6.5.1 GiNaC's run-time type information system

All algebraic classes (that is, all classes that can appear in expressions) in GiNaC are direct or indirect subclasses of the class basic. So a basic * (which is essentially what an ex is) represents a generic pointer to an algebraic class. Occasionally it is necessary to find out what the class of an object pointed to by a basic * really is. Also, for the unarchiving of expressions it must be possible to find the unarchive() function of a class given the class name (as a string). A system that provides this kind of information is called a run-time type information (RTTI) system. The C++ language provides such a thing (see the standard header file ‘<typeinfo>’) but for efficiency reasons GiNaC implements its own, simpler RTTI.

The RTTI in GiNaC is based on two mechanisms:

• The basic class declares a member variable tinfo_key which holds a variable of tinfo_t type (which is actually just const void*) that identifies the object's class.
• By means of some clever tricks with static members, GiNaC maintains a list of information for all classes derived from basic. The information available includes the class names, the tinfo_keys, and pointers to the unarchiving functions. This class registry is defined in the ‘registrar.h’ header file.

The disadvantage of this proprietary RTTI implementation is that there's a little more to do when implementing new classes (C++'s RTTI works more or less automatically) but don't worry, most of the work is simplified by macros.