Function Pointers in

About Function Pointers

Just as a pointer can be made to an int or other basic data type, a pointer can be made to a function. A function pointer can be used to pass a function to another function. A function which is passed as a parameter value to another function is known as a first class function. A function which takes one or more functions as a parameter, or which returns a function, is known as a higher order function.

In the following example, is_even and is_odd are first class functions. The higher order function is test_number.

#include <stdio.h>
#include <stdbool.h>

bool is_even (int number) {
    return number & 1 != 1;
}

bool is_odd (int number) {
    return number & 1 == 1;
}

// this function takes any other function which takes a number and returns a bool
void test_number(bool (*func) (int), int number, char *text){
    printf(text, number, func(number)? "true": "false");
}

int main() {
    // prints Testing odd for 3 is true
    test_number(is_odd, 3,"Testing odd for %d is %s\n");
    // prints Testing even for 3 is false
    test_number(is_even, 3,"Testing even for %d is %s\n");
}

The function signature of bool (*func) (int) for the first argument to test_number matches the signature of the functions passed in. Both is_even and is_odd take an int parameter and return a bool. In (*func), func represents the name of the function to be used. if (*fn) were used, then it would have been called like so: printf(text, number, fn(number)? "true": "false");. To satisfy the function pointer parameter, we pass the name of a function having the expected signature when calling test_number. Thus, we see that a function name points to the address of that named function.

Calling a function pointer with the wrong signature results in undefined behavior (UB.) Depending on the compiler, it may not compile, or it may compile with or without warnings but then suffer a runtime crash.