Auto comment: topic has been updated by roycf123 (previous revision, new revision, compare).

Are you sure this is working because its not working for input like

1 2 3 4 5

but working for

5

1 2 3 4 5

The use of cin.ignore here is dubious (also, it should be stream.ignore because you're taking any stream as input, not necessarily cin). If you do cin >> a two times in a row, you will eat the line between them: e.g

int main () {
    int _;
    cin >> _;

    vector<ll> a, b;
    cin >> a;
    cin >> b;

    for (int u : a)
        cout << u << " ";
    cout << '\n';

    for (int u : b)
        cout << u << " ";
    cout << '\n';
}

0
1 2 3
4 5 6 7
8 9

1 2 3 
8 9 

Similarly, if you do cin >> a at the start of the code, you will eat the first line of the input.

I do understand why you put it there though: if you read e.g. a single integer from cin, then after that, the \n character at the end of the line has not yet been read from the stream, so your call to getline will simply be an empty string (or a string consisting of whatever was between what you already read and the \n character, possibly just whitespace).

The simplest way to fix this is to remove the cin.ignore line, and to remember to do an ignore between reading something from cin in the "regular" way and reading a line from cin that way. There are other ways to do this, but they all make assumptions about how exactly you want to read the input. The simplest way is to simply skip over the line if it is empty:

template<typename T>
istream& operator>> (istream& stream, vector<T> &a) {
  a.clear(); // remove whatever was in a before
  
  string line;
  do {
    if (stream.eof())
      return stream; // if the stream has ended, just let a be empty
    
    getline(stream, line);
  } while (line.empty());
  
  istringstream line_stream (line);
  T x;
  while(line_stream >> x)
    a.push_back(x);
  
  return stream;
}