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

Return a reference to a rule.

template<
    class Rule>
constexpr implementation-defined
ref(
    Rule const& r);
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Return a reference to a character set.

template<
    class CharSet>
constexpr implementation-defined
ref(
    CharSet const& cs);
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