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# A Peek Under the Hood

Declaring a `class_builder<T>` causes the instantiation of an `extension_class<T>` to which it forwards all member function calls and which is doing most of the real work. `extension_class<T>` is a subclass of `PyTypeObject`, the `struct` which Python's 'C' API uses to describe a type. An instance of the `extension_class<>` becomes the Python type object corresponding to `hello::world`. When we add it to the module it goes into the module's dictionary to be looked up under the name "world".

BPL uses C++'s template argument deduction mechanism to determine the types of arguments to functions (except constructors, for which we must provide an argument list because they can't be named in C++). Then, it calls the appropriate overloaded functions `PyObject* to_python(`S`)` and S'`from_python(PyObject*, type<`S`>)` which convert between any C++ type S and a `PyObject*`, the type which represents a reference to any Python object in its 'C' API. The `extension_class<T>` template defines a whole raft of these conversions (for `T, T*, T&, std::auto_ptr<T>`, etc.), using the same inline friend function technique employed by the boost operators library.

Because the `to_python` and `from_python` functions for a user-defined class are defined by `extension_class<T>`, it is important that an instantiation of `extension_class<T>` is visible to any code which wraps a C++ function with a `T, T*, const T&`, etc. parameter or return value. In particular, you may want to create all of the classes at the top of your module's init function, then `def` the member functions later to avoid problems with inter-class dependencies.

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© Copyright David Abrahams 2000. Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.

Updated: Nov 26, 2000