data corruption on pointer cast

Sun, 15 Jul 2007 22:56:56 -0700

Names matt,

I implemented a set class as follows:

template<class T>
class Element
    virtual int operator == (T) = 0;
    virtual int hash() = 0;

 * A efficient hash implementation of a Queue-set that does not allow
addition of duplicates.
    @author matt gara <>
template<class T, int M = p>
  class QueueSet


    int exists(Element<T> * elem)
        int h = elem->hash()%M;
        for (int i=0; i < size_t[h]; i++)
            if ( *((T*)elem) == *((T*)set[h][i]))
                return 1;
        return 0;

    int add(Element<T> * elem)
        int h = elem->hash()%M;
        if (size_t[h] == max[h])
            set[h] = (Element<T> **)realloc(set[h], sizeof(Element<T>
*)*(max[h] + P));
            max[h] += P;
        if (exists(elem))
            return 0; //failed to add
        set[h][size_t[h]] = elem;
        size_t[h] += 1;
        return 1;



    Element<T> ** set[M];
    int size;
    int size_t[M];
    int max[M];

And it works up until I try adding the 52nd element and it throws an

*** glibc detected *** /home/matt/sudokusolver/debug/./src/
sudokusolver: double free or corruption (fasttop): 0x0804d170 ***
======= Backtrace: =========

I've done some debugging and it looks like the exception happens in
the exists member. I'm pretty certain the exception is caused by the
following line:

            if ( *((T*)elem) == *((T*)set[h][i]))

which is weird because it works for the first 51st elements and then
throws this nutty error.

If the code doesn't speak for itself, T is a class that implements
Element to get the hash and ==. The hash is used in creating the table
and the == is supposed to be used to make sure duplicated do not
exist, but clearly its not working properly. Thanks.

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