Tag Archives: Boost.Python

FILE* and Boost.Python

One great feature of Boost.Python is the ability to write custom converters from Python types to C++ arguments. Most Boost.Python documentation, including the incredibly helpful post by misspent, show you how to write “rvalue” converters (e.g., pass-by-value or pass-by-const-reference).

However if you need to wrap a function which takes a FILE* argument the previous approach will not prove fruitful. But Boost.Python is clearly capable of handling a similar situation, namely the implicit conversion from a Python string type to a char* type. Since the conversion from the internal PyObject* to char* is likely to be done with the PyString_AsString function, I went off in search of that code.

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Calling Python from C++

In a previous post we already covered calling a C function from one Boost.Python module in another module.

What if instead we want to call a Python function directly from C? We’ll stick with the same Bird / Quacker metaphor as in the previous post, but we’ll now implement our duck class in PyDuck.py:

"""
PyDuck.py
"""

def quack():
    print "Python Quack"

Now how can we call Python from C++? It turns out that Boost.Python defines operator() for the object class, so calling Python is relatively easy.  But things become more difficult if we want to maintain a default implementation in C.

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Python Capsules

Python Capsules are useful for passing C pointers between different Python modules.  In particular, one can encapsulate a C function pointer in one module and unpack and call it in another module.

To begin, suppose we have two Boost.Python modules:

  1. Bird, which has methods setQuack() and callQuack().
  2. CppDuck, which has a method getQuack().

Our goal will be to get a function pointer with CppDuck.getQuack(), set it in another module in Bird.setQuack(), and finally call it with Bird.callQuack().

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