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Add upgrade guide to the documentation
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Dean Moldovan
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@ -60,7 +60,8 @@ v2.2.0 (Not yet released)
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* pybind11's headers and build system now more strictly enforce hidden symbol
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visibility for extension modules. This should be seamless for most users,
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but see the upgrade guide if you use a custom build system.
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but see the :doc:`upgrade` if you use a custom build system.
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`#995 <https://github.com/pybind/pybind11/pull/995>`_.
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* Support for ``py::module_local`` types which allow multiple modules to
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export the same C++ types without conflicts. This is useful for opaque
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@ -14,6 +14,7 @@ pybind11 --- Seamless operability between C++11 and Python
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intro
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changelog
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upgrade
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.. toctree::
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:caption: The Basics
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329
docs/upgrade.rst
Normal file
329
docs/upgrade.rst
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@ -0,0 +1,329 @@
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Upgrade guide
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#############
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This is a companion guide to the :doc:`changelog`. While the changelog briefly
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lists all of the new features, improvements and bug fixes, this upgrade guide
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focuses only the subset which directly impacts your experience when upgrading
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to a new version. But it goes into more detail. This includes things like
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deprecated APIs and their replacements, build system changes, general code
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modernization and other useful information.
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v2.2
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====
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Deprecation of the ``PYBIND11_PLUGIN`` macro
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--------------------------------------------
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``PYBIND11_MODULE`` is now the preferred way to create module entry points.
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The old macro emits a compile-time deprecation warning.
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.. code-block:: cpp
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// old
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PYBIND11_PLUGIN(example) {
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py::module m("example", "documentation string");
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m.def("add", [](int a, int b) { return a + b; });
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return m.ptr();
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}
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// new
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PYBIND11_MODULE(example, m) {
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m.doc() = "documentation string"; // optional
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m.def("add", [](int a, int b) { return a + b; });
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}
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Stricter enforcement of hidden symbol visibility for pybind11 modules
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---------------------------------------------------------------------
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pybind11 now tries to actively enforce hidden symbol visibility for modules.
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If you're using either one of pybind11's :doc:`CMake or Python build systems
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<compiling>` (the two example repositories) and you haven't been exporting any
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symbols, there's nothing to be concerned about. All the changes have been done
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transparently in the background. If you were building manually or relied on
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specific default visibility, read on.
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Setting default symbol visibility to *hidden* has always been recommended for
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pybind11 (see :ref:`faq:symhidden`). On Linux and macOS, hidden symbol
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visibility (in conjunction with the ``strip`` utility) yields much smaller
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module binaries. `CPython's extension docs`_ also recommend hiding symbols
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by default, with the goal of avoiding symbol name clashes between modules.
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Starting with v2.2, pybind11 enforces this more strictly: (1) by declaring
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all symbols inside the ``pybind11`` namespace as hidden and (2) by including
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the ``-fvisibility=hidden`` flag on Linux and macOS (only for extension
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modules, not for embedding the interpreter).
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.. _CPython's extension docs: https://docs.python.org/3/extending/extending.html#providing-a-c-api-for-an-extension-module
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The namespace-scope hidden visibility is done automatically in pybind11's
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headers and it's generally transparent to users. It ensures that:
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* Modules compiled with different pybind11 versions don't clash with each other.
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* Some new features, like ``py::module_local`` bindings, can work as intended.
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The ``-fvisibility=hidden`` flag applies the same visibility to user bindings
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outside of the ``pybind11`` namespace. It's now set automatic by pybind11's
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CMake and Python build systems, but this needs to be done manually by users
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of other build systems. Adding this flag:
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* Minimizes the chances of symbol conflicts between modules. E.g. if two
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unrelated modules were statically linked to different (ABI-incompatible)
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versions of the same third-party library, a symbol clash would be likely
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(and would end with unpredictable results).
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* Produces smaller binaries on Linux and macOS, as pointed out previously.
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Within pybind11's CMake build system, ``pybind11_add_module`` has always been
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setting the ``-fvisibility=hidden`` flag in release mode. From now on, it's
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being applied unconditionally, even in debug mode and it can no longer be opted
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out of with the ``NO_EXTRAS`` option. The ``pybind11::module`` target now also
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adds this flag to it's interface. The ``pybind11::embed`` target is unchanged.
