In general the class variables and methods are defined by the kernel.
However, sig_info_base also includes a member variable
reader_pointer of type void* which references the
reader value object. It is used by the VHDL processes to read the
value of a signal. Note, the pointer is of type void* because
the actual signal type is unknown to the kernel at kernel compile
time. However, the derived sig_info<SignalType> class includes
a method to cast the reader value to the actual signal type.