Clear the contents.
Erases all characters from the string. After this call, size()
returns zero but capacity()
Linear in size().
All references, pointers, or iterators referring to contained elements
are invalidated. Any past-the-end iterators are also invalidated.
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)