...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Returns the remaining content length if known.
boost::optional< std::uint64_t > content_length_remaining() const;
If the message header specifies a Content-Length, the return value will be the number of bytes remaining in the payload body have not yet been parsed.
The return value is undefined unless is_header_done
would return true
.