Rework PointerHolder transition to make it smoother

* Don't surprise people with deprecation warnings * Provide detailed instructions and support for the transition

Rework PointerHolder transition to make it smoother
* Don't surprise people with deprecation warnings * Provide detailed instructions and support for the transition
Jay Berkenbilt
1 parent df2f5c6a
Showing 6 changed files with 575 additions and 291 deletions
ChangeLog
TODO
include/qpdf/PointerHolder.hh
libtests/pointer_holder.cc
manual/design.rst
manual/release-notes.rst
@@ -47,11 +47,6 @@
 	includes so you don't have to try to include a header that won't
 	be there.
-	* PointerHolder: deprecate getPointer() and getRefcount(). If you
-	don't want to disable deprecation warnings in general but are not
-	ready to tackle this change yet, you can define
-	NO_POINTERHOLDER_DEPRECATION to suppress those.
-
 	* 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
@@ -388,35 +388,65 @@ Other notes:
 PointerHolder to std::shared_ptr
 ================================
-Remember to update the smart-pointers section of the manual in
-design.rst.
+To perform update:
-Once all deprecation warnings are cleared up (changing getPointer() to
-get() and getRefcount() to use_count()), the only real issues are that
-implicit assignment of a pointer to a shared_ptr doesn't work while it
-does for PointerHolder and containers are different. Using auto takes
-care of containers.
+Cherry-pick pointerholder branch commit
-PointerHolder<X> x = new X() -->
-auto x = std::make_shared<X>()
+Upgrade just the library. This is not necessary, but it's an added
+check that the compatibility code works since it will show that tests,
+examples, and CLI will work properly with the upgraded APIs, which
+provides some assurance that other people will have a smooth time with
+their code.
-PointerHolder<Base> x = new Derived() -->
-auto x = std::shared_ptr<Base>(new Derived())
+patrepl s/PointerHolder/std::shared_ptr/g {include,libqpdf}/qpdf/*.hh
+patrepl s/PointerHolder/std::shared_ptr/g libqpdf/*.cc
+patrepl s/make_pointer_holder/std::make_shared/g libqpdf/*.cc
+patrepl s/make_array_pointer_holder/QUtil::make_shared_array/g libqpdf/*.cc
+patrepl s,qpdf/std::shared_ptr,qpdf/PointerHolder, **/*.cc **/*.hh
+git restore include/qpdf/PointerHolder.hh
+cleanpatch
-PointerHolder x(true, new T[5]) -->
-auto x = std::shared_ptr(new T[5], std::default_delete<T[]>())
+Increase to POINTERHOLDER_TRANSITION=3
-X* x = new X(); PointerHolder<X> x_ph(x) -->
-auto x_ph = std::make_shared<X>(); X* x = x_ph.get();
+make build_libqpdf -- no errors
-Derived* x = new Derived(); PointerHolder<Base> x_ph(x) -->
-Derived* x = new Derived(); auto x_ph = std::shared_pointer<Base>(x);
+Drop back to POINTERHOLDER_TRANSITION=2
-Also remember
+make check -- everything passes
-auto x = std::shared_ptr(new T[5], std::default_delete<T[]>())
-vs.
-auto x = std::make_unique<T[]>(5)
+Then upgrade everything else. It would work to just start here.
+
+Increase to POINTERHOLDER_TRANSITION=3
+
+patrepl s/PointerHolder/std::shared_ptr/g **/*.cc **/*.hh
+patrepl s/make_pointer_holder/std::make_shared/g **/*.cc
+patrepl s/make_array_pointer_holder/QUtil::make_shared_array/g **/*.cc
+patrepl s,qpdf/std::shared_ptr,qpdf/PointerHolder, **/*.cc **/*.hh
+git restore include/qpdf/PointerHolder.hh
+git restore libtests/pointer_holder.cc
+cleanpatch
+
+Remove all references to PointerHolder.hh from everything except
+public headers and pointer_holder.cc.
+
+make check -- everything passes
+
+Increase to POINTERHOLDER_TRANSITION=4
+
+Do a clean build and make check -- everything passes
+
+Final steps:
+
+* Change to POINTERHOLDER_TRANSITION=4 in autoconf.mk.in.
+* Check code formatting
+* std::shared_ptr<Members> m can be replaced with
+  std::shared_ptr<Members> m_ph and Members* m if performance is critical
+* Could try Members indirection with Members* for QPDFObjectHandle
+
+When done:
+
+* Update the smart-pointers section of the manual in design.rst
+* Update comments in PointerHolder.hh
 PointerHolder in public API:
@@ -453,20 +483,6 @@ PointerHolder in public API:
   QPDFPageObjectHelper::addContentTokenFilter(
     PointerHolder<QPDFObjectHandle::TokenFilter>)
-Strategy:
-* Start with pointerholder branch
-* Replace each public API one at a time
-* Replace remaining internal uses; sometimes unique_ptr may be good,
-  particularly for temporary strings that are deleted in the same
-  scope and Members for non-copyable classes
-* std::shared_ptr<Members> m can be replaced with
-  std::shared_ptr<Members> m_ph and Members* m if performance is critical
-* Remove #include <PointerHolder.hh> from all headers
-
-At that point, we're in a good state to make that compatibility
-basically works. Then we can proceed to remove PointerHolder from
-everything else.
