cpp/io/basic streambuf/sputbackc

Puts back a character back to the get area.

If a putback position is available in the get area, and the character is equal to the character one position to the left of gptr (as determined by , then simply decrements the next pointer (gptr).

Otherwise, calls to either back up the get area or to modify both the get area and possibly the associated character sequence.

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

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.