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, 10 insertions, 2 deletions

diff --git a/libqpdf/QPDFObjectHandle.cc b/libqpdf/QPDFObjectHandle.cc
index fa8aae7f..556e3a1e 100644
--- a/libqpdf/QPDFObjectHandle.cc
+++ b/libqpdf/QPDFObjectHandle.cc
@@ -252,8 +252,10 @@ void
 QPDFObjectHandle::reset()
 {
     // Recursively remove association with any QPDF object. This
-    // method may only be called during final destruction. See
-    // comments in QPDF::~QPDF().
+    // method may only be called during final destruction.
+    // QPDF::~QPDF() calls it for indirect objects using the object
+    // pointer itself, so we don't do that here. Other objects call it
+    // through this method.
     if (!isIndirect()) {
         this->obj->reset();
     }
@@ -352,6 +354,12 @@ QPDFObjectHandle::asString()
 }
 
 bool
+QPDFObjectHandle::isDestroyed()
+{
+    return dereference() && (obj->getTypeCode() == ::ot_destroyed);
+}
+
+bool
 QPDFObjectHandle::isBool()
 {
     return dereference() && (obj->getTypeCode() == ::ot_boolean);