-
 ABI Changes
 ===========
@@ -22,13 +22,125 @@
 #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.
+#ifndef POINTERHOLDER_TRANSITION
+
+// In qpdf 11, #define POINTERHOLDER_IS_SHARED_POINTER
+
+// In qpdf 11, issue a warning:
+// #define POINTERHOLDER_TRANSITION 0 to suppress this warning, and see below.
+// # warn "POINTERHOLDER_TRANSITION is not defined -- see qpdf/PointerHolder.hh"
+
+// undefined = define as 0; will also issue a warning in qpdf 11
+// 0 = no deprecation warnings
+// 1 = make PointerHolder<T>(T*) explicit
+// 2 = warn for use of getPointer() and getRefcount()
+// 3 = warn for all use of PointerHolder
+// 4 = don't define PointerHolder at all
+# define POINTERHOLDER_TRANSITION 0
+#endif // !defined(POINTERHOLDER_TRANSITION)
+
+// *** WHAT IS HAPPENING ***
+
+// In qpdf 11, PointerHolder will be replaced with std::shared_ptr
+// wherever it appears in the qpdf API. The PointerHolder object will
+// be derived from std::shared_ptr to provide a backward-compatible
+// interface and will be mutually assignable with std::shared_ptr.
+// Code that uses containers of PointerHolder will require adjustment.
+
+// *** HOW TO TRANSITION ***
+
+// The symbol POINTERHOLDER_TRANSITION can be defined to help you
+// transition your code away from PointerHolder. You can define it
+// before including any qpdf header files or including its definition
+// in your build configuration. If not defined, it automatically gets
+// defined to 0, which enables full backward compatibility. That way,
+// you don't have to take action for your code to continue to work.
+
+// If you want to work gradually to transition your code away from
+// PointerHolder, you can define POINTERHOLDER_TRANSITION and fix the
+// code so it compiles without warnings and works correctly. If you
+// want to be able to continue to support old qpdf versions at the
+// same time, you can write code like this:
+
+// #ifndef POINTERHOLDER_TRANSITION
+//  ... use PointerHolder as before 10.6
+// #else
+//  ... use PointerHolder or shared_ptr as needed
+// #endif
+
+// Each level of POINTERHOLDER_TRANSITION exposes differences between
+// PointerHolder and std::shared_ptr. The easiest way to transition is
+// to increase POINTERHOLDER_TRANSITION in steps of 1 so that you can
+// test and handle changes incrementally.
+
+// *** Transitions available starting at qpdf 10.6.0 ***
+
+// POINTERHOLDER_TRANSITION = 1
+//
+// PointerHolder<T> has an implicit constructor that takes a T*, so
+// you can replace a PointerHolder<T>'s pointer by directly assigning
+// a T* to it or pass a T* to a function that expects a
+// PointerHolder<T>. std::shared_ptr does not have this (risky)
+// behavior. When POINTERHOLDER_TRANSITION = 1, PointerHolder<T>'s T*
+// constructor is declared explicit. For compatibility with
+// std::shared_ptr, you can still assign nullptr to a PointerHolder.
+// Constructing all your PointerHolder<T> instances explicitly is
+// backward compatible, so you can make this change without
+// conditional compilation and still use the changes with older qpdf
+// versions.
+//
+// Also defined is a make_pointer_holder method that acts like
+// std::make_shared. You can use this as well, but it is not
+// compatible with qpdf prior to 10.6. Like std::make_shared<T>,
+// make_pointer_holder<T> can only be used when the constructor
+// implied by its arguments is public. If you use this, you should be
+// able to just replace it with std::make_shared when qpdf 11 is out.
+
+// POINTERHOLDER_TRANSITION = 2
+//
+// std::shared_ptr as get() and use_count(). PointerHolder has
+// getPointer() and getRefcount(). In 10.6.0, get() and use_count()
+// were added as well. When POINTERHOLDER_TRANSITION = 2, getPointer()
+// and getRefcount() are deprecated. Fix deprecation warnings by
+// replacing with get() and use_count(). This breaks compatibility
+// with qpdf older than 10.6. Search for CONST BEHAVIOR for an
+// additional note.
+//
+// Once your code is clean at POINTERHOLDER_TRANSITION = 2, the only
+// remaining issues that prevent simple replacement of PointerHolder
+// with std::shared_ptr are shared arrays and containers, and neither
+// of these are used in the qpdf API.
+
+// *** Transitions available starting at qpdf 11.0.0 **
+
+// NOTE: Until qpdf 11 is released, this is a plan and is subject to
+// change. Be sure to check again after qpdf 11 is released.
+
+// POINTERHOLDER_TRANSITION = 3
+//
+// Warn for all use of PointerHolder<T>. This help you remove all use
+// of PointerHolder from your code and use std::shared_ptr instead.
+// You will also have to transition any containers of PointerHolder in
+// your code.
+
+// POINTERHOLDER_TRANSITION = 4
+//
+// Suppress definition of the PointerHolder<T> type entirely.
