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 true if ch is in the character set.

constexpr bool
operator()(
    unsigned char ch) const;
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Return true if ch is in the character set.

constexpr bool
operator()(
    char ch) const;
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