Create a new modifiable buffer that represents the given string.
template< typename Elem, typename Traits, typename Allocator> mutable_buffer buffer( std::basic_string< Elem, Traits, Allocator > & data);
mutable_buffer(data.size() ? &data[0] :
0, data.size() * sizeof(Elem))
.
The buffer is invalidated by any non-const operation called on the given string object.