+
+// CONST BEHAVIOR
+
+// PointerHolder<T> has had a long-standing bug in its const behavior.
+// const PointerHolder<T>'s getPointer() method returns a T const*.
+// This is incorrect and is not how regular pointers or standard
+// library smart pointers behave. Making a PointerHolder<T> const
+// should prevent reassignment of its pointer but not affect the thing
+// it points to. For that, use PointerHolder<T const>. The new get()
+// method behaves correctly in this respect and is therefore slightly
+// different from getPointer(). This shouldn't break any correctly
+// written code. If you are relying on the incorrect behavior, use
+// PointerHolder<T const> instead.
+
+// OLD DOCUMENTATION
 // This class is basically std::shared_ptr but predates that by
 // several years.
@@ -58,6 +170,8 @@
 // the underlying pointers provides a well-defined, if not
 // particularly meaningful, ordering.
+#include <cstddef>
+
 template <class T>
 class PointerHolder
 {
@@ -65,66 +179,78 @@ class PointerHolder
     class Data
     {
       public:
-	Data(T* pointer, bool array) :
-	    pointer(pointer),
-	    array(array),
-	    refcount(0)
-	    {
-	    }
-	~Data()
-	    {
-		if (array)
-		{
-		    delete [] this->pointer;
-		}
-		else
-		{
-		    delete this->pointer;
-		}
-	    }
-	T* pointer;
-	bool array;
-	int refcount;
+        Data(T* pointer, bool array) :
+            pointer(pointer),
+            array(array),
+            refcount(0)
+        {
+        }
+        ~Data()
+        {
+            if (array)
+            {
+                delete [] this->pointer;
+            }
+            else
+            {
+                delete this->pointer;
+            }
+        }
+        T* pointer;
+        bool array;
+        int refcount;
       private:
-	Data(Data const&) = delete;
-	Data& operator=(Data const&) = delete;
+        Data(Data const&) = delete;
+        Data& operator=(Data const&) = delete;
     };
   public:
+#if POINTERHOLDER_TRANSITION >= 1
+    explicit
+#endif // POINTERHOLDER_TRANSITION >= 1
     PointerHolder(T* pointer = 0)
-	{
-	    this->init(new Data(pointer, false));
-	}
+    {
+        this->init(new Data(pointer, false));
+    }
     // Special constructor indicating to free memory with delete []
     // instead of delete
     PointerHolder(bool, T* pointer)
-	{
-	    this->init(new Data(pointer, true));
-	}
+    {
+        this->init(new Data(pointer, true));
+    }
     PointerHolder(PointerHolder const& rhs)
-	{
-	    this->copy(rhs);
-	}
+    {
+        this->copy(rhs);
+    }
     PointerHolder& operator=(PointerHolder const& rhs)
-	{
-	    if (this != &rhs)
-	    {
-		this->destroy();
-		this->copy(rhs);
-	    }
-	    return *this;
-	}
+    {
+        if (this != &rhs)
+        {
+            this->destroy();
+            this->copy(rhs);
+        }
+        return *this;
+    }
+    PointerHolder& operator=(decltype(nullptr))
+    {
+        this->operator=(PointerHolder<T>());
+        return *this;
+    }
     ~PointerHolder()
-	{
-	    this->destroy();
-	}
+    {
+        this->destroy();
+    }
     bool operator==(PointerHolder const& rhs) const
     {
-	return this->data->pointer == rhs.data->pointer;
+        return this->data->pointer == rhs.data->pointer;
+    }
+    bool operator==(decltype(nullptr)) const
+    {
+        return this->data->pointer == nullptr;
     }
     bool operator<(PointerHolder const& rhs) const
     {
-	return this->data->pointer < rhs.data->pointer;
+        return this->data->pointer < rhs.data->pointer;
     }
     // get() is for interface compatibility with std::shared_ptr
@@ -135,33 +261,33 @@ class PointerHolder
     // NOTE: The pointer returned by getPointer turns into a pumpkin
     // when the last PointerHolder that contains it disappears.
