In this paper, we describe an approach to query-based
debugging for C++ programs that uses queries
formulated in the object constraint language, OCL.
By using OCL, together with UML, to formulate queries,
debugging can begin early in the design phase.
Queries can be formulated to verify design constraints
such as invariants on classes and pre and post-conditions
for member functions. These queries can be
reused after code generation to verify the design contract,
as part of the testing process, and to facilitate fault
detection.