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

This is an older version of Boost and was released in 2013. The current version is 1.89.0.

buffered_stream::async_fill

Start an asynchronous fill.

template<
    typename ReadHandler>
void async_fill(
    ReadHandler handler);