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diff --git a/ext/pybind11/docs/advanced/cast/overview.rst b/ext/pybind11/docs/advanced/cast/overview.rst new file mode 100644 index 000000000..ab37b90be --- /dev/null +++ b/ext/pybind11/docs/advanced/cast/overview.rst @@ -0,0 +1,146 @@ +Overview +######## + +.. rubric:: 1. Native type in C++, wrapper in Python + +Exposing a custom C++ type using :class:`py::class_` was covered in detail +in the :doc:`/classes` section. There, the underlying data structure is +always the original C++ class while the :class:`py::class_` wrapper provides +a Python interface. Internally, when an object like this is sent from C++ to +Python, pybind11 will just add the outer wrapper layer over the native C++ +object. Getting it back from Python is just a matter of peeling off the +wrapper. + +.. rubric:: 2. Wrapper in C++, native type in Python + +This is the exact opposite situation. Now, we have a type which is native to +Python, like a ``tuple`` or a ``list``. One way to get this data into C++ is +with the :class:`py::object` family of wrappers. These are explained in more +detail in the :doc:`/advanced/pycpp/object` section. We'll just give a quick +example here: + +.. code-block:: cpp + + void print_list(py::list my_list) { + for (auto item : my_list) + std::cout << item << " "; + } + +.. code-block:: pycon + + >>> print_list([1, 2, 3]) + 1 2 3 + +The Python ``list`` is not converted in any way -- it's just wrapped in a C++ +:class:`py::list` class. At its core it's still a Python object. Copying a +:class:`py::list` will do the usual reference-counting like in Python. +Returning the object to Python will just remove the thin wrapper. + +.. rubric:: 3. Converting between native C++ and Python types + +In the previous two cases we had a native type in one language and a wrapper in +the other. Now, we have native types on both sides and we convert between them. + +.. code-block:: cpp + + void print_vector(const std::vector<int> &v) { + for (auto item : v) + std::cout << item << "\n"; + } + +.. code-block:: pycon + + >>> print_vector([1, 2, 3]) + 1 2 3 + +In this case, pybind11 will construct a new ``std::vector<int>`` and copy each +element from the Python ``list``. The newly constructed object will be passed +to ``print_vector``. The same thing happens in the other direction: a new +``list`` is made to match the value returned from C++. + +Lots of these conversions are supported out of the box, as shown in the table +below. They are very convenient, but keep in mind that these conversions are +fundamentally based on copying data. This is perfectly fine for small immutable +types but it may become quite expensive for large data structures. This can be +avoided by overriding the automatic conversion with a custom wrapper (i.e. the +above-mentioned approach 1). This requires some manual effort and more details +are available in the :ref:`opaque` section. + +.. _conversion_table: + +List of all builtin conversions +------------------------------- + +The following basic data types are supported out of the box (some may require +an additional extension header to be included). To pass other data structures +as arguments and return values, refer to the section on binding :ref:`classes`. + ++------------------------------------+---------------------------+-------------------------------+ +| Data type | Description | Header file | ++====================================+===========================+===============================+ +| ``int8_t``, ``uint8_t`` | 8-bit integers | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``int16_t``, ``uint16_t`` | 16-bit integers | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``int32_t``, ``uint32_t`` | 32-bit integers | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``int64_t``, ``uint64_t`` | 64-bit integers | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``ssize_t``, ``size_t`` | Platform-dependent size | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``float``, ``double`` | Floating point types | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``bool`` | Two-state Boolean type | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``char`` | Character literal | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``wchar_t`` | Wide character literal | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``const char *`` | UTF-8 string literal | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``const wchar_t *`` | Wide string literal | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::string`` | STL dynamic UTF-8 string | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::wstring`` | STL dynamic wide string | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::pair<T1, T2>`` | Pair of two custom types | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::tuple<...>`` | Arbitrary tuple of types | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::reference_wrapper<...>`` | Reference type wrapper | :file:`pybind11/pybind11.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::complex<T>`` | Complex numbers | :file:`pybind11/complex.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::array<T, Size>`` | STL static array | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::vector<T>`` | STL dynamic array | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::valarray<T>`` | STL value array | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::list<T>`` | STL linked list | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::map<T1, T2>`` | STL ordered map | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::unordered_map<T1, T2>`` | STL unordered map | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::set<T>`` | STL ordered set | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::unordered_set<T>`` | STL unordered set | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::optional<T>`` | STL optional type (C++17) | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::experimental::optional<T>`` | STL optional type (exp.) | :file:`pybind11/stl.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::function<...>`` | STL polymorphic function | :file:`pybind11/functional.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::chrono::duration<...>`` | STL time duration | :file:`pybind11/chrono.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``std::chrono::time_point<...>`` | STL date/time | :file:`pybind11/chrono.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``Eigen::Matrix<...>`` | Eigen: dense matrix | :file:`pybind11/eigen.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``Eigen::Map<...>`` | Eigen: mapped memory | :file:`pybind11/eigen.h` | ++------------------------------------+---------------------------+-------------------------------+ +| ``Eigen::SparseMatrix<...>`` | Eigen: sparse matrix | :file:`pybind11/eigen.h` | ++------------------------------------+---------------------------+-------------------------------+ |