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 Frequently asked questions
 ##########################
 
 "ImportError: dynamic module does not define init function"
 ===========================================================
 
 1. Make sure that the name specified in PYBIND11_MODULE is identical to the
 filename of the extension library (without suffixes such as ``.so``).
 
 2. If the above did not fix the issue, you are likely using an incompatible
 version of Python that does not match what you compiled with.
 
 "Symbol not found: ``__Py_ZeroStruct`` / ``_PyInstanceMethod_Type``"
 ========================================================================
 
 See the first answer.
 
 "SystemError: dynamic module not initialized properly"
 ======================================================
 
 See the first answer.
 
 The Python interpreter immediately crashes when importing my module
 ===================================================================
 
 See the first answer.
 
 .. _faq_reference_arguments:
 
 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); });
 
 
 How can I reduce the build time?
 ================================
 
 It's good practice to split binding code over multiple files, as in the
 following example:
 
 :file:`example.cpp`:
 
 .. code-block:: cpp
 
     void init_ex1(py::module_ &);
     void init_ex2(py::module_ &);
     /* ... */
 
     PYBIND11_MODULE(example, m) {
         init_ex1(m);
         init_ex2(m);
         /* ... */
     }
 
 :file:`ex1.cpp`:
 
 .. code-block:: cpp
 
     void init_ex1(py::module_ &m) {
         m.def("add", [](int a, int b) { return a + b; });
     }
 
 :file:`ex2.cpp`:
 
 .. code-block:: cpp
 
     void init_ex2(py::module_ &m) {
         m.def("sub", [](int a, int b) { return a - b; });
     }
 
 :command:`python`:
 
 .. code-block:: pycon
 
     >>> import example
     >>> example.add(1, 2)
     3
     >>> example.sub(1, 1)
     0
 
 As shown above, the various ``init_ex`` functions should be contained in
 separate files that can be compiled independently from one another, and then
 linked together into the same final shared object.  Following this approach
 will:
 
 1. reduce memory requirements per compilation unit.
 
 2. enable parallel builds (if desired).
 
 3. allow for faster incremental builds. For instance, when a single class
    definition is changed, only a subset of the binding code will generally need
    to be recompiled.
 
 "recursive template instantiation exceeded maximum depth of 256"
 ================================================================
 
 If you receive an error about excessive recursive template evaluation, try
 specifying a larger value, e.g. ``-ftemplate-depth=1024`` on GCC/Clang. The
 culprit is generally the generation of function signatures at compile time
 using C++14 template metaprogramming.
 
 .. _`faq:hidden_visibility`:
 
 "'SomeClass' declared with greater visibility than the type of its field 'SomeClass::member' [-Wattributes]"
 ============================================================================================================
 
 This error typically indicates that you are compiling without the required
 ``-fvisibility`` flag.  pybind11 code internally forces hidden visibility on
 all internal code, but if non-hidden (and thus *exported*) code attempts to
 include a pybind type (for example, ``py::object`` or ``py::list``) you can run
 into this warning.
 
 To avoid it, make sure you are specifying ``-fvisibility=hidden`` when
 compiling pybind code.
 
 As to why ``-fvisibility=hidden`` is necessary, because pybind modules could
 have been compiled under different versions of pybind itself, it is also
 important that the symbols defined in one module do not clash with the
 potentially-incompatible symbols defined in another.  While Python extension
 modules are usually loaded with localized symbols (under POSIX systems
 typically using ``dlopen`` with the ``RTLD_LOCAL`` flag), this Python default
 can be changed, but even if it isn't it is not always enough to guarantee
 complete independence of the symbols involved when not using
 ``-fvisibility=hidden``.
 
 Additionally, ``-fvisibility=hidden`` can deliver considerably binary size
 savings. (See the following section for more details.)
 
 
 .. _`faq:symhidden`:
 
 How can I create smaller binaries?
 ==================================
 
 To do its job, pybind11 extensively relies on a programming technique known as
 *template metaprogramming*, which is a way of performing computation at compile
 time using type information. Template metaprogramming usually instantiates code
 involving significant numbers of deeply nested types that are either completely
 removed or reduced to just a few instructions during the compiler's optimization
 phase. However, due to the nested nature of these types, the resulting symbol
 names in the compiled extension library can be extremely long. For instance,
 the included test suite contains the following symbol:
 
 .. only:: html
 
     .. code-block:: none
 
         _​_​Z​N​8​p​y​b​i​n​d​1​1​1​2​c​p​p​_​f​u​n​c​t​i​o​n​C​1​I​v​8​E​x​a​m​p​l​e​2​J​R​N​S​t​3​_​_​1​6​v​e​c​t​o​r​I​N​S​3​_​1​2​b​a​s​i​c​_​s​t​r​i​n​g​I​w​N​S​3​_​1​1​c​h​a​r​_​t​r​a​i​t​s​I​w​E​E​N​S​3​_​9​a​l​l​o​c​a​t​o​r​I​w​E​E​E​E​N​S​8​_​I​S​A​_​E​E​E​E​E​J​N​S​_​4​n​a​m​e​E​N​S​_​7​s​i​b​l​i​n​g​E​N​S​_​9​i​s​_​m​e​t​h​o​d​E​A​2​8​_​c​E​E​E​M​T​0​_​F​T​_​D​p​T​1​_​E​D​p​R​K​T​2​_
 
 .. only:: not html
 
     .. code-block:: cpp
 
         __ZN8pybind1112cpp_functionC1Iv8Example2JRNSt3__16vectorINS3_12basic_stringIwNS3_11char_traitsIwEENS3_9allocatorIwEEEENS8_ISA_EEEEEJNS_4nameENS_7siblingENS_9is_methodEA28_cEEEMT0_FT_DpT1_EDpRKT2_
 
 which is the mangled form of the following function type:
 
