How to pass a method to qsort? - c++

Question

There is a class that contains some data and it sorts them at some point of time. I use qsort() and I'd like to keep the comparing function within the class as a method. The question is how to pass a method to qsort() so that the compiler (g++) don't throw any warnings?
Attempt 1:
int Data::compare_records(void * rec_1, void * rec_2){
// [...]
}
void Data::sort(){
qsort(records, count, sizeof(*records), &Data::compare_records);
}
This way generates an error:
error: cannot convert ‘int (Data::*)(const void*, const void*)’ to ‘int (*)(const void*, const void*)’ for argument ‘4’ to ‘void qsort(void*, size_t, size_t, int (*)(const void*, const void*))’
Attempt 2 :
void Data::sort(){
qsort(
records, count, sizeof(*records),
(int (*)(const void*, const void*)) &Data::compare_records
);
}
This way generates a warning:
warning: converting from ‘int (Data::*)(const void*, const void*)’ to ‘int (*)(const void*, const void*)’
How to do it the right way then?

Answer 1

If you must use qsort and not std::sort (recommended), declaring the member method as static should be enough.

Answer 2

You pass the function as &Data::compare_records, but you should pass it as Data::compare_records and also make it static

Answer 3

Don't use qsort in C++. Use std::sort and boost/std::bind. Member function-pointer cannot been converted to function-pointer. Your method should be static, or it should be free function.
see Is the type of “pointer-to-member-function” different from “pointer-to-function”? for an explanation.

Answer 4

This code may also help as a hint, for std::sort despite the fact I Use Qt's qSort()
Functors can be very cool.
struct randomWSort
{
SatoshiGame* This;
randomWSort(SatoshiGame* g){This=g;}
bool operator()(QString& a, QString& b)
{
return This->randomWSort(a,b);
}
};
bool SatoshiGame::randomWSort(QString& a, QString& b)
{
return rand->rnd() %2;
}
QString SatoshiGame::getRandomString(QStringList words)
{
qSort(words.begin(), words.end(), ::randomWSort(this) );
return words.at(0);
}