Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Lexicographic comparison manipulator implementation.

namespace boost {
  namespace test_tools {
    struct lexicographic;
    int operator<<(unit_test::lazy_ostream const &, lexicographic);
  }
}