In this paper, we present techniques that exploit two design 
patterns, the Visitor pattern and the Decorator pattern, to 
automatically validate invariants about the data attributes 
in a C++ class.  To investigate the pragmatics involved in 
using the two patterns, we perform a study of an existing, 
well-tested application, keystone, a parser and front-end for
the C++ language.  We show that these two patterns provide 
flexibility in terms of the frequency and level of granularity 
of validation of the class invariants, which are expressed in 
the Object Constraint Language, OCL.  Our quantitative results 
measure the impact of these approaches and the additional faults 
uncovered through validation of our case study.