Question

Let’s say I have a Rectangle class like so:

class Rectangle {
public:
    double width;
    double height;
}

Now I wish to store two possibly different lists of Rectangles as heaps, except that the first heap should be based on width and the second on height. Also, I wish to use the stl’s make_heap function to heapify. Ideally, I should be able to call .heapify() on a heap, and based on the class that it belongs to, the heap in question should heapify itself by passing the correct comparison function to make_heap, perhaps using dynamic dispatch. What I have is the following:

class Heap {
public:
    vector<Rectangle> data;
    virtual bool comp(Rectangle a, Rectangle b);
    void heapify() { make_heap(data.begin(), data.end(), comp); }
    // other methods that make use of comp directly
}

class WidthHeap : public Heap {
    bool comp(Rectangle a, Rectangle b); // compares by width
}

class HeightHeap : public Heap {
    bool comp(Rectangle a, Rectangle b); // compares by height
}

This is all wrong because I guess I just don’t understand functions in C++, which is why I would like your help.

Answer 1

Because comp is a member function pointer, it cannot be called without passing this. You need to bind this to it:

std::make_heap(data.begin(), data.end(), std::bind(&Heap::comp, this, _1, _2));

std::bind can be found in the <functional> header in C++11, and also available as std::tr1::bind in <tr1/functional> using TR1. If you can’t use TR1 or C++11, there’s a Boost library for it.

Demo: http://ideone.com/5zhmg