-#ifndef NO_POINTERHOLDER_DEPRECATION
+#if POINTERHOLDER_TRANSITION >= 2
     [[deprecated("use PointerHolder<T>::get() instead of getPointer()")]]
-#endif
+#endif // POINTERHOLDER_TRANSITION >= 2
     T* getPointer()
-	{
-	    return this->data->pointer;
-	}
-#ifndef NO_POINTERHOLDER_DEPRECATION
+    {
+        return this->data->pointer;
+    }
+#if POINTERHOLDER_TRANSITION >= 2
     [[deprecated("use PointerHolder<T>::get() instead of getPointer()")]]
-#endif
+#endif // POINTERHOLDER_TRANSITION >= 2
     T const* getPointer() const
-	{
-	    return this->data->pointer;
-	}
-#ifndef NO_POINTERHOLDER_DEPRECATION
+    {
+        return this->data->pointer;
+    }
+#if POINTERHOLDER_TRANSITION >= 2
     [[deprecated("use use_count() instead of getRefcount()")]]
-#endif
+#endif // POINTERHOLDER_TRANSITION >= 2
     int getRefcount() const
-	{
-	    return this->data->refcount;
-	}
+    {
+        return this->data->refcount;
+    }
     // use_count() is for compatibility with std::shared_ptr
     long use_count()
-        {
-            return static_cast<long>(this->data->refcount);
-        }
+    {
+        return static_cast<long>(this->data->refcount);
+    }
     T const& operator*() const
     {
@@ -183,30 +309,44 @@ class PointerHolder
   private:
     void init(Data* data)
-	{
-	    this->data = data;
-            ++this->data->refcount;
-	}
+    {
+        this->data = data;
+        ++this->data->refcount;
+    }
     void copy(PointerHolder const& rhs)
-	{
-	    this->init(rhs.data);
-	}
+    {
+        this->init(rhs.data);
+    }
     void destroy()
-	{
-	    bool gone = false;
-	    {
-		if (--this->data->refcount == 0)
-		{
-		    gone = true;
-		}
-	    }
-	    if (gone)
-	    {
-		delete this->data;
-	    }
-	}
+    {
+        bool gone = false;
+        {
+            if (--this->data->refcount == 0)
+            {
+                gone = true;
+            }
+        }
+        if (gone)
+        {
+            delete this->data;
+        }
+    }
     Data* data;
 };
+template<typename T, typename... _Args>
+inline PointerHolder<T>
+make_pointer_holder(_Args&&... __args)
+{
+    return PointerHolder<T>(new T(__args...));
+}
+
+template <typename T>
+PointerHolder<T>
+make_array_pointer_holder(size_t n)
+{
+    return PointerHolder<T>(true, new T[n]);
+}
+
 #endif // POINTERHOLDER_HH
-#define NO_POINTERHOLDER_DEPRECATION // we need to test the deprecated API
+// We need to test the deprecated API
+#ifdef POINTERHOLDER_TRANSITION
+# undef POINTERHOLDER_TRANSITION
+#endif
+#define POINTERHOLDER_TRANSITION 0
 #include <qpdf/PointerHolder.hh>
 #include <iostream>
@@ -751,16 +751,26 @@ actually quite rare and largely avoidable.
 Smart Pointers
 --------------
-This section describes changes to the use of smart pointers in qpdf in
-versions 10.6.0 and 11.0.0.
+This section describes changes to the use of smart pointers there were
+made in qpdf 10.6.0 as well as some planned for 11.0.0.
 Starting in qpdf 11, ``PointerHolder`` will be replaced with
 ``std::shared_ptr`` in qpdf's public API. A backward-compatible
-``PointerHolder`` will be provided that should make it possible for
-most code to remain unchanged. This new ``PointerHolder`` will be
-marked deprecated but will provide a way to suppress the deprecation
-warnings. Code that works with containers of ``PointerHolder`` may
-have to be modified, though no qpdf interfaces do this.
+``PointerHolder`` class will be provided that should make it possible
+for most code to remain unchanged. ``PointerHolder`` may eventually be
+removed from qpdf entirely, but this will not happen for a while to
+make it easier for people who need to support multiple versions of
+qpdf.
+
+The ``POINTERHOLDER_TRANSITION`` preprocessor symbol has been
+introduced to help people transition from ``PointerHolder`` to
+``std::shared_ptr``. After qpdf 11 is released, to prepare for a
+future qpdf without ``PointerHolder`` and to let them know that it is
+no longer needed, a warning will be issued if
+``<qpdf/PointerHolder.hh>`` is included, though it will be possible to
+suppress the warning by defining ``POINTERHOLDER_TRANSITION``. In
+10.6.0, there are some steps you can perform to prepare, but no action
+is required.
 The remainder of this section describes how to prepare if you want to
 eliminate ``PointerHolder`` from your code or what to do if you want
@@ -769,25 +779,31 @@ to stick with the old interfaces.
 Changes in 10.6.0
 ~~~~~~~~~~~~~~~~~
-In qpdf 10.6.0, two ``PointerHolder`` methods have been deprecated and
-replaced with methods that are compatible with ``std::shared_ptr``:
+In qpdf 10.6.0, the following changes have been made to
+``PointerHolder`` to make its behavior closer to that of
+``std::shared_ptr``:
-- ``getPointer()`` -- use ``get()`` instead
+- ``get()`` has been added as an alternative to ``getPointer()``
-- ``getRefcount()`` -- use ``use_count()`` instead
+- ``use_count()`` has been added as an alternative to ``getRefcount()``
-If you build your code with deprecation warnings enabled and you want
-to suppress these deprecation warnings for now, you can ``#define
-NO_POINTERHOLDER_DEPRECATION`` before including any qpdf header files.
-It may be possible to leave it this way long-term to facilitate
-supporting older versions of qpdf without conditional compilation.
+- A new global helper function ``make_pointer_holder`` behaves
+  similarly to ``std::make_shared``, so you can use
+  ``make_pointer_holder<T>(args...)`` to create a ``PointerHolder<T>``
+  with ``new T(args...)`` as the pointer.
+
+- A new global helper function ``make_array_pointer_holder`` takes a
+  size and creates a ``PointerHolder`` to an array. It is a
+  counterpart to the newly added ``QUtil::make_shared_array`` method,
+  which does the same thing with a ``std::shared_ptr``.
