docs/upgrade.rst

Upgrade guide
#############

This is a companion guide to the :doc:`changelog`. While the changelog briefly
lists all of the new features, improvements and bug fixes, this upgrade guide
focuses only the subset which directly impacts your experience when upgrading
to a new version. But it goes into more detail. This includes things like
deprecated APIs and their replacements, build system changes, general code
modernization and other useful information.

.. _upgrade-guide-3.0:

v3.0
====

pybind11 v3.0 introduces major new features, but the vast majority of
existing extensions are expected to build and run without modification. Minor
adjustments may be needed in rare cases, and any such changes can be easily
wrapped in preprocessor conditionals to maintain compatibility with the
2.x series.

However, due to new features and modernizations, extensions built with
pybind11 v3.0 are not ABI-compatible with those built using v2.13. To ensure
cross-extension-module compatibility, it is recommended to rebuild all
pybind11-based extensions with v3.0.

CMake support now defaults to the modern FindPython module. If you haven't
updated yet, we provide some backward compatibility for ``PYTHON_*`` variables,
but you should switch to using ``Python_*`` variables instead. Note that
setting ``PYTHON_*`` variables no longer affects the build.

A major new feature in this release is the integration of
``py::smart_holder``, which improves support for ``std::unique_ptr``
and ``std::shared_ptr``, resolving several long-standing issues. See
:ref:`smart_holder` for details. Closely related is the addition
of ``py::trampoline_self_life_support``, documented under
:ref:`overriding_virtuals`.

This release includes a major modernization of cross-extension-module
ABI compatibility handling. The new implementation reflects actual ABI
compatibility much more accurately than in previous versions. The details
are subtle and complex; see
`#4953 <https://github.com/pybind/pybind11/pull/4953>`_ and
`#5439 <https://github.com/pybind/pybind11/pull/5439>`_.

Also new in v3.0 is ``py::native_enum``, a modern API for exposing
C++ enumerations as native Python types — typically standard-library
``enum.Enum`` or related subclasses. This provides improved integration with
Python's enum system, compared to the older (now deprecated) ``py::enum_``.
See `#5555 <https://github.com/pybind/pybind11/pull/5555>`_ for details.

Functions exposed with pybind11 are now pickleable. This removes a
long-standing obstacle when using pybind11-bound functions with Python features
that rely on pickling, such as multiprocessing and caching tools.
See `#5580 <https://github.com/pybind/pybind11/pull/5580>`_ for details.

Anything producing a deprecation warning in the 2.x series may be removed in a
future minor release of 3.x. Most of these are still present in 3.0 in order to ease
transition. The new :ref:`deprecated` page details deprecations.

Migration Recommendations
-------------------------

We recommend migrating to pybind11 v3.0 promptly, while keeping initial
changes to a minimum. Most projects can upgrade simply by updating the
pybind11 version, without altering existing binding code.

After a short stabilization period — enough to surface any subtle issues —
you may incrementally adopt new features where appropriate:

* Use ``py::smart_holder`` and ``py::trampoline_self_life_support`` as needed,
  or to improve code health. Note that ``py::classh`` is available as a
  shortcut — for example, ``py::classh<Pet>`` is shorthand for
  ``py::class_<Pet, py::smart_holder>``. This is designed to enable easy
  experimentation with ``py::smart_holder`` without introducing distracting
  whitespace changes. In many cases, a global replacement of ``py::class_``
  with ``py::classh`` can be an effective first step. Build failures will
  quickly identify places where ``std::shared_ptr<...>`` holders need to be
  removed. Runtime failures (assuming good unit test coverage) will highlight
  base-and-derived class situations that require coordinated changes.

  Note that ``py::bind_vector`` and ``py::bind_map`` (in pybind11/stl_bind.h)
  have a ``holder_type`` template parameter that defaults to
  ``std::unique_ptr``. If ``py::smart_holder`` functionality is desired or
  required, use e.g. ``py::bind_vector<VecType, py::smart_holder>``.

* Gradually migrate from ``py::enum_`` to ``py::native_enum`` to improve
  integration with Python's standard enum types.

There is no urgency to refactor existing, working bindings — adopt new
features as the need arises or as part of ongoing maintenance efforts.

