/* unspecified */ operator++(int);
Advances the iterator by calling sbuf_->sbumpc() where
sbuf_ is the stored pointer to the stream buffer.
The behavior is undefined if the iterator is end-of-stream iterator.
 Return value
2) An unspecified struct
proxyholding the current character obtained via operator*() for later retrieval.
proxyis implicitly convertible to
proxyalso holds a pointer to the stream buffer.
istreambuf_iteratorcan be constructed from
proxy, which is equivalent to constructing the
istreambuf_iteratorfrom the stream buffer pointer held by