Test Boost.Function to implement a private callback function in a class

callback_example.cpp

#include <iostream>
#include <boost/function.hpp>
#include <vector>
using namespace std;

// Simple Point class
class Point
{
    int _value;

public:
    Point(int val) : _value(val) {}
    ~Point() {}
    void setValue(int val) { _value = val; }
    int getValue() { return _value; }
};

typedef boost::function<int (Point x, Point y)> CostFunction;

// Simple Graph class
// Is hard coded to contain two points
class Graph
{
    CostFunction _cost_function;
    vector<Point> _trajectory;
    void _fill_trajectory();

public:
    Graph();
    Graph(CostFunction input_function);
    ~Graph();
    int add_two_points();

};

// Graph class implementation
void Graph::_fill_trajectory()
{
    Point o1(2), o2(9);
    _trajectory.push_back(o1);
    _trajectory.push_back(o2);
}
Graph::Graph() {
    _fill_trajectory();
}
Graph::~Graph() {}
Graph::Graph(CostFunction input_function)
{
    _cost_function = input_function;
    _fill_trajectory();
}

int Graph::add_two_points()
{
    if(_cost_function)
    {
        cout << "Graph::There is a callback defined!" << endl;
        int a = _trajectory[0].getValue();
        int b = _trajectory[1].getValue();
        return _cost_function(a, b);
    }
    else
    {
        cout << "Graph::No callback defined!" << endl;
        return 99;
    }
}

// Define callback function to pass to Graph constructor
int add_two_points_callback(Point &p1, Point &p2)
{
    return p1.getValue() + p2.getValue();
}

// Main
int main(  )
{
    // Create graph without given a callback
    // Graph::add_two_points will return 99
    Graph g1;
    cout << "main::Non callback: " << g1.add_two_points() << endl;

    // Create graph with callback function
    // Graph::add_two_points will return sum of two values of points in graph
    Graph g2(add_two_points_callback);
    cout << "main::Callback is defined " << g2.add_two_points() << endl;

    return 0;
}

I know, there is no copy constructor...