From cppreference.com
basic_istream&putback(char_typech);Puts the character ch back to the input stream so the next extracted character will be ch.
First clears eofbit, then behaves as
. After constructing and checking the sentry object, if
is not null, calls rdbuf()->sputbackc(ch), which calls rdbuf()->pbackfail(ch) if ch does not equal the most recently extracted character.
If rdbuf() is null or if rdbuf->sputbackc(ch) returns Traits::eof(), calls setstate(badbit).
In any case, sets the
counter to zero.
Parameters
ch - character to put into input stream Return value
*this
Exceptions
if an error occurred (the error state flag is not
) and
is set to throw for that state.
If an internal operation throws an exception, it is caught and
is set. If
is set for badbit, the exception is rethrown.
Example
Demonstrates the difference between modifying and non-modifying putback().
Run this code
#include<iostream>#include<sstream>intmain(){std::stringstreams1("Hello, world");// IO streams1.get();if(s1.putback('Y'))// modifies the bufferstd::cout<<s1.rdbuf()<<'\n';elsestd::cout<<"putback failed\n";std::cout<<"--\n";std::istringstreams2("Hello, world");// input-only streams2.get();if(s2.putback('Y'))// cannot modify input-only bufferstd::cout<<s2.rdbuf()<<'\n';elsestd::cout<<"putback failed\n";s2.clear();std::cout<<"--\n";if(s2.putback('H'))// non-modifying putbackstd::cout<<s2.rdbuf()<<'\n';elsestd::cout<<"putback failed\n";}Output:
Yello, world -- putback failed -- Hello, world Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
(
) C++98 putback doesn't clear eofbitclears eofbit before doing anything else
C++98 sputbackc() was called without any argument called with chSee also
puts one character back in the input sequence
(public member function of std::basic_streambuf<CharT,Traits>)
unextracts a character
(public member function)
reads the next character without extracting it
(public member function)