priority_queue class field using a comparator that access another class field

Question

Hello I need to create a class which contains a priority_queue field whose comparator function needs to access another field in the class. In short I need to write something like this:

class A
{
    B foo;
    priority_queue<C,vector<C>,comparator> bar;
}

where the comparator definition is something like

bool comparator(const C& c1, const C& c2)
{
    //compute the boolean value using c1, c2 and the field foo
}

Is it possible to obtain this result in some way and where I have to define the comparator function?

Answer 1

There are two steps to do this.

First, your comparator needs a constructor that saves a reference to an instance of A

class comparator {

     A &a;

public:
     comparator(A &a_arg) : a(a_arg)
     {
     }

     bool operator()(const C &first, const C &second) const
     {
          // The comparator can use "a" to access the contents of the
          // A class.
     }
};

And the second step is for A's constructor to initialize its priority_queue member using an explicit comparator that gets constructed from *this:

A::A() : bar(comparator(*this))
{
   // ...
}

Note: keep in mind the reference to *this in the comparator. If the instance of A gets copied, the this reference will not be valid in the copy. Your A class should have either a deleted copy-constructor, or an explicit copy-constructor that initializes the copy-constructed A priority_queue, accordingly.