How to use typedef to an array of function pointers

Question

I have an array of function pointer:

int (*collection[2]) (int input1, int input 2) = {&fct1,&fct2}

I can get values by calling both functions from the array:

*collection[0](1,2);
*collection[1](1,2);

Using typedef, I want another way to invoke the array of function pointer. So far, I am doing:

typedef int (*alternameName)(int input1, int input 2);
alternateName p = &collection[2];
int result1 = (*p[0])(1,2);
int result2 = (*p[1])(1,2);
printf("results are: %d, %d",result1, result2);

My issue is I do not think I properly defined the variable p as I keep getting 0 as my results.

alternateName p = &collection[2]; Assuming that this is the same collection as above, you take the address of the third element, which does not exist. Try alternateName p = collection; or alternateName p = &collection[0]; — Gerhardh
– Gerhardh, Commented Feb 9, 2022 at 6:23

tstanisl · Accepted Answer · 2022-02-09 10:41:39Z

It is generally cleaner to typedef function types, not function pointers. It results in cleaner syntax:

typedef int collection_f(int, int);

Now you can define collection, simply as an array of pointer to collection_f.

collection_f* collection[2] = {&fct1,&fct2};

A typical call syntax would be:

collection[0](1,2);
collection[1](1,2);

Don't de-reference function pointer prior to calling. Actually, call operator takes a function pointer as an operand, not a function. Functions decay to function pointer in all context except & operator ... which returns a function pointer.

Next, I'm not sure what:

alternateName p = &collection[2];

is supposed to mean. I assume that you want p to point to the first element of collection. Moreover, indexing p[1] and p[2] looks like out of bound access because accessing collection is only defined for indices 0 and 1.

Now your code could be rewritten can:

collection_f** p = collection; // pointer to first element which is a pointer to collection_f
int result1 = p[0](1,2);
int result2 = p[1](1,2);
printf("results are: %d, %d",result1, result2);

I hope it fixes the problem.

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