CODEWITHSUNDEEP
carraysfunction-pointers
Gradient
Gradient

Reputation: 2343

C array of function pointers

I have the following piece of code that I don't fully understand :

void (*foo[ABC]) (int *i) {
    [A] = function1,
    [B] = function2,
    [C] = function3
}

Where A, B and C are integer constants.

1- Is it a valid declaration if ABC has not been defined before?

2- What is this way of initializing called? ([A] = function1;)

3- What is the value of foo[D]? Is it a null pointer?

Upvotes: 2

Views: 440

Answers (2)

user529758
user529758

Reputation:

"I don't think it's C" is not equivalent with "it is not C".

Thanks for the link, @Kninnug -- this is a horrible C99 feature (and a GNU extension to C90), and the code has an error: it is a misspelled initialization of an array of three function pointers. I could imagine the fixed code like this:

#define ABC 3

#define A 0
#define B 1
#define C 2

void function1(int *i)
{
}

void function2(int *i)
{
}

void function3(int *i)
{
}

int main(int argc, char *argv[])
{
    void (*foo[ABC]) (int *i) = {
        [A] = function1,
        [B] = function2,
        [C] = function3
    };

    return 0;
}

This compiles.

Also:

What is the value of foo[D]? Is it a null pointer?

Well, what's D? If D >= ABC (assuming that they both are non-negative integers), then that element doesn't even exist. If D < ABC, then it is a NULL pointer, since aggregate (structure, union and array) initialization implicitly zero-initializes elements that have no corresponding initializer expression in the initializer list.

(More precisely, they are initialized "as if they had static storage duration", which is initialization to NULL in the case of pointers.)

Upvotes: 2

Kninnug
Kninnug

Reputation: 8053

  1. No, it won't work if either ABC or A, B or C are not defined.
  2. The initializers are so called designated initializers (for C90 a GNU extension and standard since C99, thanks AndreyT)
  3. As long as D < ABC, foo[D] will be 0 (equivalent to a NULL-pointer), otherwise it will be undefined.

EDIT: as the question is now about what it would mean if ABCRandom and the other indexers are strings the answer is a lot shorter: it won't mean anything, using strings as array indexes is undefined behaviour.

Upvotes: 3

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