Gecode features a generic kernel on top of which modules such as search, finite domain integer constraints, and so on are provided.
Kernel
Gecode's kernel provides a comprehensive programming interface to construct new variable domains (including propagators as implementations of constraints and branchings) and search engines. It is slim (around 1700 lines of code) and requires no modification or hacking for adding new variable domains or search engines.
Search
Search in Gecode is based on recomputation and copying. Advanced techniques include adaptive (speeds up further search) and batch recomputation (drastically reduces propagation during recomputation). Currently, Gecode supports search for some solutions, optimization (branch-and-bound), and limited discrepancy search (more details). Parallel thread-based search is currently under development.
Finite domain constraints
Gecode comes with finite domain constraints implemented on top of the generic kernel. It offers standard constraints such as arithmetics, Boolean, linear equations, and global constraints such as: distinct (alldifferent, both bounds and domain consistent), count (global cardinality, both bounds and domain consistent), element, cumulatives, table and regular as extensional constraints, sorted, circuit, channel, and lex (more details). It is simple to add new constraints and branchings (in particular as the complete source code with the above constraints as examples is available).
Finite set constraints
Gecode also provides finite integer set variables. The standard set relations and operations are available as constraints, plus some specialized constraints such as convexity, global reasoning for distinctness of sets, selection constraints, weighted sets, and constraints connecting finite domain and finite set variables (more details). As for the finite domain constraints, the library can be easily extended with new constraints and branchings.
As a more experimental (but fully functional) feature, Gecode supports complete set variables and constraints over them, where the complete set variable domains are represented by Boolean decision diagrams (BDDs).
Basic modeling support
Even though Gecode has been designed to be easy to interface to and not to be easy to model with, it comes with some basic modeling support. This supports regular expressions for extensional constraints and expressing linear and Boolean constraints in the standard way as expressions build from numbers and operators (more details).