使用qsort对2D数组进行排序-正确的类型转换

Question

I am having a problem with using c++ qsort compare function to correctly lexicographically sort 2d array of ints. I have already read many similar questions here but with no success. When making the custom compare function, its arguments are in form of

int compar (const void* p1, const void* p2)

now, I know that it the arguments are pointers to my variables, that are each ponters to one row of my array. I would like to index and compare my array in the normal conventional way using something like the following.

if(p1[0] <= p2[0]) {...}

Because I know, that the format

p1[i]

is just a shortcut for pointer arithmetic, I guess when I am recieving an argument "pointer to pointer to int", I should typecast and use it this way:

if(*(int**)p1[0] <= *(int**)p2[0]) {...}

Compiler however gives me many these errors

main.cpp:8:20: error: ‘const void*’ is not a pointer-to-object type
main.cpp:8:37: warning: pointer of type ‘void *’ used in arithmetic  [-Wpointer-arith]

My question is, how to correctly typecast this, so it would allow me to compare my rows. Also, I would like to understand, what am I doing wrong here to avoid making these pointer related mistakes in the future.

Answer 1

It's because operator[] takes precedence over cast. b[0] is trying to be resolved first. So even if it's b[0] it complains you are doing pointer arithmetic on void* type. You need to cast 1st.

#include <iostream>
using namespace std;

int main() {
    int a[10];
    a[0] = 1337;
    a[1] = 42;
    void* b;
    b = (void*)a;

    //  cout << (int*)b[0] << '\n'; // will break
    cout << ((int*)b)[0] << '\n';

    // dedicated c++ cast imposes more verbose syntax - i.e. you can't miss the ()
    cout << static_cast<int*>(b)[1] << '\n';
    return 0;
}

That being said I still would encourage to use c++ dedicated features for what you're doing =).