Resetting the State of a Stream

Question

I have a question which is slightly similar to this question on stackoverflow std::cin.clear() fails to restore input stream in a good state, but the answer provided there does not work for me.

The question is: how can I reset the state of a stream to 'good' again?

Here is my code how I try it, but the state is never set to good again. I used both of the lines ignore separately.

int _tmain(int argc, _TCHAR* argv[])
{
    int result;
    while ( std::cin.good() )
    {
        std::cout << "Choose a number: ";
        std::cin >> result;

        // Check if input is valid
        if (std::cin.bad())
        {
            throw std::runtime_error("IO stream corrupted");
        }
        else if (std::cin.fail())
        {
            std::cerr << "Invalid input: input must be a number." << std::endl;
            std::cin.clear(std::istream::failbit);
            std::cin.ignore();
            std::cin.ignore(INT_MAX,'\n');
            continue;
        }
        else
        {
            std::cout << "You input the number: " << result << std::endl;
        }
    }
    return 0;
}

Solution

The code here

std::cin.clear(std::istream::failbit);

doesn't actually clear the failbit, it replaces the current state of the stream with failbit.

To clear all the bits, just call clear().

---

The description in the standard is a bit convoluted, stated as the result of other functions

void clear(iostate state = goodbit);

Postcondition: If rdbuf()!=0 then state == rdstate(); otherwise rdstate()==(state | ios_base::badbit).

Which basically means that the next call to rdstate() will return the value passed to clear(). Except when there are some other problems, in which case you might get a badbit as well.

Also, goodbit actually isn't a bit at all, but has the value zero to clear out all the other bits.

To clear just the one specific bit, you can use this call

cin.clear(cin.rdstate() & ~ios::failbit);

However, if you clear one flag and others remain, you still cannot read from the stream. So this use is rather limited.

Comments

[physicalattraction]

In my book it says: clear(flag) sets the specified condition state to valid. I interpreted this as: clears the specified error bit. This is incorrect then?

[Bo Persson]

It is just clear() to clear all flags. To clear a single flag is a bit more complicated (and not too useful). I have added a new part to my answer.