16.2.9 Runtime Type Identification (RTTI)

Run-time Type Identification, or RTTI, is a mechanism for interrogating the type of an object at runtime. Such a mechanism is useful for avoiding the dreaded switch-on-type technique used before RTTI was incorporated into the language. Until recently, some C++ compilers did not support RTTI, so it is necessary to assume that it may not be widely available.

Switch-on-type involves giving all classes a method that returns a special type token that an object can use to discover its own type. For example:

 class Shape         {         public:           enum types { TYPE_CIRCLE, TYPE_SQUARE };           virtual enum types type () = 0;         };         class Circle: public Shape         {         public:          enum types type () { return TYPE_CIRCLE; }         };         class Square: public Shape         {         public:           enum types type () { return TYPE_SQUARE; }         }; 

Although switch-on-type is not elegant, RTTI isn't particularly object-oriented either. Given the limited number of times you ought to be using RTTI, the switch-on-type technique may be reasonable.

This document was generated by Gary V. Vaughan on May, 24 2001 using texi2html