Using a function pointer from one class in a funct

Question

I have looked at the following threads, but they do not seem to help resolve the issue:

Function pointer to member function - this thread does not help because the access is made within main and not another class

one class should invoke method of another class using a function pointer - I could not follow the accepted answer as it is not clear to me how to use <functional> header.

I have a Useful Function class, UF, in which I declare and define all common utility functions for my program. One of these functions is

int UF::compar_dbl_asc(const void *a, const void *b)//comparator for sorting 
{                                                   //array of double in ascending order
    int aa = *((int *)a), bb = *((int *)b);
    if (base_arr_dbl[aa] < base_arr_dbl[bb])
        return -1;
    if (base_arr_dbl[aa] == base_arr_dbl[bb])
        return 0;
    if (base_arr_dbl[aa] > base_arr_dbl[bb])
        return 1;
}

This is a comparator function that I intend to use within qsort() for sorting doubles in ascending order.

Within UF class definition, I also have double* base_arr_dbl; as a declaration. This is the array of values (doubles) that qsort will use.

Now, in a different class of mine named SEP, I have a function, fn1 wherein I would like to have base_arr_dbl point to a local (to fn1) double array and I would like to invoke qsort using the comparator function in UF. To be very specific although this is possibly not needed, I do not sort the actual values' array, but will sort an array of indices sortedindices[], such that sortedindices[0] will hold the index of the smallest entry in base_arr_dbl[] array. That is, the sorted order is base_arr_dbl[sortedindices[0]], base_arr_dbl[sortedindices[1]], etc.

So, I do this:

void SEP::fn1(UF *uf) {

    double ratio[100];
    //populate ratio's entries
    //Now, sort ratio's entries.
    uf->set_base_arr_dbl(ratio); //This function is defined as
   //void UF::set_base_arr_dbl(double *ptr) { base_arr_dbl = ptr; }
    qsort(sortedindices, 100, sizeof(double), uf->compar_dbl_asc);

}

However, the line qsort(sortedindices, 100, sizeof(double), uf->compar_dbl_asc); throws the following compile time error:

error C3867: 'USEFUL_FUNCTIONS::compar_dbl_asc': non-standard syntax; use '&' to create a pointer to member

I tried having &uf->compar_dbl_asc but that gives the error:

error C2276: '&': illegal operation on bound member function expression

Any way to help resolve this is appreciated.

Answer 1

As the compiler clearly tells in the error messages, neither uf->compar_dbl_asc nor &(uf->compar_dbl_asc) is appropriate for use as an argument to qsort.

You can use compar_dbl_asc as an argument to qsort using one of the following approaches.

1. Make compar_dbl_asc a static member function of the class.
2. Better yet, create a namespace for your app and define compar_dbl_asc in the namespace. UF could be that namespace unless it must be a class for some other reasons.

Another choice would be forego use of qsort in favor of std::sort. The latter gives you more options. You can use a functor or a lambda function when you use std::sort, which will allow you to use UF::compar_dbl_asc.

std::sort(sortedindices, sortedindices+100,
          [=uf](int a, int b) { return uf->compar_dbl_asc(a, b); });

One thing to note is that if you choose the last approach, the signature of UF::compar_dbl_asc can be changed to a more user friendly variation.

bool UF::compar_dbl_asc(int a, int b)
   return (base_arr_dbl[a] < base_arr_dbl[b]);
}