CODEWITHSUNDEEP
c++cfunctionsyntax
Can Bayar
Can Bayar

Reputation: 541

What is the relationship between callbacks and function pointers?

I am a little bit confused about this subject. I know what callback is but function pointers are a little bit more confusing to me. Can they mean the same thing (at least in C/C++ context)? Or is one of them including the other, like function pointers can be used as a callback or vice versa.

Is there a relationship between function pointers and callbacks, or are they completely different subjects?

Upvotes: 3

Views: 611

Answers (1)

dbush
dbush

Reputation: 223699

Function pointers are often used to implement callbacks. A classic example is the qsort function:

void qsort(void *base, size_t nmemb, size_t size,
           int (*compar)(const void *, const void *));

The 4th argument to this function is a function pointer. This will typically point to a function you wrote yourself that the qsort function will call. Here's an example of how it's used:

int cmp(const void *v1, const void *v2)
{
    const int *i1 = v1;    // C++ requires cast, C doesn't
    const int *i2 = v2;    // C++ requires cast, C doesn't
    if (*i1 < *i2) {
        return -1;
    } else if (*i1 > *i2) {
        return 1;
    } else {
        return 0;
    }
}

int main()
{
    int array[] = { 3, 7, 5, 1, 2, 9, 0, 4 };
    qsort(array, 8, sizeof(int), cmp);
}

In this example, cmp is used to tell the qsort function how the elements of the list are to be ordered.

Another example of function pointers that is not a callback is if you want to call a particular type of function based on some flag. For example:

void foo(int case_sensitive)
{
    int (*cmpfunc)(const char *, const char *);
    if (case_sensitive) { 
        cmpfunc = strcmp;
    } else {
        cmpfunc = strcasecmp;
    }

    ...
    // set strings str1 and str2
    ...
    if (cmpfunc(str1, str2) == 0)) {
        printf("strings are equal\n");
    } else {
        printf("strings are not equal\n");
    }
    ...
    // set strings str3 and str4
    ...
    if (cmpfunc(str3, str4) == 0)) {
        printf("strings are equal\n");
    } else {
        printf("strings are not equal\n");
    }
    ...
}

Upvotes: 6

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