following places:
+  * QPDFWriter.cc::enqueueObject near the call to getOwningQPDF
+  * QPDFValueProxy::reset and QPDFValueProxy::destroy
+  * QPDF::~QPDF()
+  * test 92 in test_driver.cc
+  * QPDFObjectHandle.hh near isDestroyed
+  All these references were from the release of qpdf 11 (in case they
+  have moved by such time as this might be resurrected).