Question

int main()
{
    int i;

    if (cin >> i)
    {
        //ok
    }
    else
    {
        //error
        cin.setstate(std::ios_base::goodbit);
    }
}

Why can’t I set goodbit through setstate() to clear out the failbit instead of cin.clear()?

Answer 1

Because setstate combines the current state with whatever state you’re passing it with a bitwise OR, ergo the fail bit doesn’t get cleared (set to zero).

So assume:

Assume a very simple state mechanism:
00
^^
||
|\
| the fail bit
 \
   the ok bit

Doing setstate(okbit) when your state is 01 is just going to give you 11 (look ma, the fail bit is still set) so really all you’re doing is screwing up the internal stream state. You should really avoid using setstate altogether.

Click for reference.