AspectC++ is an aspect-oriented extension of C and C++ languages. It has a source-to-source compiler, which translates AspectC++ source code into compilable C++. The compiler is available under the GNU GPL, though some extensions specific to Microsoft Windows are only available through pure-systems GmbH. Aspect-oriented programming allows modularizing cross-cutting concerns in a single module, an aspect. Aspects can modify existing classes, but most commonly they provide 'advice' that runs before, after, or around existing functionality.