A Function Pointer Example

Listing 7.18 demonstrates using function pointers in a program. It calls the estimate() function twice, once passing the betsy() function address and once passing the pam() function address. In the first case, estimate() uses betsy() to calculate the number of hours necessary, and in the second case, estimate() uses pam() for the calculation. This design facilitates future program development. When Ralph develops his own algorithm for estimating time, he doesn’t have to rewrite estimate(). Instead, he merely needs to supply his own ralph() function, making sure it has the correct signature and return type. Of course, rewriting estimate() isn’t a difficult task, but the same principle applies to more complex code. Also the function pointer method allows Ralph to modify the behavior of estimate(), even if he doesn’t have access to the source code for estimate().

Listing 7.18. fun_ptr.cpp

// 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: ";
    estimate(code, betsy);
    cout << "Here's Pam's estimate: ";
    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) ";
}

Here is a sample run of the program in Listing 7.18:

How many lines of code do you need? 30
Here's Betsy's estimate:
30 lines will take 1.5 hour(s)
Here's Pam's estimate:
30 lines will take 1.26 hour(s)

Here is a second sample run of the program:

How many lines of code do you need? 100
Here's Betsy's estimate:
100 lines will take 5 hour(s)
Here's Pam's estimate:
100 lines will take 7 hour(s)