std::cin gets skipped over even after using std::cin.ignore()-CodePudding

I'm trying to find a way to keep asking for user input even after an invalid input is taken by using std::cin.fail(). However, when the input is invalid, the program simply runs in a loop and does not pause on std::cin even with std::cin.ignore().

I tried moving std::cin.ignore() to before the std::cin >> input and although it does work, the program first stops at the std::cin.ignore() line (requiring an input first that won't be used) before going to the std::cin input line which is not what I want(I want it to immediately ask for input).

Is there a reason why and how do I fix it?

double input;
do {
    std::cin.clear(); 
    std::cout << "Enter a double value : ";
    std::cin >> input;
    if (std::cin.fail()) {
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
        std::cout << "Invalid input!\n";
    }   
} while (std::cin.fail());

CodePudding user response：

You must add the line

std::cin.clear();

immediately before the line

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

otherwise the call to std::cin.ignore will fail, because the stream is in a failed state.

However, this will cause the loop condition of the outer loop to not work. Therefore, the entire code should be changed to the following:

double input;

while( true ) {
    std::cout << "Enter a double value : ";
    std::cin >> input;
    if ( std::cin.fail() ) {
        std::cin.clear();
        std::cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );
        std::cout << "Invalid input!\n";
        continue;
    }

    //input is ok, so break out of infinite loop
    break;
}