If you are using CMake, update to FindPython variables (mostly changing
variables from ``PYTHON_*`` -> ``Python_*``). You should see if you can use
``set(PYBIND11_FINDPYTHON ON)``, which has been supported for years and will
avoid setting the compatibility mode variables (and will avoid a warning).

Potential stumbling blocks when migrating to v3.0
-------------------------------------------------

The following issues are very unlikely to arise, and easy to work around:

* In rare cases, a C++ enum may be bound to Python via a
  :ref:`custom type caster <custom_type_caster>`. In such cases, a
  template specialization like this may be required:

  .. code-block:: cpp

      #if defined(PYBIND11_HAS_NATIVE_ENUM)
      namespace pybind11::detail {
      template <typename FancyEnum>
      struct type_caster_enum_type_enabled<
          FancyEnum,
          enable_if_t<is_fancy_enum<FancyEnum>::value>> : std::false_type {};
      }
      #endif

  This specialization is needed only if the custom type caster is templated.

  The ``PYBIND11_HAS_NATIVE_ENUM`` guard is needed only
  if backward compatibility with pybind11v2 is required.

* Similarly, template specializations like the following may be required
  if there are custom

  * ``pybind11::detail::copyable_holder_caster`` or

  * ``pybind11::detail::move_only_holder_caster``

  implementations that are used for ``std::shared_ptr`` or ``std::unique_ptr``
  conversions:

  .. code-block:: cpp

      #if defined(PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT)
      namespace pybind11::detail {
      template <typename ExampleType>
      struct copyable_holder_caster_shared_ptr_with_smart_holder_support_enabled<
          ExampleType,
          enable_if_t<is_example_type<ExampleType>::value>> : std::false_type {};
      }
      #endif

  .. code-block:: cpp

      #if defined(PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT)
      namespace pybind11::detail {
      template <typename ExampleType>
      struct move_only_holder_caster_unique_ptr_with_smart_holder_support_enabled<
          ExampleType,
          enable_if_t<is_example_type<ExampleType>::value>> : std::false_type {};
      }
      #endif

  The ``PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT`` guard is needed only
  if backward compatibility with pybind11v2 is required.

  (Note that ``copyable_holder_caster`` and ``move_only_holder_caster`` are not
  documented, although they existed since 2017.)


.. _upgrade-guide-2.12:

v2.12
=====

NumPy support has been upgraded to support the 2.x series too. The two relevant
changes are that:

* ``dtype.flags()`` is now a ``uint64`` and ``dtype.alignment()`` an
  ``ssize_t`` (and NumPy may return an larger than integer value for
  ``itemsize()`` in NumPy 2.x).

* The long deprecated NumPy function ``PyArray_GetArrayParamsFromObject``
  function is not available anymore.

Due to NumPy changes, you may experience difficulties updating to NumPy 2.
Please see the `NumPy 2 migration guide <https://numpy.org/devdocs/numpy_2_0_migration_guide.html>`_
for details.
For example, a more direct change could be that the default integer ``"int_"``
(and ``"uint"``) is now ``ssize_t`` and not ``long`` (affects 64bit windows).

If you want to only support NumPy 1.x for now and are having problems due to
the two internal changes listed above, you can define
``PYBIND11_NUMPY_1_ONLY`` to disable the new support for now. Make sure you
define this on all pybind11 compile units, since it could be a source of ODR
violations if used inconsistently. This option will be removed in the future,
so adapting your code is highly recommended.


.. _upgrade-guide-2.11:

v2.11
=====

* The minimum version of CMake is now 3.5. A future version will likely move to
  requiring something like CMake 3.15. Note that CMake 3.27 is removing the
  long-deprecated support for ``FindPythonInterp`` if you set 3.27 as the
  minimum or maximum supported version. To prepare for that future, CMake 3.15+
  using ``FindPython`` or setting ``PYBIND11_FINDPYTHON`` is highly recommended,
  otherwise pybind11 will automatically switch to using ``FindPython`` if
  ``FindPythonInterp`` is not available.


.. _upgrade-guide-2.9:

v2.9
====

* Any usage of the recently added ``py::make_simple_namespace`` should be
  converted to using ``py::module_::import("types").attr("SimpleNamespace")``
  instead.

