mirror of
https://github.com/pybind/pybind11.git
synced 2024-12-01 17:37:15 +00:00
67 lines
2.5 KiB
ReStructuredText
67 lines
2.5 KiB
ReStructuredText
Frequently asked questions
|
|
##########################
|
|
|
|
(under construction)
|
|
|
|
Limitations involving reference arguments
|
|
=========================================
|
|
|
|
In C++, it's fairly common to pass arguments using mutable references or
|
|
mutable pointers, which allows both read and write access to the value
|
|
supplied by the caller. This is sometimes done for efficiency reasons, or to
|
|
realize functions that have multiple return values. Here are two very basic
|
|
examples:
|
|
|
|
.. code-block:: cpp
|
|
|
|
void increment(int &i) { i++; }
|
|
void increment_ptr(int *i) { (*i)++; }
|
|
|
|
In Python, all arguments are passed by reference, so there is no general
|
|
issue in binding such code from Python.
|
|
|
|
However, certain basic Python types (like ``str``, ``int``, ``bool``,
|
|
``float``, etc.) are **immutable**. This means that the following attempt
|
|
to port the function to Python doesn't have the same effect on the value
|
|
provided by the caller -- in fact, it does nothing at all.
|
|
|
|
.. code-block:: python
|
|
|
|
def increment(i):
|
|
i += 1 # nope..
|
|
|
|
pybind11 is also affected by such language-level conventions, which means that
|
|
binding ``increment`` or ``increment_ptr`` will also create Python functions
|
|
that don't modify their arguments.
|
|
|
|
Although inconvenient, one workaround is to encapsulate the immutable types in
|
|
a custom type that does allow modifications.
|
|
|
|
An other alternative involves binding a small wrapper lambda function that
|
|
returns a tuple with all output arguments (see the remainder of the
|
|
documentation for examples on binding lambda functions). An example:
|
|
|
|
.. code-block:: cpp
|
|
|
|
int foo(int &i) { i++; return 123; }
|
|
|
|
and the binding code
|
|
|
|
.. code-block:: cpp
|
|
|
|
m.def("foo", [](int i) { int rv = foo(i); return std::make_tuple(rv, i); });
|
|
|
|
Working with ancient Visual Studio 2009 builds on Windows
|
|
=========================================================
|
|
|
|
The official Windows distributions of Python are compiled using truly
|
|
ancient versions of Visual Studio that lack good C++11 support. Some users
|
|
implicitly assume that it would be impossible to load a plugin built with
|
|
Visual Studio 2015 into a Python distribution that was compiled using Visual
|
|
Studio 2009. However, no such issue exists: it's perfectly legitimate to
|
|
interface DLLs that are built with different compilers and/or C libraries.
|
|
Common gotchas to watch out for involve not ``free()``-ing memory region
|
|
that that were ``malloc()``-ed in another shared library, using data
|
|
structures with incompatible ABIs, and so on. pybind11 is very careful not
|
|
to make these types of mistakes.
|