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94 lines
3.3 KiB
ReStructuredText
94 lines
3.3 KiB
ReStructuredText
Custom type casters
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===================
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In very rare cases, applications may require custom type casters that cannot be
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expressed using the abstractions provided by pybind11, thus requiring raw
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Python C API calls. This is fairly advanced usage and should only be pursued by
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experts who are familiar with the intricacies of Python reference counting.
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The following snippets demonstrate how this works for a very simple ``inty``
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type that that should be convertible from Python types that provide a
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``__int__(self)`` method.
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.. code-block:: cpp
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struct inty { long long_value; };
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void print(inty s) {
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std::cout << s.long_value << std::endl;
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}
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The following Python snippet demonstrates the intended usage from the Python side:
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.. code-block:: python
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class A:
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def __int__(self):
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return 123
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from example import print
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print(A())
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To register the necessary conversion routines, it is necessary to add an
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instantiation of the ``pybind11::detail::type_caster<T>`` template.
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Although this is an implementation detail, adding an instantiation of this
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type is explicitly allowed.
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.. code-block:: cpp
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namespace PYBIND11_NAMESPACE { namespace detail {
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template <> struct type_caster<inty> {
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public:
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/**
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* This macro establishes the name 'inty' in
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* function signatures and declares a local variable
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* 'value' of type inty
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*/
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PYBIND11_TYPE_CASTER(inty, const_name("inty"));
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/**
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* Conversion part 1 (Python->C++): convert a PyObject into a inty
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* instance or return false upon failure. The second argument
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* indicates whether implicit conversions should be applied.
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*/
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bool load(handle src, bool) {
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/* Extract PyObject from handle */
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PyObject *source = src.ptr();
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/* Try converting into a Python integer value */
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PyObject *tmp = PyNumber_Long(source);
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if (!tmp)
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return false;
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/* Now try to convert into a C++ int */
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value.long_value = PyLong_AsLong(tmp);
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Py_DECREF(tmp);
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/* Ensure return code was OK (to avoid out-of-range errors etc) */
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return !(value.long_value == -1 && !PyErr_Occurred());
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}
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/**
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* Conversion part 2 (C++ -> Python): convert an inty instance into
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* a Python object. The second and third arguments are used to
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* indicate the return value policy and parent object (for
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* ``return_value_policy::reference_internal``) and are generally
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* ignored by implicit casters.
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*/
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static handle cast(inty src, return_value_policy /* policy */, handle /* parent */) {
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return PyLong_FromLong(src.long_value);
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}
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};
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}} // namespace PYBIND11_NAMESPACE::detail
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.. note::
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A ``type_caster<T>`` defined with ``PYBIND11_TYPE_CASTER(T, ...)`` requires
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that ``T`` is default-constructible (``value`` is first default constructed
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and then ``load()`` assigns to it).
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.. warning::
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When using custom type casters, it's important to declare them consistently
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in every compilation unit of the Python extension module. Otherwise,
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undefined behavior can ensue.
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