Output a set's all sub sets

Problem: Given a set, output all its subsets. For example, for set (a,b,c), we need to output (), (a), (b), (c), (a,b), (a, c), (b,c), (a,b,c).

Solution: iterative approach is not difficult. Here give a recursive approach:

void _print(const vector<int>& set)
{
    i; i<< set[i] << " ";
    cout << endl;
}


void print_set(const vector <int>&set, int i, vector<int>& output)
{
   if(i >= set.size()) _print(output);
   else {
     output.push_back(set[i]);
     print_set(set, i+1, output);
　　　// code to handle duplicates
     int tmp = output.back();
     output.pop_back();
     
     if(output.size()>0 && tmp== output.back()) return;
          
     print_set(set, i+1, output);
   }
}

int main()
{
    vector<int> set,output;
    set.push_back(1);
    set.push_back(2);
    set.push_back(3);
    set.push_back(4);

    print_set(set, 0, output);
}