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 1.91.0.

basic_streambuf::prepare

Get a list of buffers that represents the put area, with the given size.

mutable_buffers_type prepare(
    std::size_t size);