Put the last character back to stream.
  Decrements the next-character pointer for the input buffer if c was the last character got from the stream.

Parameters.

c

The character to put back. Must have the same value as the last character extracted.

Return Value.
  The function returns *this

Example.

// istream putback
#include <iostream>
using namespace std;

int main () {
  char c;
  int n;
  char str[256];

  cout << "Enter a number or a word: ";
  c = cin.get();

  if ( (c >= '0') && (c <= '9') )
  {
    cin.putback (c);
    cin >> n;
    cout << "You have entered number " << n << endl;
  }
  else
  {
    cin.putback (c);
    cin >> str;
    cout << " You have entered word " << str << endl;
  }

  return 0;
}

Basic template member declaration ( basic_istream<charT,traits> ):

typedef charT char_type; basic_istream& putback ( char_type c );

See also.
  get, unget,
  istream class