Pointer til en funktion som parameter til en funktion?

Kode 

// fun_ptr.cpp -- pointers to functions
#include <iostream>
double betsy(int);
double pam(int);

// second argument is pointer to a type double function that
// takes a type int argument
void estimate(int lines, double (*pf)(int));

int main()
{
    using namespace std;
    int code;

    cout << "How many lines of code do you need? ";
    cin >> code;
    cout << "Here's Betsy's estimate:\n";
    estimate(code, betsy);
    cout << "Here's Pam's estimate:\n";
    estimate(code, pam);
    return 0;
}

double betsy(int lns)
{
    return 0.05 * lns;
}

double pam(int lns)
{
    return 0.03 * lns + 0.0004 * lns * lns;
}

void estimate(int lines, double (*pf)(int))
{
    using namespace std;
    cout << lines << " lines will take ";
    cout << (*pf)(lines) << " hour(s)\n";
}

Kode 

#include <iostream>
#include <string>

void my_callback(std::string txt) {
    std::cout << "my_callback: " << txt << std::endl;
}

void my_other_callback(std::string txt) {
    std::cout << "my_other_callback: " << txt << std::endl;
}

void my_caller(void(*callback)(std::string), std::string str) {
    std::cout << "Before calling callback" << std::endl;
    callback(str);
    std::cout << "After calling callback" << std::endl;

}

int main (int argc, char ** argv) {
    my_caller(my_callback, "Hello, World");
    my_caller(my_other_callback, "Hello, World");
    return 0;
}