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Directly compare 3 ways of moving data between C++ and Python
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Casting data types
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##################
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Type conversions
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################
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There are 3 mechanisms that pybind11 uses to move data between C++ and Python.
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We'll take a quick look at each one to get an overview of what's happening.
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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 in
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the :doc:`/classes` section. There, the underlying data structure is always the
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original C++ class while the :class:`py::class_` wrapper provides a Python
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interface. Internally, when an object like this is sent from C++ to Python,
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pybind11 will just add the outer wrapper layer over the native C++ object.
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Getting it back from Python is just a matter of peeling off the 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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.. rubric:: Supported automatic conversions
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.. toctree::
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:maxdepth: 1
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