Bruce Eckel's Thinking in C++, 2nd Ed Contents | Prev | Next

Priority queues

When you push( ) an object onto a priority_queue, that object is sorted into the queue according to a function or function object (you can allow the default less template to supply this, or provide one of your own). The priority_queue ensures that when you look at the top( ) element it will be the one with the highest priority. When you’re done with it, you call pop( ) to remove it and bring the next one into place. Thus, the priority_queue has nearly the same interface as a stack, but it behaves differently.

Like stack and queue, priority_queue is an adapter which is built on top of one of the basic sequences – the default is vector.

It’s trivial to make a priority_queue that works with ints:

//: C20:PriorityQueue1.cpp
#include <iostream>
#include <queue>
#include <cstdlib>
#include <ctime>
using namespace std;

int main() {
  priority_queue<int> pqi;
  srand(time(0)); // Seed random number generator
  for(int i = 0; i < 100; i++)
    pqi.push(rand() % 25);
  while(!pqi.empty()) {
    cout << pqi.top() << ' ';
    pqi.pop();
  }
} ///:~

This pushes into the priority_queue 100 random values from 0 to 24. When you run this program you’ll see that duplicates are allowed, and the highest values appear first. To show how you can change the ordering by providing your own function or function object, the following program gives lower-valued numbers the highest priority: 

//: C20:PriorityQueue2.cpp
// Changing the priority
#include <iostream>
#include <queue>
#include <cstdlib>
#include <ctime>
using namespace std;

struct Reverse {
  bool operator()(int x, int y) {
    return y < x;
  }
};

int main() {
  priority_queue<int, vector<int>, Reverse> pqi;
  // Could also say:
  // priority_queue<int, vector<int>, 
  //   greater<int> > pqi;
  srand(time(0));
  for(int i = 0; i < 100; i++)
    pqi.push(rand() % 25);
  while(!pqi.empty()) {
    cout << pqi.top() << ' ';
    pqi.pop();
  }
} ///:~

Although you can easily use the Standard Library greater template to produce the predicate, I went to the trouble of creating Reverse so you could see how to do it in case you have a more complex scheme for ordering your objects.

If you look at the description for priority_queue, you see that the constructor can be handed a “Compare” object, as shown above. If you don’t use your own “Compare” object, the default template behavior the less template function. You might think (as I did) that it would make sense to leave the template instantiation as priority_queue<int>, thus using the default template arguments of vector<int> and less<int>. Then you could inherit a new class from less<int>, redefine operator( ) and hand an object of that type to the priority_queue constructor. I tried this, and got it to compile, but the resulting program produced the same old less<int> behavior. The answer lies in the less< > template:

template <class T>
struct less : binary_function<T, T, bool> {
  // Other stuff...
  bool operator()(const T& x, const T& y) const {
    return x < y;
  }
};

The operator( ) is not virtual, so even though the constructor takes your subclass of less<int> by reference (thus it doesn’t slice it down to a plain less<int>), when operator( ) is called, it is the base-class version that is used. While it is generally reasonable to expect ordinary classes to behave polymorphically, you cannot make this assumption when using the STL.

Of course, a priority_queue of int is trivial. A more interesting problem is a to-do list, where each object contains a string and a primary and secondary priority value: 

//: C20:PriorityQueue3.cpp
// A more complex use of priority_queue
#include <iostream>
#include <queue>
#include <string>
using namespace std;

class ToDoItem {
  char primary;
  int secondary;
  string item;
public:
  ToDoItem(string td, char pri ='A', int sec =1)
    : item(td), primary(pri), secondary(sec) {}
  friend bool operator<(
    const ToDoItem& x, const ToDoItem& y) {
    if(x.primary > y.primary) 
      return true;
    if(x.primary == y.primary)
      if(x.secondary > y.secondary) 
        return true;
    return false;
  }
  friend ostream& 
  operator<<(ostream& os, const ToDoItem& td) {
    return os << td.primary << td.secondary 
      << ": " << td.item;
  }
};

int main() {
  priority_queue<ToDoItem> toDoList;
  toDoList.push(ToDoItem("Empty trash", 'C', 4));
  toDoList.push(ToDoItem("Feed dog", 'A', 2));
  toDoList.push(ToDoItem("Feed bird", 'B', 7));
  toDoList.push(ToDoItem("Mow lawn", 'C', 3));
  toDoList.push(ToDoItem("Water lawn", 'A', 1));
  toDoList.push(ToDoItem("Feed cat", 'B', 1));
  while(!toDoList.empty()) {
    cout << toDoList.top() << endl;
    toDoList.pop();
  }
} ///:~

