How do I sort based on a vector of type template

How do I pass in / use a vector <T> vect in sort() function ?

Thanks for the help.

I have a few classes:

Class A {
  int x;
  int getX() {return x;}
}

Class B : public A {
  int x;
}

and a few vectors :

vector< A > a_vector;
vector< B > b_vector;

and I would like to call sortByX for these 2 classes

template <typename T>
void sortByX(const vector<T> &vectorName, int num)
{
    switch (num)
    {
        // Sort In Ascending
    case 0:
        sort(vectorName.begin(), vectorName.end(), sortByX_ascending);
        break;
        // Sort In Descending
    case 1:
        sort(vectorName.begin(), vectorName.end(), sortByX_descending);
        break;
    }
}

Below are the comparator functions:

bool sortByX_ascending(const A &lhs, const A &rhs) { return lhs.getX() < rhs.getX(); }

bool sortByX_descending(const A &lhs, const A &rhs) { return lhs.getX() > rhs.getX(); }

and when I invoke

sortByX(a_vector, 0);

I got an error

/Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:4016:17: error: no matching function for call to 'swap'
                swap(*__first, *__last);
                ^~~~
/Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:4201:5: note: in instantiation of function template specialization 'std::__1::__sort<bool (*&)(const A &, const A
      &), const A *>' requested here
    __sort<_Comp_ref>(__first, __last, __comp);