Add QPDFObjectHandle::unsafeShallowCopy

ChangeLog

@@ -1,5 +1,9 @@
 2020-04-02  Jay Berkenbilt  <ejb@ql.org>
 
+	* Add method QPDFObjectHandle::unsafeShallowCopy for copying only
+	top-level dictionary keys or array items. See comments in
+	QPDFObjectHandle.hh for when this should be used.
+
 	* Remove Members class indirection for QPDFObjectHandle. Those are
 	copied and assigned too often, and that change caused a very
 	substantial performance hit.

include/qpdf/QPDFObjectHandle.hh

@@ -670,10 +670,24 @@ class QPDFObjectHandle
     // traverse across indirect object boundaries. That means that,
     // for dictionaries and arrays, any keys or items that were
     // indirect objects will still be indirect objects that point to
-    // the same place.
+    // the same place. In the strictest sense, this is not a shallow
+    // copy because it recursively descends arrays and dictionaries;
+    // it just doesn't cross over indirect objects. See also
+    // unsafeShallowCopy().
     QPDF_DLL
     QPDFObjectHandle shallowCopy();
 
+    // Create a true shallow copy of an array or dictionary, just
+    // copying the immediate items (array) or keys (dictionary). This
+    // is "unsafe" because, if you *modify* any of the items in the
+    // copy, you are modifying the original, which is almost never
+    // what you want. However, if your intention is merely to
+    // *replace* top-level items or keys and not to modify lower-level
+    // items in the copy, this method is much faster than
+    // shallowCopy().
+    QPDF_DLL
+    QPDFObjectHandle unsafeShallowCopy();
+
     // Mutator methods.  Use with caution.
 
     // Recursively copy this object, making it direct.  Throws an
@@ -1053,7 +1067,9 @@ class QPDFObjectHandle
     void objectWarning(std::string const& warning);
     void assertType(char const* type_name, bool istype);
     void dereference();
-    void copyObject(std::set<QPDFObjGen>& visited, bool cross_indirect);
+    void copyObject(std::set<QPDFObjGen>& visited, bool cross_indirect,
+                    bool first_level_only);
+    void shallowCopyInternal(QPDFObjectHandle& oh, bool first_level_only);
     void releaseResolved();
     static void setObjectDescriptionFromInput(
         QPDFObjectHandle, QPDF*, std::string const&,

libqpdf/QPDFObjectHandle.cc

@@ -2460,6 +2460,23 @@ QPDFObjectHandle::hasObjectDescription()
 
 QPDFObjectHandle
 QPDFObjectHandle::shallowCopy()
+{
+    QPDFObjectHandle result;
+    shallowCopyInternal(result, false);
+    return result;
+}
+
+QPDFObjectHandle
+QPDFObjectHandle::unsafeShallowCopy()
+{
+    QPDFObjectHandle result;
+    shallowCopyInternal(result, true);
+    return result;
+}
+
+void
+QPDFObjectHandle::shallowCopyInternal(QPDFObjectHandle& new_obj,
+                                      bool first_level_only)
 {
     assertInitialized();
 
@@ -2470,7 +2487,6 @@ QPDFObjectHandle::shallowCopy()
 	    "attempt to make a shallow copy of a stream");
     }
 
-    QPDFObjectHandle new_obj;
     if (isArray())
     {
 	QTC::TC("qpdf", "QPDFObjectHandle shallow copy array");
@@ -2491,13 +2507,12 @@ QPDFObjectHandle::shallowCopy()
     }
 
     std::set<QPDFObjGen> visited;
-    new_obj.copyObject(visited, false);
+    new_obj.copyObject(visited, false, first_level_only);
-    return new_obj;
 }
 
 void
 QPDFObjectHandle::copyObject(std::set<QPDFObjGen>& visited,
-                             bool cross_indirect)
+                             bool cross_indirect, bool first_level_only)
 {
     assertInitialized();
 
@@ -2573,9 +2588,11 @@ QPDFObjectHandle::copyObject(std::set<QPDFObjGen>& visited,
 	for (int i = 0; i < n; ++i)
 	{
 	    items.push_back(getArrayItem(i));
-            if (cross_indirect || (! items.back().isIndirect()))
+            if ((! first_level_only) &&
+                (cross_indirect || (! items.back().isIndirect())))
             {
-                items.back().copyObject(visited, cross_indirect);
+                items.back().copyObject(
+                    visited, cross_indirect, first_level_only);
             }
 	}
 	new_obj = new QPDF_Array(items);
@@ -2589,9 +2606,11 @@ QPDFObjectHandle::copyObject(std::set<QPDFObjGen>& visited,
 	     iter != keys.end(); ++iter)
 	{
 	    items[*iter] = getKey(*iter);
-            if (cross_indirect || (! items[*iter].isIndirect()))
+            if ((! first_level_only) &&
+                (cross_indirect || (! items[*iter].isIndirect())))
             {
-                items[*iter].copyObject(visited, cross_indirect);
+                items[*iter].copyObject(
+                    visited, cross_indirect, first_level_only);
             }
 	}
 	new_obj = new QPDF_Dictionary(items);
@@ -2614,7 +2633,7 @@ void
 QPDFObjectHandle::makeDirect()
 {
     std::set<QPDFObjGen> visited;
-    copyObject(visited, true);
+    copyObject(visited, true, false);
 }
 
 void