bool  even(int  i)  {
     return  (i  %  2  ==  0);
 }
 bool  positive(int  i)  {
     return  (i  >  0);
 }

 vector<int>  v;
 std::cout  <<  find_first(v,  even)  <<  endl;
 std::cout  <<  find_first(v,  positive)  <<  endl;