std::istreambuf_iterator::operator++, operator++(int)

From cppreference.com
istreambuf_iterator& operator++();
(1)
/* unspecified */ operator++(int);
(2)

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.

Parameters

(none)

Return value

1) *this.
2) An unspecified struct proxy holding the current character obtained via operator*() for later retrieval. proxy is implicitly convertible to CharT.
proxy also holds a pointer to the stream buffer. istreambuf_iterator can be constructed from proxy, which is equivalent to constructing the istreambuf_iterator from the stream buffer pointer held by proxy.

Exceptions

(none)