TOMS515, a C++ code which lists the subsets of size K selected from a set of size N.
The text of many ACM TOMS algorithms is available online through ACM: http://calgo.acm.org/ or NETLIB: http://www.netlib.org/toms/index.html.
The computer code and data files made available on this web page are distributed under the GNU LGPL license.
TOMS515 is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
SUBSET, a C++ library which enumerates combinations, partitions, subsets, index sets, and other combinatorial objects.