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

Inequality operator.

constexpr bool operator!=(
    const any_completion_handler & a,
    nullptr_t );

Header: boost/asio/any_completion_handler.hpp

Convenience header: boost/asio.hpp