initialization for STL priority queue

Question

I\'m still confused about priority queue in STL. Here is the objective I wanna achieve, say: I have a structure called Record, which contains a string word and a int counter. Fo

Answer 1

std::priority_queue cannot magically know how to sort the elements. You must tell it how to do so. The way to do that is to give priority_queue a functor type which, when called with two objects, returns whether the first argument is "less than" the second, however you want to define that. This functor is a template parameter to the priority_queue.

The default parameter is std::less<type>, which requires that type (what you're putting in the queue) has an overloaded operator<. If it doesn't, then you either have to provide one or you have to provide a proper comparison functor.

For example:

struct Comparator
{
  bool operator()(const Record& lhs, const Record& rhs)
  {
    return lhs.count>rhs.count;
  }
};

std::priority_queue<Record, std::vector<Record>, Comparator> myQ;

The reason that doesn't work with just an overload on Record is because you didn't tell the priority_queue that it was the comparison. Also, the type used for comparison needs to be default constructable, so that the priority_queue can create and destroy the objects at will.

Though to be honest, I don't know why you don't just stick them in a std::set if you want to sort them. Or just run std::sort on the std::vector of items.

Answer 2

Your code does work, with two small changes:

• Uncomment the definition of Record::operator<(), since that's needed by the priority queue's default comparator.
• Change the declaration to bool operator<(const Record &) const (note the extra const), since the priority queue has to compare using references to const objects.

Alternatively, declare it as a free function, outside the class definition:

bool operator<(const Record &l, const Record &r) {return l.count > r.count;}

or define your own functor, and provide that as the appropriate template argument:

struct CompareRecords
{
    bool operator()(const Record &l, const Record &r) {return l.count > r.count;}
};

typedef priority_queue<Record, vector<Record>, CompareRecords> RecordQueue;