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

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


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


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.



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.


// 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";


  Nothing to unget after 'a'

See also

          puts one character back in the input sequence
sputbackc (public member function)
          unextracts a character
unget (public member function of std::basic_istream<CharT,Traits>)