聊聊C++标准库中优先队列priority_queue的源码

C++标准库提供了优先队列priority_queue，顾名思义，就是可以按照优先级出队的队列，而且时间复杂度为$O(logn)$，算法中有很多优化项就是用优先队列来优化的。

C++11的标准库是怎么构造出优先队列的呢？优先队列是用堆来构造的。所以，优先队列其实并不能叫队列，它的完整定义应该是这样的：优先队列是用堆来构造，包装成队列的适配器。

其实想一想，堆确实适合构造优先队列，优先队列每次需要弹出优先级最大的元素，而堆的堆顶恰巧就是优先级最大的元素，关于C++11中的堆，可以参考聊聊C++11标准库中堆(heap)算法的源码。

那就一起来看看MinGW中C++11的标准库stl_queue.h中的priority_queue的源码。

先来看看标准库对优先队列的介绍：

/**
   *  @brief  A standard container automatically sorting its contents.
   *
   *  @ingroup sequences
   *
   *  @tparam _Tp  Type of element.
   *  @tparam _Sequence  Type of underlying sequence, defaults to vector<_Tp>.
   *  @tparam _Compare  Comparison function object type, defaults to 
   *                    less<_Sequence::value_type>.
   *
   *  This is not a true container, but an @e adaptor.  It holds
   *  another container, and provides a wrapper interface to that
   *  container.  The wrapper is what enforces priority-based sorting 
   *  and %queue behavior.  Very few of the standard container/sequence
   *  interface requirements are met (e.g., iterators).
   *
   *  The second template parameter defines the type of the underlying
   *  sequence/container.  It defaults to std::vector, but it can be
   *  any type that supports @c front(), @c push_back, @c pop_back,
   *  and random-access iterators, such as std::deque or an
   *  appropriate user-defined type.
   *
   *  The third template parameter supplies the means of making
   *  priority comparisons.  It defaults to @c less<value_type> but
   *  can be anything defining a strict weak ordering.
   *
   *  Members not found in @a normal containers are @c container_type,
   *  which is a typedef for the second Sequence parameter, and @c
   *  push, @c pop, and @c top, which are standard %queue operations.
   *
   *  @note No equality/comparison operators are provided for
   *  %priority_queue.
   *
   *  @note Sorting of the elements takes place as they are added to,
   *  and removed from, the %priority_queue using the
   *  %priority_queue's member functions.  If you access the elements
   *  by other means, and change their data such that the sorting
   *  order would be different, the %priority_queue will not re-sort
   *  the elements for you.  (How could it know to do so?)
  */

这里面大致说了优先队列支持哪些成员函数，用了哪个数据结构来存储元素，排序规则等等。

template <typename _Tp, typename _Sequence = vector<_Tp>,
          typename _Compare = less<typename _Sequence::value_type>>
class priority_queue
{
public:
    typedef typename _Sequence::value_type value_type;
    typedef typename _Sequence::reference reference;
    typedef typename _Sequence::const_reference const_reference;
    typedef typename _Sequence::size_type size_type;
    typedef _Sequence container_type;

protected:
    //  See queue::c for notes on these names.
    _Sequence c;
    _Compare comp;
}

模板参数中，_Sequence默认是vector<_Tp>，也就是说priority_queue默认以vector作为容器，但是支持任意有push_back，pop_back，front成员函数，并且提供随机迭代器的容器；

然后就是容器必须的五个类型重定义；

成员函数只有序列c和比较函数comp。

从类的模板参数可以看出来，如果想要自定义优先级，那么就必须把第二个模板参数显示地写出来，即std::priority_queue<int, std::vector<int>, std::greater<int>>

public:
    explicit priority_queue(const _Compare &__x,
                            const _Sequence &__s)
        : c(__s), comp(__x)
    {
        std::make_heap(c.begin(), c.end(), comp);
    }

    explicit priority_queue(const _Compare &__x = _Compare(),
                            _Sequence &&__s = _Sequence())
        : c(std::move(__s)), comp(__x)
    {
        std::make_heap(c.begin(), c.end(), comp);
    }

    template <typename _InputIterator>
    priority_queue(_InputIterator __first, _InputIterator __last,
                   const _Compare &__x,
                   const _Sequence &__s)
        : c(__s), comp(__x)
    {
        c.insert(c.end(), __first, __last);
        std::make_heap(c.begin(), c.end(), comp);
    }

    template <typename _InputIterator>
    priority_queue(_InputIterator __first, _InputIterator __last,
                   const _Compare &__x = _Compare(),
                   _Sequence &&__s = _Sequence())
        : c(std::move(__s)), comp(__x)
    {
        c.insert(c.end(), __first, __last);
        std::make_heap(c.begin(), c.end(), comp);
    }

可以看到，priority_queue的构造函数就是在造一个堆。

    bool empty() const
    {
        return c.empty();
    }

    size_type size() const
    {
        return c.size();
    }

    const_reference top() const
    {
        return c.front();
    }

    void push(const value_type &__x)
    {
        c.push_back(__x);
        std::push_heap(c.begin(), c.end(), comp);
    }

    void push(value_type &&__x)
    {
        c.push_back(std::move(__x));
        std::push_heap(c.begin(), c.end(), comp);
    }

    template <typename... _Args>
    void emplace(_Args &&... __args)
    {
        c.emplace_back(std::forward<_Args>(__args)...);
        std::push_heap(c.begin(), c.end(), comp);
    }

    void pop()
    {
        std::pop_heap(c.begin(), c.end(), comp);
        c.pop_back();
    }
    void swap(priority_queue &__pq) noexcept(noexcept(swap(c, __pq.c)) && noexcept(swap(comp, __pq.comp)))
    {
        using std::swap;
        swap(c, __pq.c);
        swap(comp, __pq.comp);
    }

这些都是priority_queue的成员函数，其中emplace和swap是C++11新增的成员函数，emplace默认调用std::vector::emplace_back来构造元素；

出队就std::pop_heap; std::vector::pop_back;，入队就std::vector::push_back; std::push_heap;。

template <typename _Tp, typename _Sequence, typename _Compare>
inline void swap(priority_queue<_Tp, _Sequence, _Compare> &__x,
                 priority_queue<_Tp, _Sequence, _Compare> &__y) noexcept(noexcept(__x.swap(__y)))
{
    __x.swap(__y);
}

template <typename _Tp, typename _Sequence, typename _Compare, typename _Alloc>
struct uses_allocator<priority_queue<_Tp, _Sequence, _Compare>, _Alloc>
    : public uses_allocator<_Sequence, _Alloc>::type
{
};

这两个是C++11为priority_queue做的两个特化。