Use the std::tr1::function Class to Generalize Callbacks-3 : Page 3

using namespace std;

bool adjacent(int x, int y){return x == y-1 || x == y+1;}

class Data 
{
private:
 vector <pair<int, int> > records;
public:
 bool operator()(int x, int y)
 {
   records.push_back(make_pair(x, y));
   return x < y;
 }
};
    
function <bool(int, int)> func2;
    
func2 = &adjacent
func2(2, 3); // calls adjacent(2, 3), returns true
    
Data rec;
func2 = ref(rec); //see note 
func2 (5, 2)); // adds  pair<5, 2> to rec.records

You may also assign two std::tr1::function objects whose return types and arguments are different if there is an implicit conversion between those types:

func2=func;//OK, long return value is convertible to bool

std::tr1::function f<int (int,int>) f;
assert (!f); //OK, f is empty
f=&add
assert (f); //OK, f isn't empty

void func(function <int (int,int>) &f1);
{
 if(f1)
 {
  f1(2,3);
 }
 else
 {
  f1=&subtract
  f1(3,2);
 }
}