Question

istream &func(istream &in)
{
    string data;

    while (in >> data, !in.eof()) {
        if (in.bad())
            throw runtime_error("IO stream corrupted");
        if (in.fail()) {
            cerr << "bad data, try again" << endl;
            in.clear();
            in.ignore(200);
            continue;
        }   

        cout << data << endl;;
    }   

    //in.clear(istream::eofbit | istream::failbit);
    in.clear();

    return in; 
}

why in.clear(istream::eofbit | istream::failbit); can not reset the cin?

but in.clear can make it.

PS: I use this function in main(), and use cin as its parameter.

Answer 1

clear is defined like this:

void clear(iostate state = goodbit);

So, effectively, in.clear(); is doing this: in.clear(istream::goodbit); which resets the stream. Calling in.clear(istream::eofbit | istream::failbit); would set both the eofbit and failbit, I doubt that is what you want.

std::cin.clear(std::istream::eofbit | std::istream::failbit);
std::ios_base::iostate state = std::cin.rdstate();
if( state == (std::istream::eofbit | std::istream::failbit))
{
    std::cout << "eofbit and failbit set" << std::endl;
}