qsort Comparator

[flegmik]

Hi, i would like to sort an array of object by their atribute, this is my testing project, the biggest problem is in compare function

Thanks for help

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <algorithm>
#include "string.h"
#include <cstdlib>


using namespace std;
using std::sort;

class A{
public:
     int size;


public :
     int  get(){
          return  size;
     }
};

int compare (const void *a, const void *b) {
    A *aO = *(A **)a;
    A *bO = *(A **)b;
    return ( aO.get() - bO.get());   // now i gets error request for member of get in aO which  is none-class A*  dunno why
}
int main()
{
    A **aray;
    array= new A *[4];


    for(int i=0; i < 4; i++)
    {
         array[i] = NULL;
    }


    A* a= new A();
    A* b= new A();
    A* c= new A();
    A* d= new A();
    a->size=15;


    array[0]= b;
    array[1]= a;
    array[2]= c;
    array[3]= d;
    array[0]->size=12;
    array[1]->size=10;
    array[2]->size=20;
    array[3]->size=15;


    qsort (pole[0],4,sizeof(A*),compare);


    for(int i=0; i < 3; i++)
    {
       if(array[i]!= NULL)
        cout << array[i]->get()  << endl;
    }
    return 0;
}

[grumpy]

a0 and b0 are pointers. Use the -> syntax to access members via pointers, not the . operator.

Also, your obvious attempt at hacking around with defining a0 and b0 in the form "A *a0 = *(A **)a" is unnecessary (and, worse, means your code has undefined behaviour, since it dereferences a pointer to a type it isn't). "A *a0 = (A *)a" will suffice.

[laserlight]

Out of curiosity, but you seem aware of std::sort, so is there any special reason why you want to use qsort? Just for experimentation?

[iMalc]

If your goal is to sort things, then just use std::sort, not qsort.
You need only define a less-than operator:

bool operator < (const A *left, const A *right)
{
    return left->get() < right->get();
}