* The use of ``_`` in custom type casters can now be replaced with the more
  readable ``const_name`` instead. The old ``_`` shortcut has been retained
  unless it is being used as a macro (like for gettext).


.. _upgrade-guide-2.7:

v2.7
====

*Before* v2.7, ``py::str`` can hold ``PyUnicodeObject`` or ``PyBytesObject``,
and ``py::isinstance<str>()`` is ``true`` for both ``py::str`` and
``py::bytes``. Starting with v2.7, ``py::str`` exclusively holds
``PyUnicodeObject`` (`#2409 <https://github.com/pybind/pybind11/pull/2409>`_),
and ``py::isinstance<str>()`` is ``true`` only for ``py::str``. To help in
the transition of user code, the ``PYBIND11_STR_LEGACY_PERMISSIVE`` macro
is provided as an escape hatch to go back to the legacy behavior. This macro
will be removed in future releases. Two types of required fixes are expected
to be common:

* Accidental use of ``py::str`` instead of ``py::bytes``, masked by the legacy
  behavior. These are probably very easy to fix, by changing from
  ``py::str`` to ``py::bytes``.

* Reliance on py::isinstance<str>(obj) being ``true`` for
  ``py::bytes``. This is likely to be easy to fix in most cases by adding
  ``|| py::isinstance<bytes>(obj)``, but a fix may be more involved, e.g. if
  ``py::isinstance<T>`` appears in a template. Such situations will require
  careful review and custom fixes.


.. _upgrade-guide-2.6:

v2.6
====

Usage of the ``PYBIND11_OVERLOAD*`` macros and ``get_overload`` function should
be replaced by ``PYBIND11_OVERRIDE*`` and ``get_override``. In the future, the
old macros may be deprecated and removed.

``py::module`` has been renamed ``py::module_``, but a backward compatible
typedef has been included. This change was to avoid a language change in C++20
that requires unqualified ``module`` not be placed at the start of a logical
line. Qualified usage is unaffected and the typedef will remain unless the
C++ language rules change again.

The public constructors of ``py::module_`` have been deprecated. Use
``PYBIND11_MODULE`` or ``module_::create_extension_module`` instead.

An error is now thrown when ``__init__`` is forgotten on subclasses. This was
incorrect before, but was not checked. Add a call to ``__init__`` if it is
missing.

A ``py::type_error`` is now thrown when casting to a subclass (like
``py::bytes`` from ``py::object``) if the conversion is not valid. Make a valid
conversion instead.

The undocumented ``h.get_type()`` method has been deprecated and replaced by
``py::type::of(h)``.

Enums now have a ``__str__`` method pre-defined; if you want to override it,
the simplest fix is to add the new ``py::prepend()`` tag when defining
``"__str__"``.

If ``__eq__`` defined but not ``__hash__``, ``__hash__`` is now set to
``None``, as in normal CPython. You should add ``__hash__`` if you intended the
class to be hashable, possibly using the new ``py::hash`` shortcut.

The constructors for ``py::array`` now always take signed integers for size,
for consistency. This may lead to compiler warnings on some systems. Cast to
``py::ssize_t`` instead of ``std::size_t``.

The ``tools/clang`` submodule and ``tools/mkdoc.py`` have been moved to a
standalone package, `pybind11-mkdoc`_. If you were using those tools, please
use them via a pip install from the new location.

The ``pybind11`` package on PyPI no longer fills the wheel "headers" slot - if
you were using the headers from this slot, they are available by requesting the
``global`` extra, that is, ``pip install "pybind11[global]"``. (Most users will
be unaffected, as the ``pybind11/include`` location is reported by ``python -m
pybind11 --includes`` and ``pybind11.get_include()`` is still correct and has
not changed since 2.5).

.. _pybind11-mkdoc: https://github.com/pybind/pybind11-mkdoc

CMake support:
--------------

The minimum required version of CMake is now 3.4.  Several details of the CMake
support have been deprecated; warnings will be shown if you need to change
something. The changes are:

* ``PYBIND11_CPP_STANDARD=<platform-flag>`` is deprecated, please use
  ``CMAKE_CXX_STANDARD=<number>`` instead, or any other valid CMake CXX or CUDA
  standard selection method, like ``target_compile_features``.

