Solved

More CPtrArray sorting problems!

Posted on 2008-06-18
5
804 Views
Last Modified: 2013-11-20
Ah hello.

A follow up to a previous question about sorting CPtrArrays.  Consider this code:


class CMyClass
{
public:
      CMyClass ( DWORD d ) : m_dw ( d ) {}
      ~CMyClass() {}
      DWORD m_dw;
      CString s;
};

typedef int (__cdecl *GENERICCOMPAREFN)(const void * elem1, const void * elem2);
typedef int (__cdecl *STRINGCOMPAREFN)(const CMyClass * elem1, const CMyClass * elem2);

class CMyArray : public CPtrArray
{
public:
      void Sort(GENERICCOMPAREFN pfnCompare = Compare)
      {
            qsort ( GetData(), GetSize(), sizeof ( CMyClass ), ( GENERICCOMPAREFN ) pfnCompare );
      }
protected:

      static int __cdecl Compare(const void * pMyClass1, const void * pMyClass2 )
      {
            if ( !pMyClass1 || !pMyClass2 ) return 0;
            CMyClass* pActualMyClass1 = *(CMyClass**)pMyClass1;
            CMyClass* pActualMyClass2 = *(CMyClass**)pMyClass2;

            return pActualMyClass1 ->m_dw < pActualMyClass2 ->m_dw;
      }
};

int main()
{
      CMyArray m_array;

      m_array.Add ( ( void*) new CMyClass ( 1 ) );
      m_array.Add ( ( void*) new CMyClass ( 3 ) );
      m_array.Add ( ( void*) new CMyClass ( 2 ) );

            m_array.Sort();

            return 0;
}

I am finding that whenever I have a CString member in the class object contained in my array, the code generates an access violation in the Compare() functoin.  More specifically, the first time the function is called by qsort(), I get my two pointers pActualMyClass1 and pActualMyClass2 both valid.  The second time however, pMyClass1 is always 0xcdcdcdcd.  So the attempt to access m_dw fails.

If I remove the CString member, this does not happen.

Can someone please tell me What is going on here, and how to get around it?  I need my CString member.

TIA
0
Comment
Question by:mrwad99
  • 2
  • 2
5 Comments
 
LVL 30

Accepted Solution

by:
Zoppo earned 125 total points
ID: 21813646
Hi mrwad99,

I guess this line is wrong:

>             qsort ( GetData(), GetSize(), sizeof ( CMyClass ), ( GENERICCOMPAREFN ) pfnCompare );

instead of 'sizeof ( CMyClass )' you should pass 'sizeof ( CMyClass* )' since you compare pointers to CMyClass, not instances of CMyClass.

Hope that helps,

ZOPPO
0
 
LVL 19

Author Comment

by:mrwad99
ID: 21813883
Thank you ZOPPO :o)
0
 
LVL 30

Expert Comment

by:Zoppo
ID: 21813907
You're welcome ...
0
 
LVL 19

Expert Comment

by:alb66
ID: 21813914
Zoppo is right. The previous case(without CString) worked only because
sizeof(DWORD) == sizeof(void*)
0
 
LVL 19

Author Comment

by:mrwad99
ID: 21813932
Ah, I see!  Thanks for the follow up alb66.
0

Featured Post

What Security Threats Are You Missing?

Enhance your security with threat intelligence from the web. Get trending threat insights on hackers, exploits, and suspicious IP addresses delivered to your inbox with our free Cyber Daily.

Join & Write a Comment

In this article, I'll describe -- and show pictures of -- some of the significant additions that have been made available to programmers in the MFC Feature Pack for Visual C++ 2008.  These same feature are in the MFC libraries that come with Visual …
Introduction: Finishing the grid – keyboard support for arrow keys to manoeuvre, entering the numbers.  The PreTranslateMessage function is to be used to intercept and respond to keyboard events. Continuing from the fourth article about sudoku. …
The goal of the video will be to teach the user the difference and consequence of passing data by value vs passing data by reference in C++. An example of passing data by value as well as an example of passing data by reference will be be given. Bot…
The viewer will be introduced to the member functions push_back and pop_back of the vector class. The video will teach the difference between the two as well as how to use each one along with its functionality.

746 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

12 Experts available now in Live!

Get 1:1 Help Now