Tukukkk

The program tried to compile, but I found out that the error displays, 'control may reach end of non-void function'. I suppose how to do with if condition after putting the return compare. I have been figuring out how to solve this issue.

   int compare(const void *a, const void *b)

  {

    if (*(int *)a < *(int *)b)

        return -1;

    if (*(int *)a == *(int *)b)

        return 0;

    if (*(int *)a > *(int *)b)

        return 1;

  }

The 'control may reach end of non-void function' shouldn't be detected as there is actually not such possibility. May be, the data flow analyze doesn't detect this properly.

However, there is a much nicer alternative to implement this and it will solve your issue as well:

int compare(const void *a, const void *b)

{

  return (*(const int*)b < *(const int*)a) - (*(const int*)a < *(const int*)b);

}

Btw. the performance of this code is better as you don't need any branches anymore.

A small MCVE for demonstration:

#include <iostream>



int compare(const void *a, const void *b)

{

  return (*(const int*)b < *(const int*)a) - (*(const int*)a < *(const int*)b);

}



int main()

{

  int a = 1, b = 2;

  std::cout << "compare(&a, &b): " << compare(&a, &b) << 'n';

  std::cout << "compare(&b, &a): " << compare(&b, &a) << 'n';

  std::cout << "compare(&a, &a): " << compare(&a, &a) << 'n';

  return 0;

}

Output:

compare(&a, &b): -1

compare(&b, &a): 1

compare(&a, &a): 0

Live Demo on coliru

I must admit that somebody “forced” me to this nice comparison trick when I answered

SO: Sorting an array of integers in alternate fashion using qsort function..

This is the explanation I gave there:

How it works:

In case a < b: (a > b) - (a < b) ⇒ 0 - 1 ⇒ -1

In case a == b: (a > b) - (a < b) ⇒ 0 - 0 ⇒ 0

In case a > b: (a > b) - (a < b) ⇒ 1 - 0 ⇒ 1

Don't use C-style casts in C++.

As you know that you covered all conditions, the last test is redundant:

int compare(const void *a, const void *b)

{

int a1 = *static_cast<const int *>(a);

int b1 = *static_cast<const int *>(b);

if (a1 < b1)

    return -1;

if (a1 == b1)

    return 0;

return 1;

}

If you don't know what is the actual type of the objects behind those pointers, you may want to do:

#include <cstring>



int f(const void *a, const void *b)

{

    int ai;

    int bi;

    std::memcpy(&ai, a, sizeof(int));

    std::memcpy(&bi, b, sizeof(int));

    return (bi < ai) - (ai < bi);

}

The main point here is that the compiler can’t recognize that your conditions cover all possible cases. => if none of your conditions is met, the function ends without a return statement. You should use if - else if - else here:

if (…) { … }

else if (…) { … }

else { … }

This will help compiler understand your code better. And you drop once of the conditions.

Avoiding branches is a good idea, usually at the price of loosing readability. Check out @Scheff’s answer for that.