The HepMC package is an object oriented event record written in C++
for High Energy Physics Monte Carlo Generators. Many extensions from
HEPEVT, the Fortran HEP standard, are supported: the number of entries
is unlimited, spin density matrices can be stored with each vertex,
flow patterns (such as color) can be stored and traced, integers
representing random number generator states can be stored, and an
arbitrary number of event weights can be included. Particles and
vertices are kept separate in a graph structure, physically similar to
a physics event. The added information supports the modularisation of
event generators. Event information is accessed by means of iterators
supplied with the package.