Why doesn't cin re-prompt if input value exceeds target type size? [duplicate]

Question

This question already has answers here:

How do I prevent a runaway input loop when I request a number but the user enters a non-number? (6 answers)

Closed 8 years ago.

I noticed while I was implementing a simple linked-list stack that cin doesn't re-prompt if I enter a value larger than the max. size of an integer. For example:

int input;
do {
    cin >> input;
    if (input == -1)
        break;
    else if (input == 0)
        pop();
    else
        push(input);
} while (input != -1);

would loop forever if I entered a number which exceeds the size of an int (note that the type of input is int).

This is likely the most dangerous behaviour that I have encountered so far of a relatively well isolated C++ program in the short time that I have been programming in this language. Because the loop pushed the input on every iteration, my program's memory usage sharply increased to over 1GB in less than 30 seconds... my computer nearly crashed.

So I'm wondering: why would cin exhibit this behaviour?

P.S. I resolved this infinite-loop problem by adding input = -1 before reading the input. Is this the best way to prevent the looping?

Answer 1

Your loop should be checking the state of cin after reading from it. If the failbit is set, the input operation failed. You could re-prompt after that.

E.g., if (!cin) { ... }

Answer 2

I would consider re-writing like this:

int input;

while(!(std::cin >> input)) {
    cout << "you entered in the wrong type, please try again" << endl;
    cin.clear();// clear error flags from cin
    cin.ignore(numeric_limits<streamsize>::max(), '\n'); //extracts characters from the stream and discards them until a newline is found 
}

if(input == 0){
    pop();
}else{
    push(input);
}

Note that entering in a value that's larger than the maximum allowed int will cause the fail bit for cin to be set. This solution relies on this behaviour to work.

Answer 3

Signed integer types just wrap around if you exceed their maximum value.

To counteract it you should only push the input to the stack if it is within the acceptable bounds