Function Pointers

Just like you can point to a variable with a pointer, you can also point to a function. A function pointer stores the address of a function, letting you call that function through the pointer. This is very useful for passing a function as an argument to another function, such as a sort function that accepts a custom comparison, or a map-style helper that applies a function to every element.

The syntax is return_type (*name)(parameters). For example, a pointer to a function that takes two ints and returns an int looks like:

int (*operation)(int, int);

You can assign a matching function to it, and call it just like a normal function:

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

int (*operation)(int, int);
operation = add;               // point to add
printf("%d\n", operation(3, 4));   // call through the pointer

Function pointers are especially handy with arrays. An array of function pointers lets you select behaviour by an index:

int (*operations[2])(int, int);
operations[0] = add;
printf("%d\n", operations[0](2, 5));

Exercise

Write a function apply that takes two integers and a function pointer op to a function that maps two ints to one int, calls op on the two integers, and returns the result. Then use it to compute 3 + 4 with add.