 ``PointerHolder`` has had a long-standing bug: a ``const
 PointerHolder<T>`` would only provide a ``T const*`` with its
-``getPointer`` method. This is incorrect and is not how standard C++
-smart pointers or regular pointers behave. The correct semantics
-would be that a ``const PointerHolder<T>`` would not accept a new
-pointer after being created but would still allow you to modify the
+``getPointer`` method. This is incorrect and is not how standard
+library C++ smart pointers or regular pointers behave. The correct
+semantics would be that a ``const PointerHolder<T>`` would not accept
+a new pointer after being created (``PointerHolder`` has always
+behaved correctly in this way) but would still allow you to modify the
 item being pointed to. If you don't want to mutate the thing it points
 to, use ``PointerHolder<T const>`` instead. The new ``get()`` method
 behaves correctly. It is therefore not exactly the same as
@@ -795,157 +811,290 @@ behaves correctly. It is therefore not exactly the same as
 ``std::shared_ptr``. This shouldn't make any difference to any
 correctly written code.
+Differences between ``PointerHolder`` and ``std::shared_ptr``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Here is a list of things you need to think about when migrating from
+``PointerHolder`` to ``std::shared_ptr``. After the list, we will
+discuss how to address each one using the ``POINTERHOLDER_TRANSITION``
+preprocessor symbol or other C++ coding techniques.
+
+- ``PointerHolder<T>`` has an *implicit* constructor that takes a
+  ``T*``, which means you can assign a ``T*`` directly to a
+  ``PointerHolder<T>`` or pass a ``T*`` to a function that expects a
+  ``PointerHolder<T>`` as a parameter. ``std::shared_ptr<T>`` does not
+  have this behavior, though you can still assign ``nullptr`` to a
+  ``std::shared_ptr<T>`` and compare ``nullptr`` with a
+  ``std::shared_ptr<T>``. Here are some examples of how you might need
+  to change your code:
+
+  Old code:
+   .. code-block:: c++
+
+      PointerHolder<X> x_p;
+      X* x = new X();
+      x_p = x;
+
+  New code:
+   .. code-block:: c++
+
+      auto x_p = std::make_shared<X>();
+      X* x = x_p.get();
+      // or, less safe, but closer:
+      std::shared_ptr<X> x_p;
+      X* x = new X();
+      x_p = std::shared_ptr<X>(x);
+
+  Old code:
+   .. code-block:: c++
+
+      PointerHolder<Base> base_p;
+      Derived* derived = new Derived();
+      base_p = derived;
+
+  New code:
+   .. code-block:: c++
+
+      std::shared_ptr<Base> base_p;
+      Derived* derived = new Derived();
+      base_p = std::shared_ptr<Base>(derived);
+
+- ``PointerHolder<T>`` has ``getPointer()`` to get the underlying
+  pointer. It also has the seldom-used ``getRefcount()`` method to get
+  the reference count. ``std::shared_ptr<T>`` has ``get()`` and
+  ``use_count()``. In qpdf 10.6, ``PointerHolder<T>`` also has
+  would not be an issue unless you did this in your own code.
+
+Addressing the Differences
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you need to support versions of qpdf prior to qpdf 10.6, you don't
+*need* to take any action at this time, but it is recommended that you
+at least address the implicit constructor issue since this can be done
+without breaking backward compatibility. (Explicit construction of
+``PointerHolder<T>`` is and always has been allowed.)
+
+There are two significant things you can do to minimize the impact of
+switching from ``PointerHolder`` to ``std::shared_ptr``:
+
+- Use ``auto`` and ``decltype`` whenever possible when working with
+  ``PointerHolder`` variables that are exchanged with the qpdf API.
+
+- Use the ``POINTERHOLDER_TRANSITION`` preprocessor symbol to identify
+  and resolve the differences described above.
+
+To use ``POINTERHOLDER_TRANSITION``, you will need to ``#define`` it
+before including any qpdf header files or specify its value as part of
+your build. The table below describes the values of
+``POINTERHOLDER_TRANSITION``. This informatoin is also summarized in
+:file:`include/qpdf/PointerHolder.hh`, so you will have it handy
+without consulting this manual.
+
+.. list-table:: POINTERHOLDER_TRANSITION values
+   :widths: 5 80
+   :header-rows: 1
+
+   - - value
+     - meaning
+
+   - - undefined
+     - same as ``0``, but start with qpdf 11.0, issues a warning
+
+   - - ``0``
+     - provide a backward compatible ``PointerHolder`` and suppress
+       all deprecation warnings
+
+   - - ``1``
+     - Make the ``PointerHolder<T>(T*)`` constructor explicit
+
+   - - ``2``
+     - Deprecate ``getPointer()`` and ``getRefcount()``
+
+   - - ``3``
+     - Starting in qpdf 11, deprecate all uses of ``PointerHolder``
+
+   - - ``4``
+     - Starting in qpdf 11, disable all functionality from
+       ``qpdf/PointerHolder.hh`` so that ``#include``-ing it has no
+       effect.
+
+Based on the above, here is a procedure for preparing your code. This
+is the procedure that was used for the qpdf code itself.
