Count common elements of sorted C arrays using STL

count.cpp

/* A model of output iterator that will count every time it gets incremented. */
class Counter {
public:
  /* Implement a getter if you want. */
  int n;

  Counter(int n = 0) : n(n) {}
  Counter(Counter const& other) : n(other.n) {}
  Counter& operator* () { return *this; }
  template <typename T>
  Counter& operator= (T const&) { return *this; }
  Counter& operator= (Counter const& other) { n = other.n; return *this; }
  Counter& operator++ () { ++n; return *this; }
  Counter& operator++ (int) { return ++(*this); }
};

int M = ...;
int a_followers[M];

int N = ...;
int b_followers[N];

/* After populating and sorting... */

Counter counter;
int overlap = std::set_intersection(
  a_followers, a_followers + M,
  b_followers, b_followers + N,
  counter);

/* Hereafter, counter.n has the count you want. */