Why does while (true) skip cin when it received invalid input? [duplicate]

问题

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cin input (input is an int) when I input a letter, instead of printing back incorrect once, it prints correct once then inc for the rest of the loop (2 answers)

Closed 1 year ago.

This while-loop does not wait for input from cin after receiving wrong input (non-integer). Does cin somehow stay in a false state?

while (true) {
    int x {0};
    cout << "> ";
    cin >> x;
    cout << "= " << x << endl;
}

I would expect this while-loop to wait for input everytime around, but that no longer happens when it receives wrong input.

回答1:

Once cin fails, it stays in an invalid state until it's cleared.

clear() without arguments can be used to unset the failbit after unexpected input

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.

As @Peter points out, you also have to clear the buffer.

Your example would be like this:

while (true) {
  int x{0};
  cout << "> ";
  if (!cin) {
    // unset failbit
    cin.clear();
    // clear the buffer
    cin.ignore(numeric_limits<streamsize>::max(), '\n');
  }
  cin >> x;
  cout << "= " << x << endl;
}

来源：https://stackoverflow.com/questions/56303342/why-does-while-true-skip-cin-when-it-received-invalid-input