std::basic_istream::putback

 

C++

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std::basic_istream

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operator>>(std::basic_istream)

 

basic_istream& putback( char_type ch );

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

ch

-

character to put into input stream

Return value

*this

Exceptions

failure if an error occurred (the error state flag is not goodbit) and exceptions() is set to throw for that state.

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

demonstrates the difference between modifying and non-modifying putback()

Run this code

#include <sstream>
#include <iostream>
 
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::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();
    if (s2.putback('H')) // non-modifying putback
        std::cout << s2.rdbuf() << '\n';
    else
        std::cout << "putback failed\n";
}

Output:

Yello, world
putback failed
Hello, world

See also

sputbackc	puts one character back in the input sequence (public member function of std::basic_streambuf)
unget	unextracts a character (public member function)
peek	reads the next character without extracting it (public member function)