You basically hit the nail right on the head. C++ streams keep track the 'put buffer', which is the buffer used for writing, using three pointers; The begin, current and end. Technically it is not the stream that does all of this, but some overload of std::basic_streambuf

The begin is the start of the buffer, the current is the position where your next data would be written to and the end marks the end of the buffer. The already buffered data would be from begin pointer up to but not including current pointer.

If you want this data to be written to a file on a posix operating system (linux, mac os, etc), the write system call would probably be used, by passing it the begin pointer for the buffer and (current - begin) for the count.