+
+If you need to support versions of qpdf prior to 10.6, you can still
+do these steps:
+
+- Find all occurrences of ``PointerHolder`` in the code. See whether
+  any of them can just be outright replaced with ``std::shared_ptr``
+  or ``std::unique_ptr``. If you have been using qpdf prior to
+  adopting C++11 and were using ``PointerHolder`` as a general-purpose
+  smart pointer, you may have cases that can be replaced in this way.
+
+  For example:
+
+  - Simple ``PointerHolder<T>`` construction can be replaced with
+    either the equivalent ``std::shared_ptr<T>`` construction or, if
+    the constructor is public, with ``std::make_shared<T>(args...)``.
+    If you are creating a smart pointer that is never copied, you may
+    be able to use ``std::unique_ptr<T>`` instead.
+
+  - Array allocations will have to be rewritten.
-How to Prepare
-~~~~~~~~~~~~~~
+    Allocating a ``PointerHolder`` to an array looked like this:
-If you don't need to support versions of qpdf prior to 10.6, you can
-just replace all occurrences of ``getPointer()`` with ``get()`` and
-all occurrences of ``getRefcount()`` with ``use_count()``. That's
-about all you will be able to do prior to qpdf 11.
+    .. code-block:: c++
-If you need to support older versions, you have two choices:
+       PointerHolder<X> p(true, new X[n]);
-- ``#define NO_POINTERHOLDER_DEPRECATION`` and leave everything the
-  way it was. You can just wait until qpdf 11.
+    To allocate a ``std::shared_ptr`` to an array:
-- Write code that uses ``get()`` but falls back to ``getPointer()`` if
-  ``QPDF_MAJOR_VERSION`` is not defined. The symbols
-  ``QPDF_MAJOR_VERSION``, ``QPDF_MINOR_VERSION``, and
-  ``QPDF_PATCH_VERSION`` were introduced with 10.6.0, so just checking
-  for whether ``QPDF_MAJOR_VERSION`` is defined is sufficient for
-  telling if you're running a version before 10.6.0. If you do this,
-  once qpdf 11 comes out, you will already know all the places that
-  have to be handled specially.
+    .. code-block:: c++
-If you are somehow relying on the fact that a ``const
-PointerHolder<T>`` always gave back a ``T const*`` and are
-dereferencing a ``const PointerHolder<T>`` to call methods that only
-have ``const`` versions in ``T``, you may have to change from
-``const PointerHolder<T>`` to ``PointerHolder<T const>``. This won't
-be an issue for anything in the qpdf API, and if you are using qpdf
-``PointerHolder`` objects for any other reason, you should just
-replace them with ``std::shared_ptr``.
+       auto p = std::shared_ptr<X>(new X[n], std::default_delete<X[]>());
+       // If you don't mind using QUtil, there's QUtil::make_shared_array<X>(n).
+       // If you are using c++20, you can use std::make_shared<X[]>(n)
+       // to get a std::shared_ptr<X[]> instead of a std::shared_ptr<X>.
-What to Expect
-~~~~~~~~~~~~~~
+    To allocate a ``std::unique_ptr`` to an array:
-Note: if you are reading this in the 10.6 manual and 11 is out, you
-should read it in the manual for qpdf 11 instead. Some early tests
-have been done to try to ensure the accuracy of this information, but
-it may change once the work is actually completed.
+    .. code-block:: c++
-When ``PointerHolder`` disappears from qpdf's API in qpdf 11, you will
-have a few options:
+       auto p = std::make_unique<X[]>(n);
+       // or, if X has a private constructor:
+       auto p = std::unique_ptr<X[]>(new X[n]);
-- Use the new ``PointerHolder``, which is derived from
-  ``std::shared_ptr`` and which has methods to make it
-  interchangeable. For things that use ``PointerHolder<T>`` directly,
-  this should "just work," though you will have to ``#define
-  NO_POINTERHOLDER_DEPRECATION`` if you don't want deprecation
-  warnings.
+- If a ``PointerHolder<T>`` can't be replaced with a standard library
+  smart pointer, perhaps it can be declared using ``auto`` or
+  ``decltype`` so that, when the qpdf API changes, your code will just
+  need to be recompiled.
-- Replace all uses of ``PointerHolder<T>`` with ``std::shared_ptr<T>``
-  and deal with the required changes, outlined below. This is the
-  recommended course of action. You will need conditional compilation
-  if you want to simultaneously support order code. Stay tuned for the
-  qpdf 11 documentation for specifics.
+- ``#define POINTERHOLDER_TRANSITION 1`` to enable deprecation
+  warnings for all implicit constructions of ``PointerHolder<T>`` from
+  a plain ``T*``. When you find one, explicitly construct the
+  ``PointerHolder<T>``.
-While ``PointerHolder<T>`` and ``std::shared_ptr<T>`` will be mutually
-assignable and convertible, this does not apply to containers of those
-objects. The qpdf API doesn't have any containers of
-``PointerHolder``, so this would have to be in your own code. You can
-prepare yourself for the change by using ``auto`` and ``decltype``
-whenever possible so that a change to the underlying type of something
-won't require source changes.