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The most significant change here is for the ``pybind11::module`` target. If you
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were previously relying on default visibility, i.e. if your Python module was
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doubling as a shared library with dependents, you'll need to either export
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symbols manually (recommended for cross-platform libraries) or factor out the
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shared library (and have the Python module link to it like the other
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dependents). As a temporary workaround, you can also restore default visibility
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using the CMake code below, but this is not recommended in the long run:
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.. code-block:: cmake
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target_link_libraries(mymodule PRIVATE pybind11::module)
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add_library(restore_default_visibility INTERFACE)
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target_compile_options(restore_default_visibility INTERFACE -fvisibility=default)
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target_link_libraries(mymodule PRIVATE restore_default_visibility)
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Local STL container bindings
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----------------------------
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Previous pybind11 versions could only bind types globally -- all pybind11
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modules, even unrelated ones, would have access to the same exported types.
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However, this would also result in a conflict if two modules exported the
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same C++ type, which is especially problematic for very common types, e.g.
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``std::vector<int>``. :ref:`module_local` were added to resolve this (see
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that section for a complete usage guide).
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``py::class_`` still defaults to global bindings (because these types are
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usually unique across modules), however in order to avoid clashes of opaque
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types, ``py::bind_vector`` and ``py::bind_map`` will now bind STL containers
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as ``py::module_local`` if their elements are: builtins (``int``, ``float``,
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etc.), not bound using ``py::class_``, or bound as ``py::module_local``. For
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example, this change allows multiple modules to bind ``std::vector<int>``
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without causing conflicts. See :ref:`stl_bind` for more details.
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When upgrading to this version, if you have multiple modules which depend on
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a single global binding of an STL container, note that all modules can still
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accept foreign ``py::module_local`` types in the direction of Python-to-C++.
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The locality only affects the C++-to-Python direction. If this is needed in
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multiple modules, you'll need to either:
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* Add a copy of the same STL binding to all of the modules which need it.
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* Restore the global status of that single binding by marking it
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``py::module_local(false)``.
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The latter is an easy workaround, but in the long run it would be best to
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localize all common type bindings in order to avoid conflicts with
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third-party modules.
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New syntax for custom constructors
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----------------------------------
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The old placement-new custom constructors are still valid, but the new approach
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greatly improves type safety. Placement-new can be called accidentally with an
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incompatible type (without any compiler errors or warnings), or it can initialize
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the same object multiple times if not careful with the Python-side ``__init__``
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calls. The new-style ``py::init()`` custom constructors prevent such mistakes.
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See :ref:`custom_constructors` for details.
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.. code-block:: cpp
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// old
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py::class<Foo>(m, "Foo")
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.def("__init__", [](Foo &self, ...) {
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new (&self) Foo(...); // uses placement-new
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});
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// new
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py::class<Foo>(m, "Foo")
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.def(py::init([](...) { // Note: no `self` argument
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return new Foo(...); // return by raw pointer
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// or: return std::make_unique<Foo>(...); // return by holder
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// or: return Foo(...); // return by value (move constructor)
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}));
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Deprecation of some ``py::object`` APIs
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---------------------------------------
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To compare ``py::object`` instances by pointer, you should now use
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``obj1.is(obj2)`` which is equivalent to ``obj1 is obj2`` in Python.
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Previously, pybind11 used ``operator==`` for this (``obj1 == obj2``), but
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that could be confusing and is now deprecated (so that it can eventually
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be replaced with proper rich object comparison in a future release).
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For classes which inherit from ``py::object``, ``borrowed`` and ``stolen``
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were previously available as protected constructor tags. Now the types
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should be used directly instead: ``borrowed_t{}`` and ``stolen_t{}``
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(`#771 <https://github.com/pybind/pybind11/pull/771>`_).
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v2.1
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====
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Minimum compiler versions are enforced at compile time
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------------------------------------------------------
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The minimums also apply to v2.0 but the check is now explicit and a compile-time
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error is raised if the compiler does not meet the requirements:
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* GCC >= 4.8
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* clang >= 3.3 (appleclang >= 5.0)
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* MSVC >= 2015u3
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* Intel C++ >= 15.0
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The ``py::metaclass`` attribute is not required for static properties
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---------------------------------------------------------------------
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Binding classes with static properties is now possible by default. The
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zero-parameter version of ``py::metaclass()`` is deprecated. However, a new
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one-parameter ``py::metaclass(python_type)`` version was added for rare
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cases when a custom metaclass is needed to override pybind11's default.
