Linear constraints in object-oriented finite element programming

One of the major reasons why finite element codes are difficult to keep clean and maintainable is that operations involving nodal information (and in particular, boundary conditions) show up in many sections of the programs and in many different contexts. The matter is further complicated by the inadequacy of the unstructured procedural programming approach promoted by the Fortran language. This article addresses the management of linear constraints, and does so from an object-oriented programming viewpoint. Emphasis is laid on both the model design and the practical programming details.