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C++ istream::unget() function

The C++ std::istream::unget() function is used to put back the last character extracted from an input stream, moving the streams internal position indicator one character back. This allows the character to be read again in subsequent input operations.

Following is the syntax for std::istream::unget() function.

istream& unget();

It does not accept any parameter.

This function returns the basic_istream object (*this).

If an exception is thrown, the object is in a valid state.

Modifies the stream object.

In the following example, we are going to consider the basic usage of the unget() function.

#include <iostream>
#include <sstream>
int main()
{
    std::istringstream x("Namaste");
    char a;
    x.get(a);
    std::cout << "Read: " << a << std::endl;
    x.unget();
    x.get(a);
    std::cout << "Read again: " << a << std::endl;
    return 0;
}

Output of the above code is as follows −

Read: N
Read again: N

Consider the following example, where we are going to use the unget() function after multiple reads.

#include <iostream>
#include <sstream>
int main()
{
    std::istringstream a("TutorialsPoint");
    char x;
    a.get(x);
    std::cout << "First Read: " << x << std::endl;
    a.get(x);
    std::cout << "Second Read: " << x << std::endl;
    a.unget();
    a.get(x);
    std::cout << "Third Read: " << x << std::endl;
    return 0;
}

Following is the output of the above code −

First Read: T
Second Read: u
Third Read: u

Let's look at the following example, where we are going to use the unget() function in a loop.

#include <iostream>
#include <sstream>
int main()
{
    std::istringstream a("1123245");
    char x;
    while (a.get(x)) {
        if (x == '4') {
            a.unget();
            break;
        }
        std::cout << x << " ";
    }
    std::cout << std::endl;
    a.get(x);
    std::cout << "Character put back: " << x << std::endl;
    return 0;
}

If we run the above code it will generate the following output −

1 1 2 3 2 
Character put back: 4

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