Function execute

include/pybind11/detail/init.h:434–460

    // class (i.e. not inherited), the alias factory when `self` is a Python-side subtype.
    template <typename Class, typename... Extra>
    void execute(Class &cl, const Extra &...extra) && {
        static_assert(Class::has_alias,
                      "The two-argument version of `py::init()` can "
                      "only be used if the class has an alias");
#if defined(PYBIND11_CPP14)
        cl.def(
            "__init__",
            [class_func = std::move(class_factory), alias_func = std::move(alias_factory)]
#else
        auto &class_func = class_factory;
        auto &alias_func = alias_factory;
        cl.def(
            "__init__",
            [class_func, alias_func]
#endif
            (value_and_holder &v_h, CArgs... args) {
                if (Py_TYPE(v_h.inst) == v_h.type->type) {
                    // If the instance type equals the registered type we don't have inheritance,
                    // so don't need the alias and can construct using the class function:
                    construct<Class>(v_h, class_func(std::forward<CArgs>(args)...), false);
                } else {
                    construct<Class>(v_h, alias_func(std::forward<CArgs>(args)...), true);
                }
            },
            is_new_style_constructor(),
            extra...);
    }
};

/// Set just the C++ state. Same as `__init__`.