Programming and Graphics

5.5 Pointers to functions 143

Assume that we need to call a function A which, in turn, calls either
function B or function C. We want the call to A to include an argument that
allows us to specify which one of the functions B or C will be called. This can
be done be introducing pointers to functions B and C, called pA and pB, and
calling A with a pointer argument p that is evaluated either as the pointer of
B or as the pointer of C – that is, p=pB or p=pC.

If the prototype of a function is:

double functionname(double, double);

its pointer is declared as:

double (*othername)(double, double) = functionname;

The function may then be called as

c = functionname(a, b);

or

c = (*othername)(a, b);

For example, consider the following code consisting of three functions and
the main program, contained in the filepointerfun.cc:

#include <iostream>

using namespace std;

double ratio(double, double); // function prototype

double (*pointratio)(double, double)=ratio; // and its pointer

double product(double, double); // function prototype

double (*pointproduct)(double, double)=product; // and pointer

/*--------------------------------------------------

The following is a function prototype; the arguments consist of two

"double" scalars and the pointer of a function that receives

two doubles and returns one double

-----------------------------------------------------*/

double operate(double, double, double(*)(double, double));

/*--------------- main program --------------------*/

int main()

{