22 Containers library [containers]

22.6 Container adaptors [container.adaptors]

22.6.5 Class template priority_­queue [priority.queue]

22.6.5.1 Overview [priqueue.overview]

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Any sequence container with random access iterator and supporting operations front(), push_­back() and pop_­back() can be used to instantiate priority_­queue.
In particular, vector and deque can be used.
Instantiating priority_­queue also involves supplying a function or function object for making priority comparisons; the library assumes that the function or function object defines a strict weak ordering.
namespace std {
  template<class T, class Container = vector<T>,
           class Compare = less<typename Container::value_type>>
  class priority_queue {
  public:
    using value_type      = typename Container::value_type;
    using reference       = typename Container::reference;
    using const_reference = typename Container::const_reference;
    using size_type       = typename Container::size_type;
    using container_type  = Container;
    using value_compare   = Compare;

  protected:
    Container c;
    Compare comp;

  public:
    priority_queue() : priority_queue(Compare()) {}
    explicit priority_queue(const Compare& x) : priority_queue(x, Container()) {}
    priority_queue(const Compare& x, const Container&);
    priority_queue(const Compare& x, Container&&);
    template<class InputIterator>
      priority_queue(InputIterator first, InputIterator last, const Compare& x,
                     const Container&);
    template<class InputIterator>
      priority_queue(InputIterator first, InputIterator last,
                     const Compare& x = Compare(), Container&& = Container());
    template<class Alloc> explicit priority_queue(const Alloc&);
    template<class Alloc> priority_queue(const Compare&, const Alloc&);
    template<class Alloc> priority_queue(const Compare&, const Container&, const Alloc&);
    template<class Alloc> priority_queue(const Compare&, Container&&, const Alloc&);
    template<class Alloc> priority_queue(const priority_queue&, const Alloc&);
    template<class Alloc> priority_queue(priority_queue&&, const Alloc&);

    [[nodiscard]] 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);
    void push(value_type&& x);
    template<class... Args> void emplace(Args&&... args);
    void pop();
    void swap(priority_queue& q) noexcept(is_nothrow_swappable_v<Container> &&
                                          is_nothrow_swappable_v<Compare>)
      { using std::swap; swap(c, q.c); swap(comp, q.comp); }
  };

  template<class Compare, class Container>
    priority_queue(Compare, Container)
      -> priority_queue<typename Container::value_type, Container, Compare>;

  template<class InputIterator,
           class Compare = less<typename iterator_traits<InputIterator>::value_type>,
           class Container = vector<typename iterator_traits<InputIterator>::value_type>>
    priority_queue(InputIterator, InputIterator, Compare = Compare(), Container = Container())
      -> priority_queue<typename iterator_traits<InputIterator>::value_type, Container, Compare>;

  template<class Compare, class Container, class Allocator>
    priority_queue(Compare, Container, Allocator)
      -> priority_queue<typename Container::value_type, Container, Compare>;

  // no equality is provided

  template<class T, class Container, class Compare>
    void swap(priority_queue<T, Container, Compare>& x,
              priority_queue<T, Container, Compare>& y) noexcept(noexcept(x.swap(y)));

  template<class T, class Container, class Compare, class Alloc>
    struct uses_allocator<priority_queue<T, Container, Compare>, Alloc>
      : uses_allocator<Container, Alloc>::type { };
}