ToDoItem’s operator< must be a non-member function for it to work with less< > . Other than that, everything happens automatically. The output is: 

A1: Water lawn
A2: Feed dog
B1: Feed cat
B7: Feed bird
C3: Mow lawn
C4: Empty trash

Note that you cannot iterate through a priority_queue. However, it is possible to emulate the behavior of a priority_queue using a vector, thus allowing you access to that vector. You can do this by looking at the implementation of priority_queue, which uses make_heap( ), push_heap( ) and pop_heap( ) (they are the soul of the priority_queue; in fact you could say that the heap is the priority queue and priority_queue is just a wrapper around it). This turns out to be reasonably straightforward, but you might think that a shortcut is possible. Since the container used by priority_queue is protected (and has the identifier, according to the Standard C++ specification, named c) you can inherit a new class which provides access to the underlying implementation: 

//: C20:PriorityQueue4.cpp
// Manipulating the underlying implementation
#include <iostream>
#include <queue>
#include <cstdlib>
#include <ctime>
using namespace std;

class PQI : public priority_queue<int> {
public:
  vector<int>& impl() { return c; }
};

int main() {
  PQI pqi;
  srand(time(0));
  for(int i = 0; i < 100; i++)
    pqi.push(rand() % 25);
  copy(pqi.impl().begin(), pqi.impl().end(),
    ostream_iterator<int>(cout, " "));
  cout << endl;
  while(!pqi.empty()) {
    cout << pqi.top() << ' ';
    pqi.pop();
  }
} ///:~

However, if you run this program you’ll discover that the vector doesn’t contain the items in the descending order that you get when you call pop( ), the order that you want from the priority queue. It would seem that if you want to create a vector that is a priority queue, you have to do it by hand, like this: 

//: C20:PriorityQueue5.cpp
// Building your own priority queue
#include <iostream>
#include <queue>
#include <cstdlib>
#include <ctime>
using namespace std;

template<class T, class Compare>
class PQV : public vector<T> {
  Compare comp;
public:
  PQV(Compare cmp = Compare()) : comp(cmp) {
    make_heap(begin(), end(), comp);
  }
  const T& top() { return front(); }
  void push(const T& x) {
    push_back(x);
    push_heap(begin(), end(), comp);
  }
  void pop() {
    pop_heap(begin(), end(), comp);
    pop_back();
  }  
};

int main() {
  PQV<int, less<int> > pqi;
  srand(time(0));
  for(int i = 0; i < 100; i++)
    pqi.push(rand() % 25);
  copy(pqi.begin(), pqi.end(),
    ostream_iterator<int>(cout, " "));
  cout << endl;
  while(!pqi.empty()) {
    cout << pqi.top() << ' ';
    pqi.pop();
  }
} ///:~

But this program behaves in the same way as the previous one! What you are seeing in the underlying vector is called a heap. This heap represents the tree of the priority queue (stored in the linear structure of the vector), but when you iterate through it you do not get a linear priority-queue order. You might think that you can simply call sort_heap( ), but that only works once, and then you don’t have a heap anymore, but instead a sorted list. This means that to go back to using it as a heap the user must remember to call make_heap( ) first. This can be encapsulated into your custom priority queue: 

//: C20:PriorityQueue6.cpp
#include <iostream>
#include <queue>
#include <algorithm>
#include <cstdlib>
#include <ctime>
using namespace std;

template<class T, class Compare>
class PQV : public vector<T> {
  Compare comp;
  bool sorted;
  void assureHeap() {
    if(sorted) {
      // Turn it back into a heap:
      make_heap(begin(), end(), comp);
      sorted = false;
    }
  }    
public:
  PQV(Compare cmp = Compare()) : comp(cmp) {
    make_heap(begin(), end(), comp);
    sorted = false;
  }
  const T& top() {
    assureHeap();
    return front(); 
  }
  void push(const T& x) {
    assureHeap();
    // Put it at the end:
    push_back(x);
    // Re-adjust the heap:
    push_heap(begin(), end(), comp);
  }
  void pop() {
    assureHeap();
    // Move the top element to the last position:
    pop_heap(begin(), end(), comp);
    // Remove that element:
    pop_back();
  }
  void sort() {
    if(!sorted) {
      sort_heap(begin(), end(), comp);
      reverse(begin(), end());
      sorted = true;
    }
  }
};

int main() {
  PQV<int, less<int> > pqi;
  srand(time(0));
  for(int i = 0; i < 100; i++) {
    pqi.push(rand() % 25);
    copy(pqi.begin(), pqi.end(),
      ostream_iterator<int>(cout, " "));
    cout << "\n-----\n";
  }
  pqi.sort();
  copy(pqi.begin(), pqi.end(),
    ostream_iterator<int>(cout, " "));
  cout << "\n-----\n";
  while(!pqi.empty()) {
    cout << pqi.top() << ' ';
    pqi.pop();
  }
} ///:~

