Quick and dirty imperative version:

    #include <climits>

    int count_common_chars(const std::string& str) {
      int n = 0, common = 0, counts[CHAR_MAX - CHAR_MIN + 1] = {};
      for (char c : str) {
        if (c == '\n') {
          common = 0;
          n++;
        }
        if (counts[c - CHAR_MIN] == n) {
          counts[c - CHAR_MIN]++;
          common++;
        }
      }
      return common;
    }

Tastes may vary, but to my eyes the imperative version has some advantages:

* Faster

* No allocations

* No dependencies

* Easy to add an early return if common == 0 (in the functional version it's much harder)