* If you do not request a standard, pybind11 targets will compile with the
  compiler default, but not less than C++11, instead of forcing C++14 always.
  If you depend on the old behavior, please use ``set(CMAKE_CXX_STANDARD 14 CACHE STRING "")``
  instead.

* Direct ``pybind11::module`` usage should always be accompanied by at least
  ``set(CMAKE_CXX_VISIBILITY_PRESET hidden)`` or similar - it used to try to
  manually force this compiler flag (but not correctly on all compilers or with
  CUDA).

* ``pybind11_add_module``'s ``SYSTEM`` argument is deprecated and does nothing;
  linking now behaves like other imported libraries consistently in both
  config and submodule mode, and behaves like a ``SYSTEM`` library by
  default.

* If ``PYTHON_EXECUTABLE`` is not set, virtual environments (``venv``,
  ``virtualenv``, and ``conda``) are prioritized over the standard search
  (similar to the new FindPython mode).

In addition, the following changes may be of interest:

* ``CMAKE_INTERPROCEDURAL_OPTIMIZATION`` will be respected by
  ``pybind11_add_module`` if set instead of linking to ``pybind11::lto`` or
  ``pybind11::thin_lto``.

* Using ``find_package(Python COMPONENTS Interpreter Development)`` before
  pybind11 will cause pybind11 to use the new Python mechanisms instead of its
  own custom search, based on a patched version of classic ``FindPythonInterp``
  / ``FindPythonLibs``. In the future, this may become the default. A recent
  (3.15+ or 3.18.2+) version of CMake is recommended.



v2.5
====

The Python package now includes the headers as data in the package itself, as
well as in the "headers" wheel slot. ``pybind11 --includes`` and
``pybind11.get_include()`` report the new location, which is always correct
regardless of how pybind11 was installed, making the old ``user=`` argument
meaningless. If you are not using the function to get the location already, you
are encouraged to switch to the package location.


v2.2
====

Deprecation of the ``PYBIND11_PLUGIN`` macro
--------------------------------------------

``PYBIND11_MODULE`` is now the preferred way to create module entry points.
The old macro emits a compile-time deprecation warning.

.. code-block:: cpp

    // old
    PYBIND11_PLUGIN(example) {
        py::module m("example", "documentation string");

        m.def("add", [](int a, int b) { return a + b; });

        return m.ptr();
    }

    // new
    PYBIND11_MODULE(example, m) {
        m.doc() = "documentation string"; // optional

        m.def("add", [](int a, int b) { return a + b; });
    }


New API for defining custom constructors and pickling functions
---------------------------------------------------------------

The old placement-new custom constructors have been deprecated. The new approach
uses ``py::init()`` and factory functions to greatly improve type safety.

Placement-new can be called accidentally with an incompatible type (without any
compiler errors or warnings), or it can initialize the same object multiple times
if not careful with the Python-side ``__init__`` calls. The new-style custom
constructors prevent such mistakes. See :ref:`custom_constructors` for details.

.. code-block:: cpp

    // old -- deprecated (runtime warning shown only in debug mode)
    py::class<Foo>(m, "Foo")
        .def("__init__", [](Foo &self, ...) {
            new (&self) Foo(...); // uses placement-new
        });

    // new
    py::class<Foo>(m, "Foo")
        .def(py::init([](...) { // Note: no `self` argument
            return new Foo(...); // return by raw pointer
            // or: return std::make_unique<Foo>(...); // return by holder
            // or: return Foo(...); // return by value (move constructor)
        }));

Mirroring the custom constructor changes, ``py::pickle()`` is now the preferred
way to get and set object state. See :ref:`pickling` for details.

