C ++使用cin.fail（）和cin.clear（）-我有一个无限循环，不知道为什么

Question

Can you not use cin and then cin.fail() in the same line like this? Is there a cleaner or more standard way to do this type of error checking upon input?--perhaps not using a do-while.

do {
     cin.clear();
     cout << "\nPlease enter the size of the array (1-10): ";
} while (cin >> array_size && array_size <= 1 || array_size >= 10 || cin.fail());

This one works:

do {
    cout << "Please input #: ";
    if (cin.fail()){
        cin.clear();
        cin.ignore(80, '\n');
    }
    cin >> kids_total;
} while (cin.fail() || kids_total <= 0);

Answer 1

cin >> array_size && array_size <= 1 || array_size >= 10 || cin.fail()

Say you've got a letter next in cin , cin >> array_size evaluates false so the && short-circuit evalusation skips over array_size <= 1 to test:

• array_size >= 10 : might be uninitialised memory read ==> undefined behaviour, otherwise presumably false
• cin.fail() - definitely true

...unless there's undefined behaviour - and maybe even then - the loop will continue without having removed the letter from cin , only to fail again immediately.

Check cin.ignore for a way to remove input that's failed parsing, or use std::getline(std::cin, my_string) , std::istringstream iss(my_string); if (iss >> array_size std::istringstream iss(my_string); if (iss >> array_size etc. for a way to guarantee the entire line's thrown away on bad input....

---

For comparison, this is pretty robust and IMHO intuitive if verbose. If doing it more than once, just create a function....

while (true)
{
    std::cout << "Please input #: ";
    if (std::cin >> kids_total)
    {
        if (kids_total > 0)
            break;
        std::cout << "Value must be > 0, please try again.\n";
    }
    else if (std::cin.eof())
    {
        // rare for keyboard input - e.g. ^D on UNIX/Linux, ^Z Windows
        // can happen with pipes/redirects - e.g. echo 10 20 | ./my_app
        std::cerr << "expected value for kids_total but encountered end of input\n";
        exit(EXIT_FAILURE);
    }
    else
    {
        std::cin.clear();
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }
}