RTOpPack: Extra C/C++ Code for Vector Reduction/Transformation Operators Version of the Day
Reduction/transformation operation class (struct).
Instantiations of this type are used as polymorphic objects for applying reduction/transformation operations on sub-vectors.
Strictly speaking, the class of the object is determined by the virtual function table that
vtbl points to while the specific object instance data is pointed to by
obj_data. This design allows complete polymorphic objects in C.