function pointer stuff

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April 8th, 2003, 12:10 PM

balance

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function pointer stuff

this function call line is confusing me a bit:

Code:

qsorty((void **) lineptr, 0, nlines-1, (int (*)(void *, void *))(fold ? strcmpulc : strcmp));

i don't understand what the (void **) part is. it doesn't match up with the qsorty decleration which is:

Code:

void qsorty(void *lineptr[], int left, int right, int (*comp)(void *, void *));

also the (int (*)(void *, void *))(fold ? strcmpulc : strcmp) part from the function call i kind of understand. it seems that it's working a bit like the %d's in a printf statement - the place holder is getting replaced. so (*) is getting replaced by the answer to fold ? which is going to be one of the two pointers to functions.

what is that makes it behave like that? is that something to do with the (void **) maybe? (i'm guessing)

also, say you've got a situation where there isn't any option about which function pointer should be passed (like in the (fold ? strcmpulc : strcmp) part), is this the only way to say that? :

Code:

qsorty((void **) lineptr, 0, nlines-1, (int (*)(void *, void *))(numcmp));

that works but it seems like it should be possible to use something like:

Code:

qsorty((void **) lineptr, 0, nlines-1, (int (*numcmp)(void *, void *)));

Code:

qsorty((void **) lineptr, 0, nlines-1, (*numcmp)(void *, void *));

but they aren't ok.

April 8th, 2003, 01:23 PM

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Quote:

i don't understand what the (void **) part is. it doesn't match up with the qsorty declaration...

Bear in mind that in a function declaration (and only in a function declaration!) pointer-to-type and array-of-type are identical; they're synonymous, and the only reason why you might want to choose array-of-type (type[]) over pointer-to-type (type*) is as a hint to the programmer that it doesn't point to a single element of type, but to the first in a sequence of type.

In the function call I assume that lineptr is an array of pointer-to-char, which is why the typecast to (void **) makes sense, being that in a function declaration, **void is identical to *void[] as I just explained.

Quote:

Let's break that up into two pieces, the first part being a typecast, and the second being the conditional operator:

The (fold ? strcmpulc : strcmp) bit is fairly straight-forward; it's the conditional operator ?: (check A7.16 on page 208 of K&R if you're unfamiliar with it).

Basically that expression evaluates to either strcmpulc or strcmp, depending on the value of fold. So what you get, is a function pointer.

The problem is that qsorty expects a pointer to a function that has a prototype of

Code:

int (*cmp)(void *, void *)

, but strcmp and strcmpulc are (probably) declared as

Code:

int (*cmp)(const char *, const char *)

That's what the typecast is for. It might look draconian, but yes, it really is a typecast.

It casts the pointer-to-function-with-two-pointer-to-char-arguments-and-returning-an-int
to a pointer-to-function-with-two-pointer-to-void-arguments-and-returning-an-int.

Code:

int (*)(void *, void *)

can be broken down as follows:

Code:

int              /* returning an int */
(*)              /* pointer-to */
(void *, void *) /* function with two pointer-to-void arguments */

It needs the parentheses around the asterisk (the pointer-to bit) to make sure it isn't associated with the return type, int.

I hope this clears things up a bit. Keep asking though; it keeps us on our toes

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April 9th, 2003, 08:50 AM

balance

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thanks for your reply.

Quote:

, but strcmp and strcmpulc are (probably) declared as

Code:

int (*cmp)(const char *, const char *)

they're just declared normally :

Code:

int strcmpulc(char *, char *);

and the other is part of string.h so it's also declared in the usual way. maybe that's why i get :

warning: pointer type mismatch in conditional expression

for that line?

probably is. but then saying that, on this function call line:

Code:

qsorty((void **) lineptr, 0, nlines-1, (int (*)(void *, void *))(numcmp));

i don't get that warning and numcmp is declared like so:

Code:

int numcmp(char *, char *);

Quote:

(int (*)(void *, void *))(fold ? strcmpulc : strcmp)

two pieces, the first part being a typecast, and the second being the conditional operator

i understand that apart from why, when there's no conditional aspect - only one possible function to call, can i not say:

Code:

(int (*strcmp)(void *, void *))

or something similar? does it have to be in two pieces even when no conitional element? (in the call to the function that is)

Quote:

I hope this clears things up a bit.

a bit, yes thanks

i need to go over it a bit more to let it sink in

Last edited by balance : April 9th, 2003 at 08:57 AM.

April 9th, 2003, 02:29 PM

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Quote:

they're just declared normally

Well, my emphasis wasn't on the consts, but the fact that it's char* instead of void*.

Quote:

I understand that apart from why, when there's no conditional aspect - only one possible function to call, can i not say:

Code:

(int (*strcmp)(void *, void *))

or something similar? does it have to be in two pieces even when no conitional element?

The code snippet above is just a semi-colon short of a declaration statement. It would declare strcmp as being a pointer-to-function. What the qsorty function call contains though, is a type cast, which has a general form of

Code:

(type-to-cast-to) (expression-to-cast)

Just like you can cast a real expression to an integer expression by saying (int)(3.14159), you can cast pointer-to-function expressions.

The thing is that not all function pointers are alike; some pointers point to a function that takes an int as an argument, others point to a function that returns a double, etc, etc. They're all different, because the functions' declarations are different.

Hence, when you've defined a function (qsorty in this case) that expects as its third parameter a pointer to a function that takes two pointer-to-void arguments, and you call it with its third argument being a pointer to a function that takes two pointer-to-char arguments (strcmp, for example), then you'll get a warning (if not error) saying that the types don't match. That's why you have to perform a type cast.

Quote:

i don't get that warning and numcmp is declared like so

Again, that would be due to the type cast