How does Boost.Python work?

Question

How is Python able to call C++ objects when the interpreter is C and has been built w/ a C compiler?

Answer 1

Boost.Python has special macros that declare functions with extern "C" so the Python interpreter will be able to call them. It's kind of complicated, but you can look at the Boost documentation for more info.

Answer 2

Python declares a C-API (see http://docs.python.org/2/c-api/ or http://docs.python.org/3/c-api/). This API defines a generic object type called PyObject which is just a normal C struct. This structure defines (nearly) everything a python object can do, e.g., what happens when do additions or comparisons on this object or simply call it like a function.

Because python types are also objects (and therefore are represented in C by a PyObject structure), defining a new type is a simple matter of defining a new PyObject struct like that one. When methods are called in Python, the interpreter forwards the call to C functions associated with this structure.

As long as a given (compiled) extension provides the correct entry points such that the Python interpreter can introspect it and find out what is available (the documentation I indicated above does explain this in details), then it can use these objects like any other object you normally have available at the prompt - which BTW, are constructed using the very same C-API. It suffices you import the compiled extension.

I hope it is somewhat clear how the Python interpreter calls stuff from compiled extensions from the above. The sole missing gap is how the C-API calls the C++ code.

Boost.Python does this by declaring C entry points in code along the lines as explained here: Elegantly call C++ from C. Every time you call, e.g., boost::python::class_, it does this for the type you declare to python, creating therefore a PyObject that represents your class, with the name you choose. As you call .def on this class you go filling in the internal slots of that structure, declaring more methods, operators and attributes of your new type. Each of these internal slots points to a C-style function that is nothing but a wrapper to the equivalent C++ call.

Answer 3

C++ can interoperate with C by extern "C" declarations.