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.. code-block:: cpp
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// old -- emits a deprecation warning
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py::class_<Foo>(m, "Foo", py::metaclass())
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.def_property_readonly_static("foo", ...);
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// new -- static properties work without the attribute
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py::class_<Foo>(m, "Foo")
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.def_property_readonly_static("foo", ...);
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// new -- advanced feature, override pybind11's default metaclass
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py::class_<Bar>(m, "Bar", py::metaclass(custom_python_type))
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...
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v2.0
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====
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Breaking changes in ``py::class_``
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----------------------------------
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These changes were necessary to make type definitions in pybind11
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future-proof, to support PyPy via its ``cpyext`` mechanism (`#527
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<https://github.com/pybind/pybind11/pull/527>`_), and to improve efficiency
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(`rev. 86d825 <https://github.com/pybind/pybind11/commit/86d825>`_).
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1. Declarations of types that provide access via the buffer protocol must
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now include the ``py::buffer_protocol()`` annotation as an argument to
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the ``py::class_`` constructor.
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.. code-block:: cpp
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py::class_<Matrix>("Matrix", py::buffer_protocol())
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.def(py::init<...>())
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.def_buffer(...);
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2. Classes which include static properties (e.g. ``def_readwrite_static()``)
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must now include the ``py::metaclass()`` attribute. Note: this requirement
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has since been removed in v2.1. If you're upgrading from 1.x, it's
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recommended to skip directly to v2.1 or newer.
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3. This version of pybind11 uses a redesigned mechanism for instantiating
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trampoline classes that are used to override virtual methods from within
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Python. This led to the following user-visible syntax change:
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.. code-block:: cpp
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// old v1.x syntax
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py::class_<TrampolineClass>("MyClass")
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.alias<MyClass>()
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...
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// new v2.x syntax
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py::class_<MyClass, TrampolineClass>("MyClass")
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...
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Importantly, both the original and the trampoline class are now specified
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as arguments to the ``py::class_`` template, and the ``alias<..>()`` call
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is gone. The new scheme has zero overhead in cases when Python doesn't
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override any functions of the underlying C++ class.
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`rev. 86d825 <https://github.com/pybind/pybind11/commit/86d825>`_.
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The class type must be the first template argument given to ``py::class_``
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while the trampoline can be mixed in arbitrary order with other arguments
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(see the following section).
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Deprecation of the ``py::base<T>()`` attribute
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----------------------------------------------
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``py::base<T>()`` was deprecated in favor of specifying ``T`` as a template
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argument to ``py::class_``. This new syntax also supports multiple inheritance.
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Note that, while the type being exported must be the first argument in the
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``py::class_<Class, ...>`` template, the order of the following types (bases,
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holder and/or trampoline) is not important.
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.. code-block:: cpp
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// old v1.x
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py::class_<Derived>("Derived", py::base<Base>());
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// new v2.x
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py::class_<Derived, Base>("Derived");
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// new -- multiple inheritance
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py::class_<Derived, Base1, Base2>("Derived");
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// new -- apart from `Derived` the argument order can be arbitrary
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py::class_<Derived, Base1, Holder, Base2, Trampoline>("Derived");
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Out-of-the-box support for ``std::shared_ptr``
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----------------------------------------------
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The relevant type caster is now built in, so it's no longer necessary to
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include a declaration of the form:
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.. code-block:: cpp
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PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>)
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Continuing to do so won’t cause an error or even a deprecation warning,
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but it's completely redundant.
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Deprecation of a few ``py::object`` APIs
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----------------------------------------
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All of the old-style calls emit deprecation warnings.
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+---------------------------------------+---------------------------------------------+
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| Old syntax | New syntax |
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+=======================================+=============================================+
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| ``obj.call(args...)`` | ``obj(args...)`` |
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+---------------------------------------+---------------------------------------------+
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| ``obj.str()`` | ``py::str(obj)`` |
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+---------------------------------------+---------------------------------------------+
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| ``auto l = py::list(obj); l.check()`` | ``py::isinstance<py::list>(obj)`` |
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+---------------------------------------+---------------------------------------------+
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| ``py::object(ptr, true)`` | ``py::reinterpret_borrow<py::object>(ptr)`` |
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+---------------------------------------+---------------------------------------------+
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| ``py::object(ptr, false)`` | ``py::reinterpret_steal<py::object>(ptr)`` |
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+---------------------------------------+---------------------------------------------+
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| ``if (obj.attr("foo"))`` | ``if (py::hasattr(obj, "foo"))`` |
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+---------------------------------------+---------------------------------------------+
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| ``if (obj["bar"])`` | ``if (obj.contains("bar"))`` |
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+---------------------------------------+---------------------------------------------+
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