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More CPtrArray sorting problems!

Posted on 2008-06-18
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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
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Question by:mrwad99
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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
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LVL 19

Author Comment

by:mrwad99
ID: 21813883
Thank you ZOPPO :o)
0
 
LVL 31

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.
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