cin.clear() doesn't reset cin object

I have the following loop. It should read numbers until EndOfFile, or the user input -999

int arr[100];

int index;

for (index = 0; index < 100; index++)
{
 cin >> arr[index];
 if (!cin)
 {
  cin.clear();
  index--;
  continue;
 }
 if (arr[index] == -999)
 {
     break;
 }
}

When the user input an invalid thing开发者_运维知识库, such as some chars, this loop is being repeated for ever without clearing the error state or stopping.

After calling clear, you must also somehow remove the invalid input from the stream. Here is one way:

 cin >> arr[index];
 if (!cin)
 {
  cin.clear();
  std::string ignoreLine; //read the invalid input into it
  std::getline(cin, ignoreLine); //read the line till next space
  index--;
  continue;
 }

It's because when cin fails to read the invalid input, it remains there in the stream. It has to be removed, by some means. I just read and ignore it.

You can also use ignore as:

cin.clear();
cin.ignore(std::numeric_limits<streamsize>::max(),' ');

which is better in my opinion provided inputs are space separated (and if you don't want to inspect the invalid input). The online doc says:

istream::ignore

istream& ignore( streamsize n = 1, int delim = EOF );

Extract and discard characters

Extracts characters from the input sequence and discards them.

The extraction ends when n characters have been extracted and discarded or when the character delim is found, whichever comes first. In the latter case, the delim character itself is also extracted.