Commit 6ffbfc5e authored by scoder's avatar scoder Committed by GitHub

Merge pull request #2260 from gabrieldemarmiesse/transfered_type_casting

Transfered type casting
parents b74716a2 bebb9b54
...@@ -596,6 +596,52 @@ detect a problem that exists. Ultimately, you need to understand the issue and ...@@ -596,6 +596,52 @@ detect a problem that exists. Ultimately, you need to understand the issue and
be careful what you do. be careful what you do.
Type Casting
------------
Where C uses ``"("`` and ``")"``, Cython uses ``"<"`` and ``">"``. For example::
cdef char *p
cdef float *q
p = <char*>q
When casting a C value to a Python object type or vice versa,
Cython will attempt a coercion. Simple examples are casts like ``<int>pyobj``,
which converts a Python number to a plain C ``int`` value, or ``<bytes>charptr``,
which copies a C ``char*`` string into a new Python bytes object.
.. note:: Cython will not prevent a redundant cast, but emits a warning for it.
To get the address of some Python object, use a cast to a pointer type
like ``<void*>`` or ``<PyObject*>``.
You can also cast a C pointer back to a Python object reference
with ``<object>``, or a more specific builtin or extension type
(e.g. ``<MyExtType>ptr``). This will increase the reference count of
the object by one, i.e. the cast returns an owned reference.
Here is an example::
from __future__ import print_function
from cpython.ref cimport PyObject
from libc.stdint cimport uintptr_t
python_string = "foo"
cdef void* ptr = <void*>python_string
cdef uintptr_t adress_in_c = <uintptr_t>ptr
address_from_void = adress_in_c # address_from_void is a python int
cdef PyObject* ptr2 = <PyObject*>python_string
cdef uintptr_t address_in_c2 = <uintptr_t>ptr2
address_from_PyObject = address_in_c2 # address_from_PyObject is a python int
assert address_from_void == address_from_PyObject == id(python_string)
print(<object>ptr) # Prints "foo"
print(<object>ptr2) # prints "foo"
The precedence of ``<...>`` is such that ``<type>a.b.c`` is interpreted as ``<type>(a.b.c)``.
Checked Type Casts Checked Type Casts
------------------ ------------------
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