std::basic_istream<CharT,Traits>:: putback

Puts the character ch back to the input stream so the next extracted character will be ch .

First clears eofbit , then behaves as UnformattedInputFunction . After constructing and checking the sentry object, if rdbuf() 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 gcount() counter to zero.

Parameters

Return value

* this

Exceptions

If an internal operation throws an exception, it is caught and badbit is set. If exceptions() is set for badbit , the exception is rethrown.

Example

#include <iostream>
#include <sstream>
 
int main()
{
    std::stringstream s1("Hello, world"); // IO stream
    s1.get();
    if (s1.putback('Y')) // modifies the buffer
        std::cout << s1.rdbuf() << '\n';
    else
        std::cout << "putback failed\n";
 
    std::cout << "--\n";
 
    std::istringstream s2("Hello, world"); // input-only stream
    s2.get();
    if (s2.putback('Y')) // cannot modify input-only buffer
        std::cout << s2.rdbuf() << '\n';
    else
        std::cout << "putback failed\n"; 
    s2.clear();
 
    std::cout << "--\n";
 
    if (s2.putback('H')) // non-modifying putback
        std::cout << s2.rdbuf() << '\n';
    else
        std::cout << "putback failed\n";
}

Yello, world
--
putback failed
--
Hello, world

See also