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172 lines
12 KiB
ReStructuredText
172 lines
12 KiB
ReStructuredText
Overview
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########
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.. rubric:: 1. Native type in C++, wrapper in Python
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Exposing a custom C++ type using :class:`py::class_` was covered in detail
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in the :doc:`/classes` section. There, the underlying data structure is
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always the original C++ class while the :class:`py::class_` wrapper provides
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a Python interface. Internally, when an object like this is sent from C++ to
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Python, pybind11 will just add the outer wrapper layer over the native C++
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object. Getting it back from Python is just a matter of peeling off the
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wrapper.
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.. rubric:: 2. Wrapper in C++, native type in Python
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This is the exact opposite situation. Now, we have a type which is native to
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Python, like a ``tuple`` or a ``list``. One way to get this data into C++ is
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with the :class:`py::object` family of wrappers. These are explained in more
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detail in the :doc:`/advanced/pycpp/object` section. We'll just give a quick
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example here:
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.. code-block:: cpp
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void print_list(py::list my_list) {
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for (auto item : my_list)
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std::cout << item << " ";
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}
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.. code-block:: pycon
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>>> print_list([1, 2, 3])
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1 2 3
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The Python ``list`` is not converted in any way -- it's just wrapped in a C++
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:class:`py::list` class. At its core it's still a Python object. Copying a
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:class:`py::list` will do the usual reference-counting like in Python.
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Returning the object to Python will just remove the thin wrapper.
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.. rubric:: 3. Converting between native C++ and Python types
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In the previous two cases we had a native type in one language and a wrapper in
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the other. Now, we have native types on both sides and we convert between them.
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.. code-block:: cpp
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void print_vector(const std::vector<int> &v) {
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for (auto item : v)
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std::cout << item << "\n";
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}
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.. code-block:: pycon
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>>> print_vector([1, 2, 3])
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1 2 3
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In this case, pybind11 will construct a new ``std::vector<int>`` and copy each
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element from the Python ``list``. The newly constructed object will be passed
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to ``print_vector``. The same thing happens in the other direction: a new
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``list`` is made to match the value returned from C++.
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Lots of these conversions are supported out of the box, as shown in the table
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below. They are very convenient, but keep in mind that these conversions are
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fundamentally based on copying data. This is perfectly fine for small immutable
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types but it may become quite expensive for large data structures. This can be
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avoided by overriding the automatic conversion with a custom wrapper (i.e. the
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above-mentioned approach 1). This requires some manual effort and more details
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are available in the :ref:`opaque` section.
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.. _conversion_table:
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List of all builtin conversions
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-------------------------------
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The following basic data types are supported out of the box (some may require
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an additional extension header to be included). To pass other data structures
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as arguments and return values, refer to the section on binding :ref:`classes`.
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+------------------------------------+---------------------------+-----------------------------------+
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| Data type | Description | Header file |
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+====================================+===========================+===================================+
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| ``int8_t``, ``uint8_t`` | 8-bit integers | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``int16_t``, ``uint16_t`` | 16-bit integers | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``int32_t``, ``uint32_t`` | 32-bit integers | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``int64_t``, ``uint64_t`` | 64-bit integers | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``ssize_t``, ``size_t`` | Platform-dependent size | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``float``, ``double`` | Floating point types | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``bool`` | Two-state Boolean type | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``char`` | Character literal | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``char16_t`` | UTF-16 character literal | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``char32_t`` | UTF-32 character literal | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``wchar_t`` | Wide character literal | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``const char *`` | UTF-8 string literal | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``const char16_t *`` | UTF-16 string literal | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``const char32_t *`` | UTF-32 string literal | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``const wchar_t *`` | Wide string literal | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::string`` | STL dynamic UTF-8 string | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::u16string`` | STL dynamic UTF-16 string | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::u32string`` | STL dynamic UTF-32 string | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::wstring`` | STL dynamic wide string | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::string_view``, | STL C++17 string views | :file:`pybind11/pybind11.h` |
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| ``std::u16string_view``, etc. | | |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::pair<T1, T2>`` | Pair of two custom types | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::tuple<...>`` | Arbitrary tuple of types | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::reference_wrapper<...>`` | Reference type wrapper | :file:`pybind11/pybind11.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::complex<T>`` | Complex numbers | :file:`pybind11/complex.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::array<T, Size>`` | STL static array | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::vector<T>`` | STL dynamic array | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::deque<T>`` | STL double-ended queue | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::valarray<T>`` | STL value array | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::list<T>`` | STL linked list | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::map<T1, T2>`` | STL ordered map | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::unordered_map<T1, T2>`` | STL unordered map | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::set<T>`` | STL ordered set | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::unordered_set<T>`` | STL unordered set | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::optional<T>`` | STL optional type (C++17) | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::experimental::optional<T>`` | STL optional type (exp.) | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::variant<...>`` | Type-safe union (C++17) | :file:`pybind11/stl.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::filesystem::path<T>`` | STL path (C++17) [#]_ | :file:`pybind11/stl/filesystem.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::function<...>`` | STL polymorphic function | :file:`pybind11/functional.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::chrono::duration<...>`` | STL time duration | :file:`pybind11/chrono.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``std::chrono::time_point<...>`` | STL date/time | :file:`pybind11/chrono.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``Eigen::Matrix<...>`` | Eigen: dense matrix | :file:`pybind11/eigen.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``Eigen::Map<...>`` | Eigen: mapped memory | :file:`pybind11/eigen.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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| ``Eigen::SparseMatrix<...>`` | Eigen: sparse matrix | :file:`pybind11/eigen.h` |
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+------------------------------------+---------------------------+-----------------------------------+
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.. [#] ``std::filesystem::path`` is converted to ``pathlib.Path`` and
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``os.PathLike`` is converted to ``std::filesystem::path``, but this requires
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Python 3.6 (for ``__fspath__`` support).
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