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, 3 insertions, 1 deletions

diff --git a/libqpdf/QPDF_Unresolved.cc b/libqpdf/QPDF_Unresolved.cc
index 503e5b84..79553c1e 100644
--- a/libqpdf/QPDF_Unresolved.cc
+++ b/libqpdf/QPDF_Unresolved.cc
@@ -17,7 +17,7 @@ std::shared_ptr<QPDFValueProxy>
 QPDF_Unresolved::shallowCopy()
 {
     throw std::logic_error(
-        "attempted to shallow copy unresolved QPDFObjectHandle");
+        "attempted to shallow copy an unresolved QPDFObjectHandle");
     return nullptr;
 }
 
@@ -32,5 +32,7 @@ QPDF_Unresolved::unparse()
 JSON
 QPDF_Unresolved::getJSON(int json_version)
 {
+    throw std::logic_error(
+        "attempted to get JSON from an unresolved QPDFObjectHandle");
     return JSON::makeNull();
 }