How to handle wrong data type input

Question

In C++, how do you handle wrong inputs? Like, if the program asks for an integer, when you type a character it should be able to do something and then loop to repeat the input but the loop goes infinite when you input a character when an integer is need and vice versa.

Answer 1

The reason the program goes into an infinite loop is because std::cin's bad input flag is set due to the input failing. The thing to do is to clear that flag and discard the bad input from the input buffer.

//executes loop if the input fails (e.g., no characters were read) while (std::cout << "Enter a number" && !(std::cin >> num)) {     std::cin.clear(); //clear bad input flag     std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); //discard input     std::cout << "Invalid input; please re-enter.\n"; } 

See the C++ FAQ for this, and other examples, including adding a minimum and/or maximum into the condition.

Another way would be to get the input as a string and convert it to an integer with std::stoi or some other method that allows checking the conversion.

Answer 2

The top voted answer covers the solution really well.

In addition to that answer, this may help visualize what's going on a little better:

int main()      int input = 1;//set to 1 for illustrative purposes     bool cinState = false;     string test = "\0";     while(input != -1){//enter -1 to exit         cout << "Please input (a) character(s): ";//input a character here as a test         cin >> input; //attempting to input a character to an int variable will cause cin to fail         cout << "input: " << input << endl;//input has changed from 1 to 0         cinState = cin;//cin is in bad state, returns false         cout << "cinState: " << cinState << endl;         cin.clear();//bad state flag cleared         cinState = cin;//cin now returns true and will input to a variable         cout << "cinState: " << cinState << endl;         cout << "Please enter character(s): ";         cin >> test;//remaining text in buffer is dumped here. cin will not pause if there is any text left in the buffer.         cout << "test: " << test << endl;     }     return 0;     } 

Dumping the text in the buffer to a variable isn't particularly useful, however it helps visualize why cin.ignore() is necessary.

I noted the change to the input variable as well because if you're using an input variable in your condition for a while loop, or a switch statement it may go into deadlock, or it may fulfill a condition you weren't expecting, which can be more confusing to debug.