Error checking cin.fail() for a string variable in C++

Question

#include <iostream>
using namespace std;

int main()
{
  int i;
  cout << "Enter: " << endl;
  cin >> i;

  cout << cin.fail() << endl;
}

This is my typical implementation for error checking to make sure I am entering a valid input and this was what I was taught. The problem with my above code is if I type in '4d' it stores the 4 in variable 'i', but leaves the 'd' in the buffer, so the cin.fail() test returns false, but I want it to return true. If I type in 'c' or 'ccc' etc... cin.fail() returns true as I want.

Is there any proper command to test for what I have described?

Thanks for the reply. I'm actually doing this as a homework assignment for a class, and we are not annoyed to use things we haven't learned (we haven't gone over isdigit()). Our instructor just wanted us to perform error testing, but didn't mention how in depth. Is there any way to do this with cin.fail()? — user5965026
– user5965026, Commented Mar 16, 2016 at 5:16
while(cin << i) { if(cin.fail()) cout << "Failed" << endl; } — Kit Fung
– Kit Fung, Commented Mar 16, 2016 at 5:22

R Sahu · Accepted Answer · 2016-03-16 05:18:23Z

1

I suspect there are more than one ways to solve your problem. I can think of the following two.

Get the next character right after you read i. If that character is not a whitespace character, you can flag that as an error.

cin >> i;
if ( cin ) // reading to i was successful.
{
   int c = cin.get();
   if (!isspace(c) )
   {
      // There was a non whitespace character right
      // after the number.
      // Treat it as a problem
   }
}

Read a token of characters and check whether any of them is not a digit.

std::string token;
cin >> token;
for ( auto c : token )
{
   if (!isdigit(c) )
   {
      // Problem
   }
}

answered Mar 16, 2016 at 5:18

R Sahu

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