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

From cppreference.com
C++
Input/output library
std::basic_istream
basic_istream & unget ( ) ;

Makes the most recently extracted character available again.

First, clears eofbit . Then, (since C++11) unget behaves as UnformattedInputFunction . After constructing and checking the sentry object, if any ios_base::iostate flags are set, the function sets failbit and returns. Otherwise, calls rdbuf ( ) - > sungetc ( ) .

If rdbuf ( ) - > sungetc ( ) returns Traits :: eof ( ) , calls setstate ( badbit ) .

In any case, sets the gcount() counter to zero.

Parameters

(none)

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

Run this code 
#include <iostream>
#include <sstream>
 
int main()
{
    std::istringstream s1("Hello, world.");
    char c1 = s1.get();
    if (s1.unget())
    {
        char c2 = s1.get();
        std::cout << "Got: '" << c1 << "'. Got again: '" << c2 << "'.\n";
    }
}

Output: 

Got: 'H'. Got again: 'H'.

See also

moves the next pointer in the input sequence back by one
(public member function of std::basic_streambuf<CharT,Traits> )
extracts characters
(public member function)
reads the next character without extracting it
(public member function)
puts a character into input stream
(public member function)
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