Help Please: Passing Functor Object with Template type parameter

From: CoolPint (coolpint_at_yahoo.co.uk)
Date: 12/23/03 

Date: 23 Dec 2003 02:28:51 -0800

I have implemented a generic priority queue below and tested it works
fine, but I have one small problem I cannot understand. I have type
parameter F which determines the priority so that users can
instantiate in the following ways

PQueue<int> pq1;
PQueue<int, Functor> pq2; // where Functor is a name of user-defined
class

I also added another constructor to accept a function pointer so that
users can use normal functions as callback too. For example,

PQueue<int, bool (*) (int, int)> pq3(function);
// where function is a user-defined function which accepts ints and a
bool

So far so good. I then got a bit curious and issued something like
this:

PQueue<int, Functor> pq4(Functor()); <------- This causes the error

thinking this would assign Functor class to F and then assign an
object of Functor class to cmp, but it got me a compiler error which I
cannot understand.
I am interested to know what the error message means and what is
causing the problem. Can somebody kindly explain what is happening?

The compiler error I got when I tried to compile above with g++ is
something like below:

pqdriver.cpp:23: request for member `enQueue' in `numq(isGreater
(*)())', which
   is of non-aggregate type `PQueue<int, isGreater> ()(isGreater
(*)())'

I would very much appreciate a kind explanation as to what the problem
is what is causing it. And maybe a solution,, but I am more interested
in knowing what's happening so that I can learn from it. Thanks a lot
in advance!

class DefaultHeapOrder {
public:
   template <typename T>
    bool operator()(const T & a, const T & b)
   { return ( a < b ? true : false); }
};

template <typename T, typename F = DefaultHeapOrder >
class PQueue {
public:
  PQueue();
  explicit PQueue(F);
  template <typename Iterator>
  PQueue(Iterator, Iterator);
  bool isEmpty() const;
  bool isFull() const;
  bool enQueue(const T &);
  bool deQueue(T &);
  bool getTop(T &);
  void clear();
private:
  int counter;
  DArray<T> heap;
  F cmp;
  void buildHeap();
  void shiftdown(int);
};

Relevant Pages