.. code-block:: cpp

    // old -- deprecated (runtime warning shown only in debug mode)
    py::class<Foo>(m, "Foo")
        ...
        .def("__getstate__", [](const Foo &self) {
            return py::make_tuple(self.value1(), self.value2(), ...);
        })
        .def("__setstate__", [](Foo &self, py::tuple t) {
            new (&self) Foo(t[0].cast<std::string>(), ...);
        });

    // new
    py::class<Foo>(m, "Foo")
        ...
        .def(py::pickle(
            [](const Foo &self) { // __getstate__
                return py::make_tuple(self.value1(), self.value2(), ...); // unchanged
            },
            [](py::tuple t) { // __setstate__, note: no `self` argument
                return new Foo(t[0].cast<std::string>(), ...);
                // or: return std::make_unique<Foo>(...); // return by holder
                // or: return Foo(...); // return by value (move constructor)
            }
        ));

For both the constructors and pickling, warnings are shown at module
initialization time (on import, not when the functions are called).
They're only visible when compiled in debug mode. Sample warning:

.. code-block:: none

    pybind11-bound class 'mymodule.Foo' is using an old-style placement-new '__init__'
    which has been deprecated. See the upgrade guide in pybind11's docs.


Stricter enforcement of hidden symbol visibility for pybind11 modules
---------------------------------------------------------------------

pybind11 now tries to actively enforce hidden symbol visibility for modules.
If you're using either one of pybind11's :doc:`CMake or Python build systems
<compiling>` (the two example repositories) and you haven't been exporting any
symbols, there's nothing to be concerned about. All the changes have been done
transparently in the background. If you were building manually or relied on
specific default visibility, read on.

Setting default symbol visibility to *hidden* has always been recommended for
pybind11 (see :ref:`faq:symhidden`). On Linux and macOS, hidden symbol
visibility (in conjunction with the ``strip`` utility) yields much smaller
module binaries. `CPython's extension docs`_ also recommend hiding symbols
by default, with the goal of avoiding symbol name clashes between modules.
Starting with v2.2, pybind11 enforces this more strictly: (1) by declaring
all symbols inside the ``pybind11`` namespace as hidden and (2) by including
the ``-fvisibility=hidden`` flag on Linux and macOS (only for extension
modules, not for embedding the interpreter).

.. _CPython's extension docs: https://docs.python.org/3/extending/extending.html#providing-a-c-api-for-an-extension-module

The namespace-scope hidden visibility is done automatically in pybind11's
headers and it's generally transparent to users. It ensures that:

* Modules compiled with different pybind11 versions don't clash with each other.

* Some new features, like ``py::module_local`` bindings, can work as intended.

The ``-fvisibility=hidden`` flag applies the same visibility to user bindings
outside of the ``pybind11`` namespace. It's now set automatic by pybind11's
CMake and Python build systems, but this needs to be done manually by users
of other build systems. Adding this flag:

* Minimizes the chances of symbol conflicts between modules. E.g. if two
  unrelated modules were statically linked to different (ABI-incompatible)
  versions of the same third-party library, a symbol clash would be likely
  (and would end with unpredictable results).

* Produces smaller binaries on Linux and macOS, as pointed out previously.

Within pybind11's CMake build system, ``pybind11_add_module`` has always been
setting the ``-fvisibility=hidden`` flag in release mode. From now on, it's
being applied unconditionally, even in debug mode and it can no longer be opted
out of with the ``NO_EXTRAS`` option. The ``pybind11::module`` target now also
adds this flag to its interface. The ``pybind11::embed`` target is unchanged.

The most significant change here is for the ``pybind11::module`` target. If you
were previously relying on default visibility, i.e. if your Python module was
doubling as a shared library with dependents, you'll need to either export
symbols manually (recommended for cross-platform libraries) or factor out the
shared library (and have the Python module link to it like the other
dependents). As a temporary workaround, you can also restore default visibility
using the CMake code below, but this is not recommended in the long run:

.. code-block:: cmake

    target_link_libraries(mymodule PRIVATE pybind11::module)

    add_library(restore_default_visibility INTERFACE)
    target_compile_options(restore_default_visibility INTERFACE -fvisibility=default)
    target_link_libraries(mymodule PRIVATE restore_default_visibility)


Local STL container bindings
----------------------------

Previous pybind11 versions could only bind types globally -- all pybind11
modules, even unrelated ones, would have access to the same exported types.
However, this would also result in a conflict if two modules exported the
same C++ type, which is especially problematic for very common types, e.g.
``std::vector<int>``. :ref:`module_local` were added to resolve this (see
that section for a complete usage guide).

