std::basic_streambuf<CharT,Traits>::sungetc (3) - Linux Manuals

std::basic_streambuf<CharT,Traits>::sungetc: std::basic_streambuf<CharT,Traits>::sungetc

NAME

std::basic_streambuf<CharT,Traits>::sungetc - std::basic_streambuf<CharT,Traits>::sungetc

Synopsis

int_type sungetc();

If a putback position is available in the get area (gptr() > eback()), 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 basic_istream::unget 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 int_type with Traits::to_int_type(*gptr()). The next single-character input from this streambuf will return this character.
If putback position was not available, returns what pbackfail() returns, which is Traits::eof() on failure.

Example

// Run this code

  #include <iostream>
  #include <sstream>

  int main()
  {
      std::stringstream s("abcdef"); // gptr() poitns to 'a'
      char c1 = s.get(); // c = 'a', gptr() now points to 'b'
      char c2 = s.rdbuf()->sungetc(); // same as s.unget(): gptr() points to 'a' again
      char c3 = s.get(); // c3 = 'a', gptr() now points to 'b'
      char c4 = s.get(); // c4 = 'b', gptr() now points to 'c'
      std::cout << c1 << c2 << c3 << c4 << '\n';

      s.rdbuf()->sungetc(); // back to 'b'
      s.rdbuf()->sungetc(); // back to 'a'
      int eof = s.rdbuf()->sungetc(); // nothing to unget: pbackfail() fails
      if (eof == EOF)
              std::cout << "Nothing to unget after 'a'\n";
  }

Output:

  aaab
  Nothing to unget after 'a'