mirror of https://github.com/pybind/pybind11.git
38 lines
1.3 KiB
ReStructuredText
38 lines
1.3 KiB
ReStructuredText
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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
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that binding ``increment`` or ``increment_ptr`` will also create Python functions that don't modify their arguments.
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Although inconvenient, one workaround is to encapsulate the immutable types
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in a custom type that does allow modifications.
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