Dynamic function members in C++

fptr.cpp

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

typedef string (*callback)(string);

string cb1 (string str)
{
    return str + "...in bed!";
}

string cb2 (string str)
{
    return str + " ...that's what she said!";
}

class Tester
{
    string (*exec)(string); 

public:    
    Tester()
    {   
    }   

    void setExec(callback cb) 
    {   
        exec = cb; 
    }   

    string run(string str)
    {   
        return exec(str);
    }   
};

int main()
{
    string foo = "Last night I ate brains.";
    Tester tester;
    tester.setExec(&cb1);
    cout << tester.run(foo) << endl;
    tester.setExec(&cb2);
    cout << tester.run(foo) << endl;

    return 0;
}