Memory consumption
In general, a wrapped C++ object with a corresponding Python object is the size of:
- a new-style class (derived from 'object' in Python) instance plus
- the extra size required to allow variable-length data in the instance, plus
- the size of the C++ object, plus
the size of a vtable pointer, plus
- a pointer to the C++ object's instanceholder, plus
- zero or more bytes of padding required to ensure that the instanceholder is properly aligned.
You can see this in boost/python/object/instance.hpp. Most Python objects are represented by instance<value_holder<T> >, for some C++ class T.
All the code for implementing C++ object wrappers is in libs/python/src/object/class.cpp. Instance dictionaries are created only "on demand", the first time the instance's dict attribute is accessed (see instance_get_dict).
If your C++ data structure contains pointers or smart pointers, you can arrange for Python objects to be created which only embed those pointers (instance<pointer_holder<Ptr> >). These Python objects will be in existence only as long as your Python code holds a reference to them.