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

Why EOF is an error

The end of a stream can cause read, async_read, read_until or async_read_until functions to violate their contract. E.g. a read of N bytes may finish early due to EOF.
An EOF error may be used to distinguish the end of a stream from a successful read of size 0.