Validating input in C++

Question

I have this code:

double input2;
cout<<"please enter one number:"<<endl;
cin>>input2;

How can I judge if the user input only about digits, like '3', '4', or '4.241'. Sometimes when users enter a character like 'n', '3.q', my program will crash.

Answer 1

You can use stringstream to convert a string safely to number:

 string input;
 int mynumber;    
 cout << "Please enter a valid number: ";
 getline(cin, input);

   // This code converts from string to number safely.
   stringstream myStream(input);
   if (myStream >> myNumber)
    cout <<"number is valid"<<endl;
   else
    cout<<"invalid number";

Answer 2

cin::<< operator returns NULL if operation went bad. Causes may be many but if interpretation fails then the failbit is set. You can check it with rdstate():

int main()
{
  double input2;
  cout<<"please enter one number:" << endl;

  cin >> input2;

  if (cin.rdstate() & ifstream::failbit)
     cout << "input badly formatted" << endl;

  return 0;
}

Answer 3

It won't crash because of that. If the cin >> input2 operation fails because of invalid input, cin is put in an invalid state, but it won't crash.

Answer 4

You need to verify that the input was auccessful

if (std::cin >> input2) {
    ...
}

.. and if it wasn't deal with the erronous input, e.g.:

std::cin.clear();
std::cin.ignore();