Section 29.1 Function Pointers
Just like
int *p; declares
p as a pointer to an
int, we can declare a pointer to a function. The syntax looks a little unusual at first:
void (*functionptr)(int);
This declares
functionptr as a pointer to a function that takes a single
int argument and returns nothing (
void). Just like arrays, function names themselves act as addresses, so both of the following work to assign a function pointer:
functionptr = &printNumber; /* explicit address-of */
functionptr = printNumber; /* implicit conversion, same effect */
To actually call the function through the pointer, we
dereference it (though C is forgiving here, and
functionptr(x) works too):
(*functionptr)(my_number);
Let’s see this in action:
A function pointer can be reassigned to point to a
different function at any time, as long as that function has a matching prototype (same return type and same parameter types). This is exactly what lets us pick a function based on user input, rather than hard-coding an
if/
else that calls one function or the other directly:
Activity 29.1.
Define two functions,
float square(float x) and
float cube(float x). Ask the user to enter a number and choose whether they want to square (2) or cube (3) it. Declare a function pointer, and use an
if/
else statement to assign it to the correct function based on the user’s choice. Finally, use your function pointer to calculate and print the result.
Activity 29.2.
Let’s use a function pointer to convert a whole
batch of temperatures! We have two functions,
float cTof(float c) and
float fToc(float f). Ask the user which conversion they want to do (1 for Celsius→Fahrenheit, 2 for Fahrenheit→Celsius), declare a function pointer, and assign it to the correct function based on the user’s choice. Then write a
for loop that iterates through the temperature array, passing each value to your function pointer and printing the result.