``py::class_`` still defaults to global bindings (because these types are
usually unique across modules), however in order to avoid clashes of opaque
types, ``py::bind_vector`` and ``py::bind_map`` will now bind STL containers
as ``py::module_local`` if their elements are: builtins (``int``, ``float``,
etc.), not bound using ``py::class_``, or bound as ``py::module_local``. For
example, this change allows multiple modules to bind ``std::vector<int>``
without causing conflicts. See :ref:`stl_bind` for more details.

When upgrading to this version, if you have multiple modules which depend on
a single global binding of an STL container, note that all modules can still
accept foreign  ``py::module_local`` types in the direction of Python-to-C++.
The locality only affects the C++-to-Python direction. If this is needed in
multiple modules, you'll need to either:

* Add a copy of the same STL binding to all of the modules which need it.

* Restore the global status of that single binding by marking it
  ``py::module_local(false)``.

The latter is an easy workaround, but in the long run it would be best to
localize all common type bindings in order to avoid conflicts with
third-party modules.


Negative strides for Python buffer objects and numpy arrays
-----------------------------------------------------------

Support for negative strides required changing the integer type from unsigned
to signed in the interfaces of ``py::buffer_info`` and ``py::array``. If you
have compiler warnings enabled, you may notice some new conversion warnings
after upgrading. These can be resolved using ``static_cast``.


Deprecation of some ``py::object`` APIs
---------------------------------------

To compare ``py::object`` instances by pointer, you should now use
``obj1.is(obj2)`` which is equivalent to ``obj1 is obj2`` in Python.
Previously, pybind11 used ``operator==`` for this (``obj1 == obj2``), but
that could be confusing and is now deprecated (so that it can eventually
be replaced with proper rich object comparison in a future release).

For classes which inherit from ``py::object``, ``borrowed`` and ``stolen``
were previously available as protected constructor tags. Now the types
should be used directly instead: ``borrowed_t{}`` and ``stolen_t{}``
(`#771 <https://github.com/pybind/pybind11/pull/771>`_).


Stricter compile-time error checking
------------------------------------

Some error checks have been moved from run time to compile time. Notably,
automatic conversion of ``std::shared_ptr<T>`` is not possible when ``T`` is
not directly registered with ``py::class_<T>`` (e.g. ``std::shared_ptr<int>``
or ``std::shared_ptr<std::vector<T>>`` are not automatically convertible).
Attempting to bind a function with such arguments now results in a compile-time
error instead of waiting to fail at run time.

``py::init<...>()`` constructor definitions are also stricter and now prevent
bindings which could cause unexpected behavior:

.. code-block:: cpp

    struct Example {
        Example(int &);
    };

    py::class_<Example>(m, "Example")
        .def(py::init<int &>()); // OK, exact match
        // .def(py::init<int>()); // compile-time error, mismatch

A non-``const`` lvalue reference is not allowed to bind to an rvalue. However,
note that a constructor taking ``const T &`` can still be registered using
``py::init<T>()`` because a ``const`` lvalue reference can bind to an rvalue.

v2.1
====

Minimum compiler versions are enforced at compile time
------------------------------------------------------

The minimums also apply to v2.0 but the check is now explicit and a compile-time
error is raised if the compiler does not meet the requirements:

* GCC >= 4.8
* clang >= 3.3 (appleclang >= 5.0)
* MSVC >= 2015u3
* Intel C++ >= 15.0


The ``py::metaclass`` attribute is not required for static properties
---------------------------------------------------------------------

Binding classes with static properties is now possible by default. The
zero-parameter version of ``py::metaclass()`` is deprecated. However, a new
one-parameter ``py::metaclass(python_type)`` version was added for rare
cases when a custom metaclass is needed to override pybind11's default.

.. code-block:: cpp

    // old -- emits a deprecation warning
    py::class_<Foo>(m, "Foo", py::metaclass())
        .def_property_readonly_static("foo", ...);

    // new -- static properties work without the attribute
    py::class_<Foo>(m, "Foo")
        .def_property_readonly_static("foo", ...);

    // new -- advanced feature, override pybind11's default metaclass
    py::class_<Bar>(m, "Bar", py::metaclass(custom_python_type))
        ...


v2.0
====

Breaking changes in ``py::class_``
----------------------------------

These changes were necessary to make type definitions in pybind11
future-proof, to support PyPy via its ``cpyext`` mechanism (`#527
<https://github.com/pybind/pybind11/pull/527>`_), and to improve efficiency
(`rev. 86d825 <https://github.com/pybind/pybind11/commit/86d825>`_).

