C/C++ Function Pointers

Key Word(s): C++, Function Pointers

Key: :large_orange_diamond: - Code Example :large_blue_diamond: - Code Exercise :red_circle: - Code Warning
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Function Pointers

A function pointer is a variable that stores the address of a function which can be used within in a program, including as a function argument.

Syntax

return_type (*function_name) (argument list);

#include <stdio.h>

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

int main(void){
    int (*foo)(int,int);   // optional argument names
    foo = my_add_function; // initialize the function pointer 

    /* optionally: we can use & */
    // foo = &my_add_function;

    int a = 5, b = 70;
    int sum = foo(a,b);
    printf("The sum of %d and %d equals %d.\n",a,b,sum);

    return 0;
}

:large_orange_diamond: Deepnote: Function Pointers

Reading Function Pointer Declarations

void *(*bar_func)(int *);

To understand this function, start reading inside out:

• (*bar_foo): pointer to the function
• void *: pointer to a void type
• (int *): integer pointer is the input argument

Functions as Arguments to Other Functions

You can pass functions as arguments to other functions which can be executed within the function.
For example, if you need a sorting function for your C/C++ program, there is a function called qsort. Let's take a look at how it works.

Function Pointers in the wild

C has a built-in sort function:

void qsort (void* base, size_t num, size_t size,  
            int (*comparator)(const void*,const void*));

int comparator(const void* p1, const void* p2);
Return value meaning
<0 The element pointed by p1 goes before the element pointed by p2
 0 The element pointed by p1 is equivalent to the element pointed by p2
>0 The element pointed by p1 goes after the element pointed by p2

:large_orange_diamond: Deepnote: Sorting Structs

Callback Functions

Another use of function pointers is in callback functions.

We can request that a user set the function we want to call abstractly.
Here we see an example where we pass the function pointer that is to be set by a user. Then we call the callback function after is set.

#include <stdio.h>

double custom_mult(double x,double y){
    return x*x + 2.0*x*y + y*y;
}

// callback prototype
typedef double (*multiply_func_t)(double,double);

void set_callback(multiply_func_t *func_ptr){
      *(void **)(func_ptr) = custom_mult;
}

int main(void){
    multiply_func_t callback_multiply;
    set_callback(&callback_multiply);

    double x = 2.6, y = 5.1;
    double result = callback_multiply(x,y);
    printf("The results from our callback Op(x=%f,y=%f) = x*x + 2.0*x*y + y*y = %f\n",x,y,result);

    return 0;
}

:large_orange_diamond: Deepnote: Callback Functions
:large_orange_diamond: Sandbox: Callback Functions Shared Library

Building a Class in C - Sort Of

Structs allow various data members of different types to be stored. However, structs don't actually have methods. But we can trick it!
We can store function pointers in structs which we can use as a method.

typedef double (*custom_print_func_t)(int in_var);

typedef struct {
    int exp;
    custom_print_func_t my_print;
} func_t;

:large_orange_diamond: Deepnote: Function Pointers in Structs!

Next: C++ Classes & Objects