If sorted is true, then the vector is not organized as a heap, but instead as a sorted sequence. assureHeap( ) guarantees that it’s put back into heap form before performing any heap operations on it.

The first for loop in main( ) now has the additional quality that it displays the heap as it’s being built.

The only drawback to this solution is that the user must remember to call sort( ) before viewing it as a sorted sequence (although one could conceivably override all the methods that produce iterators so that they guarantee sorting). Another solution is to build a priority queue that is not a vector, but will build you a vector whenever you want one: 

//: C20:PriorityQueue7.cpp
// A priority queue that will hand you a vector
#include <iostream>
#include <queue>
#include <algorithm>
#include <cstdlib>
#include <ctime>
using namespace std;

template<class T, class Compare>
class PQV {
  vector<T> v;
  Compare comp;
public:
  // Don't need to call make_heap(); it's empty:
  PQV(Compare cmp = Compare()) : comp(cmp) {}
  void push(const T& x) {
    // Put it at the end:
    v.push_back(x);
    // Re-adjust the heap:
    push_heap(v.begin(), v.end(), comp);
  }
  void pop() {
    // Move the top element to the last position:
    pop_heap(v.begin(), v.end(), comp);
    // Remove that element:
    v.pop_back();
  }
  const T& top() { return v.front(); }
  bool empty() const { return v.empty(); }
  int size() const { return v.size(); }
  typedef vector<T> TVec;
  TVec vector() {
    TVec r(v.begin(), v.end());
    // It’s already a heap
    sort_heap(r.begin(), r.end(), comp);
    // Put it into priority-queue order:
    reverse(r.begin(), r.end());
    return r;
  }
};

int main() {
  PQV<int, less<int> > pqi;
  srand(time(0));
  for(int i = 0; i < 100; i++)
    pqi.push(rand() % 25);
  const vector<int>& v = pqi.vector();
  copy(v.begin(), v.end(),
    ostream_iterator<int>(cout, " "));
  cout << "\n-----------\n"; 
  while(!pqi.empty()) {
    cout << pqi.top() << ' ';
    pqi.pop();
  }
} ///:~

PQV follows the same form as the STL’s priority_queue, but has the additional member vector( ), which creates a new vector that’s a copy of the one in PQV (which means that it’s already a heap), then sorts it (thus it leave’s PQV’s vector untouched), then reverses the order so that traversing the new vector produces the same effect as popping the elements from the priority queue.

You may observe that the approach of inheriting from priority_queue used in PriorityQueue4.cpp could be used with the above technique to produce more succinct code: 

//: C20:PriorityQueue8.cpp
// A more compact version of PriorityQueue7.cpp
#include <iostream>
#include <queue>
#include <algorithm>
#include <cstdlib>
#include <ctime>
using namespace std;

template<class T>
class PQV : public priority_queue<T> {
public:
  typedef vector<T> TVec;
  TVec vector() {
    TVec r(c.begin(), c.end());
    // c is already a heap
    sort_heap(r.begin(), r.end(), comp);
    // Put it into priority-queue order:
    reverse(r.begin(), r.end());
    return r;
  }
};

int main() {
  PQV<int> pqi;
  srand(time(0));
  for(int i = 0; i < 100; i++)
    pqi.push(rand() % 25);
  const vector<int>& v = pqi.vector();
  copy(v.begin(), v.end(),
    ostream_iterator<int>(cout, " "));
  cout << "\n-----------\n"; 
  while(!pqi.empty()) {
    cout << pqi.top() << ' ';
    pqi.pop();
  }
} ///:~

The brevity of this solution makes it the simplest and most desirable, plus it’s guaranteed that the user will not have a vector in the unsorted state. The only potential problem is that the vector( ) member function returns the vector<T> by value, which might cause some overhead issues with complex values of the parameter type T.

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