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