Generic update or insert of a key within a C++ map, efficiently

gistfile1.cpp

#include <map>
#include <functional> // C++11

// general method stolen from: http://stackoverflow.com/questions/97050/stdmap-insert-or-stdmap-find 
template <class K, class V>
void setOrUpdate(map<K, V> &tree, const K& key, const V& amount, function<void (V&)> updater) {
  auto ptr = tree.lower_bound(key);
  if(ptr != tree.end() && ptr->first == key) {
    // key already exists
    updater(ptr->second);
  } else {
    // C++11 pair syntax is the bomb!
    tree.insert(ptr, {key, amount});
  }
}

template <class T>
map<T, int> counts(const vector<T> &input) {
  map<T, int> results;
  for(const T& x : input) {
    setOrUpdate<T,int>(results, x, 1,
        [](int &prev) { prev++; });
  }
  return results;
}