oremanj fd9bc8f54d Add basic support for tag-based static polymorphism (#1326) ... * Add basic support for tag-based static polymorphism Sometimes it is possible to look at a C++ object and know what its dynamic type is, even if it doesn't use C++ polymorphism, because instances of the object and its subclasses conform to some other mechanism for being self-describing; for example, perhaps there's an enumerated "tag" or "kind" member in the base class that's always set to an indication of the correct type. This might be done for performance reasons, or to permit most-derived types to be trivially copyable. One of the most widely-known examples is in LLVM: https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html This PR permits pybind11 to be informed of such conventions via a new specializable detail::polymorphic_type_hook<> template, which generalizes the previous logic for determining the runtime type of an object based on C++ RTTI. Implementors provide a way to map from a base class object to a const std::type_info* for the dynamic type; pybind11 then uses this to ensure that casting a Base* to Python creates a Python object that knows it's wrapping the appropriate sort of Derived. There are a number of restrictions with this tag-based static polymorphism support compared to pybind11's existing support for built-in C++ polymorphism: - there is no support for this-pointer adjustment, so only single inheritance is permitted - there is no way to make C++ code call new Python-provided subclasses - when binding C++ classes that redefine a method in a subclass, the .def() must be repeated in the binding for Python to know about the update But these are not much of an issue in practice in many cases, the impact on the complexity of pybind11's innards is minimal and localized, and the support for automatic downcasting improves usability a great deal.		2018-04-14 02:13:10 +02:00
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pybind11	Add basic support for tag-based static polymorphism (#1326)	2018-04-14 02:13:10 +02:00