invalid use of non-static member function in qsort C++

Question

here is my function in class SuffixArray:

int pstrcmp(const void *a, const void *b) {

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

I used this comparison function in qsort：

qsort(ap, len1+len2, sizeof(char *),pstrcmp);

which ap is a pointer array of suffix

When I compile it, there is an error: invalid use of non-static member function

and I use notepad++ to compile it, it provide that

 error: cannot convert 'SuffixArray::pstrcmp' from type 'int (SuffixArray::)(const void*, const void*)' to type 'int (*)(const void*, const void*)'
 qsort(ap, len1+len2, sizeof(char *),pstrcmp);

is there anyone can help me?

Answer 1

In C++ you need to pass a free-standing function or a static member function (as opposed to a non-static member function) to qsort , because calling conventions of non-static member functions require an instance to be passed.

You have two solutions to this problem:

• Move the declaration of pstrcmp out of the SuffixArray class, or
• Declare pstrcmp static in the class.