 .. code-block:: cpp
 
     pybind11::cpp_function::cpp_function<void, Example2, std::__1::vector<std::__1::basic_string<wchar_t, std::__1::char_traits<wchar_t>, std::__1::allocator<wchar_t> >, std::__1::allocator<std::__1::basic_string<wchar_t, std::__1::char_traits<wchar_t>, std::__1::allocator<wchar_t> > > >&, pybind11::name, pybind11::sibling, pybind11::is_method, char [28]>(void (Example2::*)(std::__1::vector<std::__1::basic_string<wchar_t, std::__1::char_traits<wchar_t>, std::__1::allocator<wchar_t> >, std::__1::allocator<std::__1::basic_string<wchar_t, std::__1::char_traits<wchar_t>, std::__1::allocator<wchar_t> > > >&), pybind11::name const&, pybind11::sibling const&, pybind11::is_method const&, char const (&) [28])
 
 The memory needed to store just the mangled name of this function (196 bytes)
 is larger than the actual piece of code (111 bytes) it represents! On the other
 hand, it's silly to even give this function a name -- after all, it's just a
 tiny cog in a bigger piece of machinery that is not exposed to the outside
 world. So we'll generally only want to export symbols for those functions which
 are actually called from the outside.
 
 This can be achieved by specifying the parameter ``-fvisibility=hidden`` to GCC
 and Clang, which sets the default symbol visibility to *hidden*, which has a
 tremendous impact on the final binary size of the resulting extension library.
 (On Visual Studio, symbols are already hidden by default, so nothing needs to
 be done there.)
 
 In addition to decreasing binary size, ``-fvisibility=hidden`` also avoids
 potential serious issues when loading multiple modules and is required for
 proper pybind operation.  See the previous FAQ entry for more details.
 
 How can I properly handle Ctrl-C in long-running functions?
 ===========================================================
 
 Ctrl-C is received by the Python interpreter, and holds it until the GIL
 is released, so a long-running function won't be interrupted.
 
 To interrupt from inside your function, you can use the ``PyErr_CheckSignals()``
 function, that will tell if a signal has been raised on the Python side.  This
 function merely checks a flag, so its impact is negligible. When a signal has
 been received, you must either explicitly interrupt execution by throwing
 ``py::error_already_set`` (which will propagate the existing
 ``KeyboardInterrupt``), or clear the error (which you usually will not want):
 
 .. code-block:: cpp
 
     PYBIND11_MODULE(example, m)
     {
         m.def("long running_func", []()
         {
             for (;;) {
                 if (PyErr_CheckSignals() != 0)
                     throw py::error_already_set();
                 // Long running iteration
             }
         });
     }
 
 CMake doesn't detect the right Python version
 =============================================
 
 The CMake-based build system will try to automatically detect the installed
 version of Python and link against that. When this fails, or when there are
 multiple versions of Python and it finds the wrong one, delete
 ``CMakeCache.txt`` and then add ``-DPYTHON_EXECUTABLE=$(which python)`` to your
 CMake configure line. (Replace ``$(which python)`` with a path to python if
 your prefer.)
 
 You can alternatively try ``-DPYBIND11_FINDPYTHON=ON``, which will activate the
 new CMake FindPython support instead of pybind11's custom search. Requires
 CMake 3.12+, and 3.15+ or 3.18.2+ are even better. You can set this in your
 ``CMakeLists.txt`` before adding or finding pybind11, as well.
 
 Inconsistent detection of Python version in CMake and pybind11
 ==============================================================
 
 The functions ``find_package(PythonInterp)`` and ``find_package(PythonLibs)``
 provided by CMake for Python version detection are modified by pybind11 due to
 unreliability and limitations that make them unsuitable for pybind11's needs.
 Instead pybind11 provides its own, more reliable Python detection CMake code.
 Conflicts can arise, however, when using pybind11 in a project that *also* uses
 the CMake Python detection in a system with several Python versions installed.
 
 This difference may cause inconsistencies and errors if *both* mechanisms are
 used in the same project.
 
 There are three possible solutions:
 
 1. Avoid using ``find_package(PythonInterp)`` and ``find_package(PythonLibs)``
    from CMake and rely on pybind11 in detecting Python version. If this is not
    possible, the CMake machinery should be called *before* including pybind11.
 2. Set ``PYBIND11_FINDPYTHON`` to ``True`` or use ``find_package(Python
    COMPONENTS Interpreter Development)`` on modern CMake (3.12+, 3.15+ better,
    3.18.2+ best). Pybind11 in these cases uses the new CMake FindPython instead
    of the old, deprecated search tools, and these modules are much better at
    finding the correct Python.
 3. Set ``PYBIND11_NOPYTHON`` to ``TRUE``. Pybind11 will not search for Python.
    However, you will have to use the target-based system, and do more setup
    yourself, because it does not know about or include things that depend on
    Python, like ``pybind11_add_module``. This might be ideal for integrating
    into an existing system, like scikit-build's Python helpers.
 
 How to cite this project?
 =========================
 
 We suggest the following BibTeX template to cite pybind11 in scientific
 discourse:
 
 .. code-block:: bash
 
     @misc{pybind11,
        author = {Wenzel Jakob and Jason Rhinelander and Dean Moldovan},
        year = {2017},
        note = {https://github.com/pybind/pybind11},
        title = {pybind11 -- Seamless operability between C++11 and Python}
     }

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