Apart from enabling cross-language function calls, a fundamental problem that a binding tool like pybind11 must address is to provide access to native Python types in C++ and vice versa. There are three fundamentally different ways to do this—which approach is preferable for a particular type depends on the situation at hand.
Use a native C++ type everywhere. In this case, the type must be wrapped using pybind11-generated bindings so that Python can interact with it.
Use a native Python type everywhere. It will need to be wrapped so that C++ functions can interact with it.
Use a native C++ type on the C++ side and a native Python type on the Python side. pybind11 refers to this as a type conversion.
Type conversions are the most “natural” option in the sense that native (non-wrapped) types are used everywhere. The main downside is that a copy of the data must be made on every Python ↔ C++ transition: this is needed since the C++ and Python versions of the same type generally won’t have the same memory layout.
pybind11 can perform many kinds of conversions automatically. An overview is provided in the table “List of all builtin conversions”.
The following subsections discuss the differences between these options in more detail. The main focus in this section is on type conversions, which represent the last case of the above list.