Using peek() and putback() with cin in C++

By: Henry in C++ Tutorials on 2007-09-17

The input object cin has two additional methods that can come in rather handy: peek(), which looks at but does not extract the next character, and putback(), which inserts a character into the input stream. Listing below illustrates how these might be used.

1:     // Using peek() and putback()
2:     #include <iostream.h>
3:
4:     int main()
5:     {
6:        char ch;
7:        cout << "enter a phrase: ";
8:        while ( cin.get(ch) )
9:        {
10:          if (ch == `!')
11:             cin.putback(`$');
12:          else
13:             cout << ch;
14:          while (cin.peek() == `#')
15:             cin.ignore(1,'#');
16:       }
17:     return 0;
18: }

Output: enter a phrase: Now!is#the!time#for!fun#!
Now$isthe$timefor$fun$

Analysis: On line 6, a character variable, ch, is declared, and on line 7, the user is prompted to enter a phrase. The purpose of this program is to turn any exclamation marks (!) into dollar signs ($) and to remove any pound symbols (#).

The program loops as long as it is getting characters other than the end of file (remember that cin.get() returns 0 for end of file). If the current character is an exclamation point, it is thrown away and the $ symbol is put back into the input buffer; it will be read the next time through. If the current item is not an exclamation point, it is printed. The next character is "peeked" at, and when pound symbols are found, they are removed.

This is not the most efficient way to do either of these things (and it won't find a pound symbol if it is the first character), but it does illustrate how these methods work. They are relatively obscure, so don't spend a lot of time worrying about when you might really use them. Put them into your bag of tricks; they'll come in handy sooner or later.