MFC Programmer's SourceBook : Thinking in C++
Bruce Eckel's Thinking in C++, 2nd Ed Contents | Prev | Next

Summary

RTTI is a convenient extra feature, a bit of icing on the cake. Although normally you upcast a pointer to a base class and then use the generic interface of that base class (via virtual functions), occasionally you get into a corner where things can be more effective if you know the exact type of the object pointed to by the base pointer, and that’s what RTTI provides. Because some form of virtual-function-based RTTI has appeared in almost all class libraries, this is a useful feature because it means

  1. You don’t have to build it into your own libraries.
  2. You don’t have to worry whether it will be built into someone else’s library.
  3. You don’t have the extra programming overhead of maintaining an RTTI scheme during inheritance.
  4. The syntax is consistent, so you don’t have to figure out a new one for each library.
While RTTI is a convenience, like most features in C++ it can be misused by either a naive or determined programmer. The most common misuse may come from the programmer who doesn’t understand virtual functions and uses RTTI to do type-check coding instead. The philosophy of C++ seems to be to provide you with powerful tools and guard for type violations and integrity, but if you want to deliberately misuse or get around a language feature, there’s nothing to stop you. Sometimes a slight burn is the fastest way to gain experience.

The explicit cast syntax will be a big help during debugging because casting opens a hole into your type system and allows errors to slip in. The explicit cast syntax will allow you to more easily locate these error entryways.

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