+  - Old code:
-Required Changes in qpdf 11
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    .. code-block:: c++
-This section describes unavoidable changes when replacing
-``PointerHolder`` with ``std::shared_ptr`` rather than continuing to
-use the backward compatible API. Nothing here is needed (or can be
-done) prior to qpdf 11, so consider this to be a preview.
+       PointerHolder<X> x = new X();
-- Change ``getPointer`` to ``get`` and ``getRefcount`` to
-  ``use_count`` as above. If your starting point is no deprecation
-  warnings with qpdf 10.6, this will already be true.
+  - New code:
-- Array allocations will have to be rewritten.
+    .. code-block:: c++
-  To allocate a ``PointerHolder`` to an array:
+       auto x = PointerHolder<X>(new X(...)); // all versions of qpdf
+       // or, if X(...) is public:
+       auto x = make_pointer_holder<X>(...); // only 10.6 and above
-  .. code-block:: c++
+    Other examples appear above.
-     PointerHolder<X> p(true, new X[n]);
+If you need to support older versions of qpdf than 10.6, this is as
+far as you can go until qpdf 11 comes out.
-  To allocate a ``std::shared_ptr`` to an array:
+If you only need to support the latest version of qpdf, proceed as
+follows:
-  .. code-block:: c++
+- ``#define POINTERHOLDER_TRANSITION 2`` to enable deprecation of
+  ``getPointer()`` and ``getRefcount()``
-     auto p = std::shared_ptr<X>(new X[n], std::default_delete<X[]>());
+- Replace ``getPointer()`` with ``get()`` and ``getRefcount()`` with
+  ``use_count()``. These methods were not present prior to 10.6.0.
-  To allocate a ``std::unique_ptr`` to an array:
+When you have gotten your code to compile cleanly with
+``POINTERHOLDER_TRANSITION=2``, you are well on your way to being
+ready for eliminating ``PointerHolder`` entirely after qpdf 11 is
+released.
-  .. code-block:: c++
+After qpdf 11 is out
+~~~~~~~~~~~~~~~~~~~~
-     auto p = std::make_unique<X[]>(n);
-     // or
-     auto p = std::unique_ptr<X[]>(new X[n]);
+In the 10.6 manual, this section represents a plan and is subject to
+change. However, it has been tested in practice using a version of the
+qpdf 11 ``PointerHolder`` on a branch, so it is likely to be accurate.
+In the meantime, think of this is a preview.
-  The second form may be needed if ``X`` has a private constructor
-  from this context.
+First, make sure you have done the steps in the 10.6 section. (Note:
+once qpdf 11 comes out, the goal is to not have to migrate to 10.6
+first, so it is likely that these sections will be combined.)
-  C++-17 has a better way to allocate ``std::shared_ptr`` to an array,
-  but qpdf is still allowing C++-14 to be used. You can use whatever
-  method to handle shared arrays that is supported in your
-  environment. There are no shared arrays in qpdf's public API except
-  for some ``QUtil`` helper methods that are not essential for use of
-  qpdf features.
+If you are explicitly choosing to stick with the backward compatible
+``PointerHolder`` for now, you should define
+``POINTERHOLDER_TRANSITION`` to ``0`` to suppress the warning from
+including ``qpdf/PointerHolder.hh``. Be aware that you may eventually
+have to deal with the transition, though the intention is to leave the
+compatibility layer in place for a while. You should rebuild and test
+your code. There may be compiler errors if you have containers of
+``PointerHolder``, but most code should compile without any changes.
+Even if you have errors, use of ``auto`` or ``decltype`` may enable
+you to write code that works with the old and new API without having
+to use conditional compilation. The
+``POINTERHOLDER_IS_SHARED_POINTER`` is defined in qpdf 11 if you
+``#include <qpdf/PointerHolder.hh>``.
-- ``PointerHolder<T>`` can have plain pointers directly assigned to
-  it, while ``std::shared_ptr<T>`` cannot. This makes code like this
-  possible:
+If you want to support older versions of qpdf and still transition so
+that the backward-compatible ``PointerHolder`` is not in use, you can
+separate old code and new code by testing with the
+``POINTERHOLDER_IS_SHARED_POINTER`` preprocessor symbol, as in
-  .. code-block:: c++
+.. code-block:: c++
-     PointerHolder<X> x_p;
-     X* x = new X();
-     x_p = x;
+   #ifdef POINTERHOLDER_IS_SHARED_POINTER
+   std::shared_ptr<X> x;
+   #else
+   PointerHolder<X> x;
+   #endif // POINTERHOLDER_IS_SHARED_POINTER
+   x = decltype(x)(new X())
-  It also makes it possible to pass a plain pointer to a function
-  expecting a ``PointerHolder``, thereby transferring "ownership" of
-  the pointer into the function.
