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Showing content from https://en.cppreference.com/w/cpp/language/../io/basic_streambuf/sungetc.html below:

std::basic_streambuf<CharT,Traits>::sungetc - cppreference.com

If a putback position is available in the get area (gptr() > eback()), then decrements the next pointer (gptr()) and returns the character it now points to.

If a putback position is not available, then calls pbackfail() to back up the input sequence if possible.

The I/O stream function basic_istream::unget is implemented in terms of this function.

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If putback position was available, returns the character that the next pointer is now pointing at, converted to int_type with Traits::to_int_type(*gptr()). The next single-character input from this streambuf will return this character.

If putback position was not available, returns what pbackfail() returns, which is Traits::eof() on failure.

#include <iostream>
#include <sstream>
 
int main()
{
    std::stringstream s("abcdef"); // gptr() points to 'a'
    char c1 = s.get(); // c = 'a', gptr() now points to 'b' 
    char c2 = s.rdbuf()->sungetc(); // same as s.unget(): gptr() points to 'a' again 
    char c3 = s.get(); // c3 = 'a', gptr() now points to 'b'
    char c4 = s.get(); // c4 = 'b', gptr() now points to 'c'
    std::cout << c1 << c2 << c3 << c4 << '\n';
 
    s.rdbuf()->sungetc();  // back to 'b'
    s.rdbuf()->sungetc();  // back to 'a'
    int eof = s.rdbuf()->sungetc();  // nothing to unget: pbackfail() fails
    if (eof == EOF)
            std::cout << "Nothing to unget after 'a'\n";
}

Output:

aaab
Nothing to unget after 'a'

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