FeatureC++ is a C++ language extension that supports Feature-Oriented Programming (FOP). FOP studies feature modularity in program families. The idea of FOP is to build software by composing features. Features are basic building blocks that satisfy intuitive user-formulated requirements on the software system. Features refine other features incrementally. This step-wise refinement leads to a layered stack of features. Mixin Layers are one appropriate technique for implementing features.