std::basic_ios<CharT,Traits>::clear

void clear( std::ios_base::iostate state = std::ios_base::goodbit );

Sets the stream error state flags by assigning them the value of state. By default, assigns std::ios_base::goodbit which has the effect of clearing all error state flags.

If rdbuf() is a null pointer (i.e. there is no associated stream buffer), then state | std::ios_base::badbit is assigned.

Parameters

state

new error state flags setting. It can be a combination of the following constants:

Constant	Explanation
`goodbit`	no error
`badbit`	irrecoverable stream error
`failbit`	input/output operation failed (formatting or extraction error)
`eofbit`	associated input sequence has reached end-of-file

Return value

(none)

Exceptions

If the new error state includes a bit that is also included in the exceptions() mask, throws an exception of type failure.

Example

clear() without arguments can be used to unset the failbit after unexpected input; for std::cin.putback(c) see ungetc.

#include <iostream>
#include <string>

int main()
{
    for (char c : {'\n', '4', '1', '.', '3', '\n', 'Z', 'Y', 'X'})
        std::cin.putback(c); // emulate user's input (not portable: see ungetc Notes)

    double n;
    while (std::cout << "Please, enter a number: " && !(std::cin >> n))
    {
        std::cin.clear();
        std::string line;
        std::getline(std::cin, line);
        std::cout << line << "\nI am sorry, but '" << line << "' is not a number\n";
    }
    std::cout << n << "\nThank you for entering the number " << n << '\n';
}

Output:

Please, enter a number: XYZ
I am sorry, but 'XYZ' is not a number
Please, enter a number: 3.14
Thank you for entering the number 3.14

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 412	C++98	an excption would be thrown if the current error state includes a bit that is also included in the `exceptions()` mask	checks the new error state instead