Allowing a functor or a function to be stored in a class

From: James Aguilar (jfa1_at_cec.wustl.edu)
Date: 03/30/05 

Date: Tue, 29 Mar 2005 16:28:02 -0600

In the C++ STL, there are various places where you can call a function that
takes a functor as a parameter. This functor may be either a function object or
a function pointer. For instance, if you have a vector<char> and call for_each
on it, both of the methods below would be valid:

#include <iostream>
#include <vector>
using namespace std;

struct MyFunctionObject
{
  void operator ()(char a) {cout << a;};
};
void myFunction(char a) {cout << a;}

int main()
{
  vector<char> v;
  //fill it . . .
  for_each(v.begin(), v.end(), (MyFunctionObject()));
  for_each(v.begin(), v.end(), &myFunction);
  return 0;
}

My question pertains to a similar issue: how do I cause the same to occur in a
class template? I would like my template to look like this:

template <typename CompT>
class WhyWontThisWork
{
  CompT m_comparator;
public:
  WhyWontThisWork(CompT cmpIn) : m_comparator(cmpIn) {}
  void doThis(char a, char b) { if (m_comparator(a, b)) cout << "WIN"; }
};

Can I actually do this in C++ without writing an inordinate amount of code? If
so, how?

- JFA1

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