Functions pointers - c

Question

Hey guys I have a question: How can i call a function from an enum structure with pointers?
For example I have this structure:
typedef enum struct_e
{
FUNCTION_ONE,
FUNCTION_TWO,
FUNCTION_THREE,
FUNCTION_FOUR,
} sctruct_t;
And I have a function that receives one of these variables and the parameters of the function (for example an int)
void call_functions(struct_t action, int exemple) {...}
// -> call like this call_functions(FUNCTION_ONE, 45);
And in that function I have to call one of the functions like this one:
void function_one(int a)
{
printf("You have %d years old", a);
}

Answer 1

Assuming each of the functions to call has type void (*)(int), you can create an array of function pointers, using the enum values as the array index:
typedef void (*call_func_type)(int);
call_func_type func_list[] = {
[FUNCTION_ONE] = function_one,
[FUNCTION_TWO] = function_two,
[FUNCTION_THREE] = function_three,
[FUNCTION_FOUR] = function_four
}
Then call_functions would just index into that array:
void call_functions(struct_t action, int example)
{
func_list[action](example);
}

Answer 2

I usually find that the first step when dealing with a function pointer is to use a typedef to make the syntax more readable. Then, such pointers can be used much like any other data type.
// declares function_ptr as a new type, pointer to function taking an integer parameter and returning void
typedef void (*function_ptr)(int);
// define function pointer p and initialize to point at function one
function_ptr p = function_one;
// call function_one passing 7 as a parameter
(*p)(7);
In this case, assuming that all of the functions take an integer as a parameter, we can use a table of pointers to represent all the functions you'd want to call:
function_ptr table[]=
{
function_one,
function_two,
function_three,
function_four,
};
At this point it's fairly easy to call any number of functions using this method.
void call_functions(struct_t action, int exemple)
{
if( action >= FUNCTION_ONE && action <= FUNCTION_FOUR )
{
(table[action])(exemple);
}
else
{
printf("Unrecognized function %i. Check that function table is up to date\n", (int)action);
}
}