CODEWITHSUNDEEP
c++pointers
Green goblin
Green goblin

Reputation: 9996

casting in qsort()

The below program sorts all the suffices of a string using qsort() library function.

int sacomp(const void *a, const void *b) 
{
    return strcmp(*(const char**)a, *(const char**)b); <------------
}

void sort(string s) 
{
    int size = s.size();
    char const *data = s.c_str();
    char const **sa = new char const *[size+1];

    for(int i = 0; i < size; i++)
        sa[i] = data+i;

    qsort(sa, size, sizeof(sa[0]), sacomp); // O(n * Lon n)
}

int main() 
{
    string s("ABCCCDEFABBABBA");
    sort(s);

    return 0;
}

I am not able to understand the casting done in the sacomp() method. 

strcmp(*(const char**)a, *(const char**)b);

Why a is casted to const char** and then being de-referenced?

Upvotes: 1

Views: 172

Answers (2)

klm123
klm123

Reputation: 12865

Your element is char *, therefore you should change void on char * and get pointer to char *, which is char **. Meanwhile strcmp needs char *.

Upvotes: 0

Rango
Rango

Reputation: 1105

qsort passes pointer of elements. You passed sa as char**, so every element is char*, and qsort passes char**, finally you got char** in the sort function.

Upvotes: 1

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