Sams Teach Yourself C in 21 Days

Pointers: Beyond the Basics 409

15

OUTPUT 43.560000 43.560000

Precision of the values might cause some numbers to not display as the

exact values entered. For example, the correct answer, 43.56, might appear

as 43.559999.

Note

Line 7 declares the function square(), and line 11 declares the pointer ptrto a function

containing a doubleargument and returning a doublevalue, matching the declaration of

square(). Line 17 sets the pointer ptrequal to square. Notice that parentheses aren’t

used with squareorptr. Line 20 prints the return values from calls to square()and

ptr().

A function name without parentheses is a pointer to the function (sounds similar to the

situation with arrays, doesn’t it?). What’s the point of declaring and using a separate

pointer to the function? Well, the function name itself is a pointer constant and can’t be

changed (again, a parallel to arrays). A pointer variable, in contrast, can be changed.

Specifically, it can be made to point to different functions as the need arises.

Listing 15.9 calls a function, passing it an integer argument. Depending on the value of

the argument, the function initializes a pointer to point to one of three other functions

and then uses the pointer to call the corresponding function. Each of these three func-

tions displays a specific message on-screen.

LISTING15.9 ptr2.c. Using a pointer to a function to call different functions

depending on program circumstances

1: /* Using a pointer to call different functions. */

2:

3: #include <stdio.h>

4:

5: /* The function prototypes. */

6:

7: void func1(int x);

8: void one(void);

9: void two(void);

10: void other(void);

11:

12: int main( void )

13: {

14: int nbr;

15:

INPUT

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