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. 
The package has been kept as simple as possible with minimal internal/external
dependencies. Event information is accessed by means of iterators supplied 
with the package. 

Reference: M. Dobbs and J.B. Hansen, Comput. Phys. Commun. 134 (2001) 41.
http://lcgapp.cern.ch/project/simu/HepMC/