qsort Comparator

This is a discussion on qsort Comparator within the C++ Programming forums, part of the General Programming Boards category; Hi, i would like to sort an array of object by their atribute, this is my testing project, the biggest ...

  1. #1
    Registered User
    Join Date
    Nov 2011

    qsort Comparator

    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{
         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();
        array[0]= b;
        array[1]= a;
        array[2]= c;
        array[3]= d;
        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;
    Last edited by flegmik; 10-19-2012 at 05:23 PM. Reason: some flows and mistakes

  2. #2
    Registered User
    Join Date
    Jun 2005
    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.
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy in reply to you, it is likely you deserve it. Suck it up, sunshine, and read this, this, and this before posting again.

  3. #3
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    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?
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  4. #4
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    New Zealand
    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();
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