+or
-  Code like that is a risky because you can leak memory if an
-  exception is thrown between creation of the X and assignment of it
-  into the ``PointerHolder``. In any case, ``std::shared_ptr`` does
-  not allow that, so you need one of these instead:
+.. code-block:: c++
-  .. code-block:: c++
+   #ifdef POINTERHOLDER_IS_SHARED_POINTER
+   auto x_p = std::make_shared<X>();
+   X* x = x_p.get();
+   #else
+   auto x_p = PointerHolder<X>(new X());
+   X* x = x_p.getPointer();
+   #endif // POINTERHOLDER_IS_SHARED_POINTER
+   x_p->doSomething();
+   x->doSomethingElse();
-     auto x_p = std::make_shared<X>();
-     X* x = x_p.get();
-     // or, less safe, but closer:
-     std::shared_ptr<X> x_p;
-     X* x = new X();
-     x_p = std::shared_ptr<X>(x);
+If you don't need to support older versions of qpdf, you can proceed
+with these steps without protecting changes with the preprocessor
+symbol. Here are the remaining changes.
-  Also, code like this:
+- Make sure you have a clean build with ``POINTERHOLDER_TRANSITION``
+  set to ``2``. This means that you are using ``PointerHolder`` in a
+  manner that is API-compatible with ``std::shared_ptr`` in all cases
+  except for array pointers.
-  .. code-block:: c++
+- Replace all occurrences of ``PointerHolder`` with
+  ``std::shared_ptr`` except in ``#include <qpdf/PointerHolder.hh>``
-     PointerHolder<Base> base_p;
-     Derived* derived = new Derived();
-     base_p = derived;
+- Replace all occurrences of ``make_pointer_holder`` with
+  ``std::make_shared``
-  needs to be replaced with something like this instead:
+- Replace all occurrences of ``make_array_pointer_holder`` with
+  ``QUtil::make_shared_array``. You will need to include
+  ``<qpdf/QUtil.hh>`` if you haven't already done so.
-  .. code-block:: c++
+- Make sure ``<memory>`` is included wherever you were including
+  ``<qpdf/PointerHolder.hh>``.
-     std::shared_ptr<Base> base_p;
-     Derived* derived = new Derived();
-     base_p = std::shared_ptr<Base>(derived);
+- If you were using any array ``PointerHolder<T>`` objects, replace
+  them as above. You can let the compiler find these for you.
+
+- ``#define POINTERHOLDER_TRANSITION 3`` to enable deprecation of
+  all ``PointerHolder<T>`` construction.
+
+- Build and test. Fix any remaining issues.
+
+- If not supporting older qpdf, remove all references to
+  ``<qpdf/PointerHolder.hh>``. Otherwise, you wil still need to
+  include it but can ``#define POINTERHOLDER_TRANSITION 4`` to prevent
+  ``PointerHolder`` from being defined. The
+  ``POINTERHOLDER_IS_SHARED_POINTER`` symbol will still be defined.
 Historical Background
 ~~~~~~~~~~~~~~~~~~~~~
@@ -953,13 +1102,6 @@ Historical Background
 Since its inception, the qpdf library used its own smart pointer
 class, ``PointerHolder``. The ``PointerHolder`` class was originally
 created long before ``std::shared_ptr`` existed, and qpdf itself
-didn't start requiring a C++-11 compiler version 9.1.0 released in
-late 2019.
-
-``PointerHolder`` is a reference-counted smart pointer with semantics
-almost identical to ``std::shared_ptr`` except that it is not
-thread-safe. It has a few interface differences that prevent
-``std::shared_ptr`` from being a drop-in replacement. However, given
-the value of using standard library smart pointers, qpdf is taking the
-plunge for version 11 and switching over to standard library smart
-pointers.
+didn't start requiring a C++11 compiler version 9.1.0 released in
+late 2019. With current C++ versions, is no longer desirable for qpdf
+to have its own smart pointer class.
@@ -7,28 +7,15 @@ For a detailed list of changes, please see the file
 :file:`ChangeLog` in the source distribution.
 10.6.0: XXX
-  - Deprecations/future replacement of ``PointerHolder``
+  - Preparation for replacement of ``PointerHolder``
     The next major release of qpdf will replace ``PointerHolder`` with
-    ``std::shared_ptr`` across all of qpdf's public API. In
-    preparation for this change, the following ``PointerHolder``
-    methods have been deprecated in favor of interfaces that more
-    closely match ``std::shared_ptr``:
-
-    - ``getPointer()`` -- use ``get()`` instead; this also fixes
-      ``const`` semantics as discussed in
-      :file:`include/qpdf/PointerHolder.hh`.
-
-    - ``getRefcount()`` -- use ``use_count()`` instead
-
-    If you build your code with deprecation warnings enabled and you
-    want to suppress these deprecation warnings for now, you can
-    ``#define NO_POINTERHOLDER_DEPRECATION`` before including any qpdf
-    header files. Code that does this will *require no changes* prior
-    to qpdf 11 and may or may not require changes after qpdf 11.
-
-    For a detailed discussion of this change and how to prepare for
-    it, see :ref:`smart-pointers`.
+    ``std::shared_ptr`` across all of qpdf's public API. No action is
+    required at this time, but if you'd like to prepare, read the
+    comments :file:`include/qpdf/PointerHolder.hh` and see
+    :ref:`smart-pointers` for details on what you can do now to create
+    code that will continue to work with older versions of qpdf and be
+    easier to switch over to qpdf 11 when it comes out.
   - Preparation for a new JSON output version