c function pointers

Complete

Concept

Function pointer

pointer to a function
refer to functions as variables
makes higher order functions possible

int f(int); // function declaration, returns int

int (*fptr)(int); // function pointer declaration

// wrong syntax
int *fptr(int); // function that returns an int

fptr = f; // assignment

the parentheses are necessary due to operator precedence

Functions returning pointers

int *g(void); // funtion that returns an int pointer

int *(*gptr)(void); // function pointer declaration

// wrong syntax
int **gptr(void); // function that returns a pointer to int pointer

gptr = g;

Returning function pointers

int (*h(void))(int);
// h, is a function 
// (void), that takes no arguments
// *, returning a pointer to
// (int), a function taking in an int
// int, and returns an int

// equivalent using typedef, easier to read
typedef int (*F)(int);

F h(void); // more familiar syntax

a way of understanding the syntax is the clockwise/spiral method, the return type is not just at the front of the declaration, but wraps it

Application

Binary operators

#include <stdio.h>

int add(int a, int b) {
	return a + b;
}

int sub(int a, int b) {
	return a - b;
}

int mul(int a, int b) {
	return a * b;
}

int divide(int a, int b) {
	return b != 0 ? a / b : 0;
}

int (*get_op(int))(int, int);
// or
typedef (*Op)(int,int);
Op get_op(int);

int (*get_op(int index))(int, int) {
// or
Op get_op(int) {
    switch (index) {
        case 0: return add;
        case 1: return sub;
        case 2: return mul;
        case 3: return divide;
        default: return 0;  // NULL function pointer
    }
}

int main(void) {
    int (*op)(int, int);

    op = get_op(2); // mul

    if (op) {
        printf("%d\n", op(6, 7));  // 42
    }

    return 0;
}