%module mycpp
%include "std_vector.i"
namespace std {
  %template(IntVector) vector<int>;
}
extern double average(std::vector<int> v);

Having the C++ file as follows:

#include <vector>
#include <algorithm>
#include <functional>
#include <numeric>

double average(std::vector<int> v) {
    return std::accumulate(v.begin(),v.end(),0.0)/v.size();
}