PointerHolder: add get() and use_count() for forward compatibility

PointerHolder will be replaced with shared_ptr, so let people start
moving.

ChangeLog

@@ -1,3 +1,20 @@
+2022-02-04  Jay Berkenbilt  <ejb@ql.org>
+
+	* PointerHolder: add a get() method and a use_count() method for
+	compatibility with std::shared_ptr. In qpdf 11, qpdf's APIs will
+	switch to using std::shared_ptr instead of PointerHolder, though
+	there will be a PointerHolder class with a backward-compatible
+	API. To ease the transition, we are adding get() now with the same
+	semantics as std::shared_ptr's get. Note that there is a
+	difference in behavior: const PointerHolder has always behaved
+	incorrectly. const PointerHolder objects only returned const
+	pointers. This is wrong. If you want a const pointer, use
+	PointerHolder<T const>. A const PointerHolder just shouldn't allow
+	its pointer to be reassigned. The new get() method behaves
+	correctly in that calling get() on a const PointerHolder to a
+	non-const pointer returns a non-const pointer. This is the way
+	regular pointers behave.
+
 2022-02-01  Jay Berkenbilt  <ejb@ql.org>
 
 	* Major refactor: all functionality from the qpdf CLI is now

include/qpdf/PointerHolder.hh

@@ -22,6 +22,14 @@
 #ifndef POINTERHOLDER_HH
 #define POINTERHOLDER_HH
 
+// In qpdf 11, PointerHolder will be derived from std::shared_ptr and
+// will also include a fix to incorrect semantics of const
+// PointerHolder objects. PointerHolder only allows a const
+// PointerHolder to return a const pointer. This is wrong. Use a
+// PointerHolder<const T> for that. A const PointerHolder should just
+// not allow you to change what it points to. This is consistent with
+// how regular pointers and standard library shared pointers work.
+
 // This class is basically std::shared_ptr but predates that by
 // several years.
 
@@ -119,6 +127,12 @@ class PointerHolder
 	return this->data->pointer < rhs.data->pointer;
     }
 
+    // get() is for interface compatibility with std::shared_ptr
+    T* get() const
+    {
+        return this->data->pointer;
+    }
+
     // NOTE: The pointer returned by getPointer turns into a pumpkin
     // when the last PointerHolder that contains it disappears.
     T* getPointer()
@@ -134,6 +148,12 @@ class PointerHolder
 	    return this->data->refcount;
 	}
 
+    // use_count() is for compatibility with std::shared_ptr
+    long use_count()
+        {
+            return static_cast<long>(this->data->refcount);
+        }
+
     T const& operator*() const
     {
         return *this->data->pointer;

libtests/pointer_holder.cc

@@ -4,8 +4,6 @@
 #include <stdlib.h>
 #include <list>
 
-#include <qpdf/QUtil.hh>
-
 class Object
 {
   public:
@@ -47,13 +45,20 @@ Object::hello() const
 
 typedef PointerHolder<Object> ObjectHolder;
 
-void callHello(ObjectHolder const& oh)
+void callHello(ObjectHolder& oh)
 {
     oh.getPointer()->hello();
     oh->hello();
     (*oh).hello();
 }
 
+void callHelloWithGet(ObjectHolder const& oh)
+{
+    oh.get()->hello();
+    oh->hello();
+    (*oh).hello();
+}
+
 int main(int argc, char* argv[])
 {
     std::list<ObjectHolder> ol1;
@@ -66,7 +71,7 @@ int main(int argc, char* argv[])
         std::cout << "oh1 refcount = " << oh1.getRefcount() << std::endl;
 	ObjectHolder oh2(oh1);
         std::cout << "oh1 refcount = " << oh1.getRefcount() << std::endl;
-        std::cout << "oh2 refcount = " << oh2.getRefcount() << std::endl;
+        std::cout << "oh2 refcount = " << oh2.use_count() << std::endl;
 	ObjectHolder oh3(new Object);
 	ObjectHolder oh4;
 	ObjectHolder oh5;
@@ -89,12 +94,15 @@ int main(int argc, char* argv[])
 	ol1.push_back(oh3);
 	Object* o3 = new Object;
 	oh0 = o3;
+        PointerHolder<Object const> oh6(new Object());
+        oh6->hello();
     }
 
     ol1.front().getPointer()->hello();
     ol1.front()->hello();
     (*ol1.front()).hello();
     callHello(ol1.front());
+    callHelloWithGet(ol1.front());
     ol1.pop_front();
     std::cout << "array" << std::endl;
     PointerHolder<Object> oarr1_ph(true, new Object[2]);

libtests/qtest/ph/ph.out

@@ -10,17 +10,23 @@ destroyed Object, id 2
 equal okay
 less than okay
 created Object, id 3
+created Object, id 4
+calling Object::hello const for 4
+destroyed Object, id 4
+calling Object::hello for 1
+calling Object::hello for 1
+calling Object::hello for 1
+calling Object::hello for 1
 calling Object::hello for 1
 calling Object::hello for 1
 calling Object::hello for 1
-calling Object::hello const for 1
 calling Object::hello const for 1
 calling Object::hello const for 1
 array
-created Object, id 4
 created Object, id 5
+created Object, id 6
 goodbye
+destroyed Object, id 6
 destroyed Object, id 5
-destroyed Object, id 4
 destroyed Object, id 3
 destroyed Object, id 1