Base class for all errors
Base class for all objects that need RTTI.
Each derived class must provide a unique string as its type. The best
way to ensure uniqueness is to build the string "<class name>::<owner
name>". To help avoid typos, define this string as a static class
constant so clients can use this constant instead of the actual string.
If they mistype the constant, the compiler will complain. If they
mistype the string, it will be a very subtle bug. In addition, using
the constant makes Is() much faster.
This was built long before compiler support for RTTI support was
stable. We could replace it with std::type_info, but since Is()
already does a pointer comparison, there is no speed benefit, and the
reduction in static data would be negligible.
