C++: how do I remove bad input from cin?

Question

I have done some cin operation and before I use cin I want to clear its state and flush it. How do I do it? I know cin.clear() clears the error state but to flush the cin buffer how do I check if it's empty and if not which of the below statements shall I use to empty it so that I can safely use cin later?

cin.ignore(std::numeric_limits<std::streamsize>::max()); 

or

cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); 

Answer 1

It seems, you detected a problem and you want to get rid of erronous input, e.g., the entered line. You could try to get rid what std::cin has in its buffer but generally, this doesn't work too well as there is no guarantee that the stream has read, e.g., a complete line. Also, to make it potentially useful you'll need to make sure that stdin and std::cin are not synchronized as otherwise std::cin will never buffer any characters anyway:

std::ios_base::sync_with_stdio(false);
// read data using std::cin

if (!std::cin) {
    // clean-up
    std::cin.clear();
    if (std::cin) {
        std::cin.ignore(std::cin.rdbuf()->in_avail()); // ignore what is buffered
    }
}

Whether in_avail() will return a non-zero value depends entirely on the used stream buffer and it may ignore more than you actually want to ignore anyway. For example, if std::cin's stream buffer is replaced to use the stream buffer of an std::istringstream it will probably consume all the character from this source.

Probably the most sensible clean-up is to ignore() the next character or the characters on the current line, using either

std::cin.ignore();

or

std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

The max() of std::streamsize is used as a magical number to indicate that the count should not be considered a termination criteria. If you don't pass a termination character the stream will ignore characters until the end of the stream which is probably not what you want.