1. Declarations of types that provide access via the buffer protocol must
   now include the ``py::buffer_protocol()`` annotation as an argument to
   the ``py::class_`` constructor.

   .. code-block:: cpp

       py::class_<Matrix>("Matrix", py::buffer_protocol())
           .def(py::init<...>())
           .def_buffer(...);

2. Classes which include static properties (e.g. ``def_readwrite_static()``)
   must now include the ``py::metaclass()`` attribute. Note: this requirement
   has since been removed in v2.1. If you're upgrading from 1.x, it's
   recommended to skip directly to v2.1 or newer.

3. This version of pybind11 uses a redesigned mechanism for instantiating
   trampoline classes that are used to override virtual methods from within
   Python. This led to the following user-visible syntax change:

   .. code-block:: cpp

       // old v1.x syntax
       py::class_<TrampolineClass>("MyClass")
           .alias<MyClass>()
           ...

       // new v2.x syntax
       py::class_<MyClass, TrampolineClass>("MyClass")
           ...

   Importantly, both the original and the trampoline class are now specified
   as arguments to the ``py::class_`` template, and the ``alias<..>()`` call
   is gone. The new scheme has zero overhead in cases when Python doesn't
   override any functions of the underlying C++ class.
   `rev. 86d825 <https://github.com/pybind/pybind11/commit/86d825>`_.

   The class type must be the first template argument given to ``py::class_``
   while the trampoline can be mixed in arbitrary order with other arguments
   (see the following section).


Deprecation of the ``py::base<T>()`` attribute
----------------------------------------------

``py::base<T>()`` was deprecated in favor of specifying ``T`` as a template
argument to ``py::class_``. This new syntax also supports multiple inheritance.
Note that, while the type being exported must be the first argument in the
``py::class_<Class, ...>`` template, the order of the following types (bases,
holder and/or trampoline) is not important.

.. code-block:: cpp

    // old v1.x
    py::class_<Derived>("Derived", py::base<Base>());

    // new v2.x
    py::class_<Derived, Base>("Derived");

    // new -- multiple inheritance
    py::class_<Derived, Base1, Base2>("Derived");

    // new -- apart from `Derived` the argument order can be arbitrary
    py::class_<Derived, Base1, Holder, Base2, Trampoline>("Derived");


Out-of-the-box support for ``std::shared_ptr``
----------------------------------------------

The relevant type caster is now built in, so it's no longer necessary to
include a declaration of the form:

.. code-block:: cpp

    PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>)

Continuing to do so won't cause an error or even a deprecation warning,
but it's completely redundant.


Deprecation of a few ``py::object`` APIs
----------------------------------------

All of the old-style calls emit deprecation warnings.

+---------------------------------------+---------------------------------------------+
|  Old syntax                           |  New syntax                                 |
+=======================================+=============================================+
| ``obj.call(args...)``                 | ``obj(args...)``                            |
+---------------------------------------+---------------------------------------------+
| ``obj.str()``                         | ``py::str(obj)``                            |
+---------------------------------------+---------------------------------------------+
| ``auto l = py::list(obj); l.check()`` | ``py::isinstance<py::list>(obj)``           |
+---------------------------------------+---------------------------------------------+
| ``py::object(ptr, true)``             | ``py::reinterpret_borrow<py::object>(ptr)`` |
+---------------------------------------+---------------------------------------------+
| ``py::object(ptr, false)``            | ``py::reinterpret_steal<py::object>(ptr)``  |
+---------------------------------------+---------------------------------------------+
| ``if (obj.attr("foo"))``              | ``if (py::hasattr(obj, "foo"))``            |
+---------------------------------------+---------------------------------------------+
| ``if (obj["bar"])``                   | ``if (obj.contains("bar"))``                |
+---------------------------------------+---------------------------------------------+