A C++ monoid trait. Example code shamelessly copied from http://www.scala-lang.org/node/114 . This is just an experiment, how the linked Scala code can be translated into C++.

monoid.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <list>
#include <string>

using namespace std;


template<class T>
struct Semigroup {
  static T add(const T& a, const T& b) {
    return a + b;
  }
};

template<class T>
struct Monoid : Semigroup<T> {
  static T unit();
};

template<>
struct Monoid<int> : Semigroup<int> {
  static int unit() { return 0; }
};

template<>
struct Monoid<string> : Semigroup<string> {
  static string unit() { return ""; }
};

template<class InputIt,
         class T = typename iterator_traits<InputIt>::value_type>
T sum(InputIt first, InputIt last) {
  return accumulate(first, last, Monoid<T>::unit(), Monoid<T>::add);
}

int main() {
  auto l1 = list<int>{1, 2, 3};
  auto l2 = list<string>{"a", "b", "c"};
  auto l3 = list<char>{'a', 'b', 'c'};
  
  cout << sum(begin(l1), end(l1)) << endl;
  cout << sum(begin(l2), end(l2)) << endl;
  
  //cout << sum(begin(l3), end(l3)) << endl; // does not compile
  // undefined reference to `Monoid<char>::unit()'
}

// $ g++ -std=c++11 monoid.cpp 
// $ ./a.out 
// 6
// abc