cpp/io/basic streambuf/sungetc

If a putback position is available in the get area, then decrements the next pointer (gptr) and returns the character it now points to.

If a putback position is not available, then calls pbackfail to back up the input sequence if possible.

The I/O stream function is implemented in terms of this function.

Parameters
(none)

Return value
If putback position was available, returns the character that the next pointer is now pointing at, converted to with. The next single-character input from this streambuf will return this character.

If putback position was not available, returns what pbackfail returns, which is  on failure.