I was trying to remind how pointers worked in c and c++ and I found this very interesting video (Pointers in C/C++). In minute 3:14:00, he begins to talk about pointer functions and callbacks, and I ended a bit confused about the real application of them.

The example case he provides consists of a sorting algorithm that takes a function pointer as argument, which defines the comparison "rule" to follow (order from geater to smaller, order from smaller to greater, from greater to smaller given the absolute values…). He eventually ends with something like this:

#include<stdio.h>
int compare(int a, int b){
    if(a > b) return -1;
    return 1;
}
void MyBubbleSort(int A[], int n, int (*compare)(int,int)){
    int i,j,temp;
    for(i=0; i<n; i++){
        for(j=0; j<n-1; j++){
            if(compare(A[j], A[j+1]) > 0{
                temp = A[j];
                A[j] = A[j+1];
                A[j+1] = temp;
            }
        }
    }
}
int main(){
    int i, A[] = {3,2,1,5,6,4};
    MyBubbleSort(A,6,compare);
    for(i=0; i<6; i++) printf("%d ",A[i]);
}

When he wants to change the comparison rule, he changes the compare(int, int) content and that’s all. My question is, why would he do that, instead of just having a separate function called compare(int, int) that just does the same as the one showed in the code snippet, and just call that function from within MyBubbleSort(int[], int). Wouldn’t it just have the same behaviour? What are the benefits then? Are there any other interesting use cases?

Thank you very much for answers!

>Solution :

If I understand your question correctly, the main point is something that he mentions in the narration but does not include in the sample code: you could have several different comparison functions in your program, and use them at different times as appropriate, while still having just one MyBubbleSort function.

An example might look like:

int compare_increasing(int a, int b){
    if(a > b) return -1;
    return 1;
}

int compare_decreasing(int a, int b){
    if(a < b) return -1;
    return 1;
}

void MyBubbleSort(int A[], int n, int (*compare)(int,int));


}
int main(){
    // ...

    // sort in increasing order
    MyBubbleSort(A,6,compare_increasing);

    // sort in decreasing order
    MyBubbleSort(A,6,compare_decreasing);
}