# Standard library header <queue>

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< cpp‎ | header

This header is part of the containers library.

### Includes
<compare> (C++20) | Three-way comparison operator support
<initializer_list> (C++11) | std::initializer_list class template
### Classes
queue | adapts a container to provide queue (FIFO data structure) (class template)
priority_queue | adapts a container to provide priority queue (class template)
std::uses_allocator<std::queue> (C++11) | specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator<std::priority_queue> (C++11) | specializes the std::uses_allocator type trait (class template specialization)
### Functions
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20) | lexicographically compares the values in the queue (function template)
std::swap(std::queue) (C++11) | specializes the std::swap algorithm (function template)
std::swap(std::priority_queue) (C++11) | specializes the std::swap algorithm (function template)

### Synopsis

    #include <compare>
    #include <initializer_list>

    namespace std {
      // class template queue
      template<class T, class Container = deque<T>> class queue;

      template<class T, class Container>
        bool operator==(const queue<T, Container>& x, const queue<T, Container>& y);
      template<class T, class Container>
        bool operator!=(const queue<T, Container>& x, const queue<T, Container>& y);
      template<class T, class Container>
        bool operator< (const queue<T, Container>& x, const queue<T, Container>& y);
      template<class T, class Container>
        bool operator> (const queue<T, Container>& x, const queue<T, Container>& y);
      template<class T, class Container>
        bool operator<=(const queue<T, Container>& x, const queue<T, Container>& y);
      template<class T, class Container>
        bool operator>=(const queue<T, Container>& x, const queue<T, Container>& y);
      template<class T, three_way_comparable Container>
        compare_three_way_result_t<Container>
          operator<=>(const queue<T, Container>& x, const queue<T, Container>& y);

      template<class T, class Container>
        void swap(queue<T, Container>& x, queue<T, Container>& y)
          noexcept(noexcept(x.swap(y)));
      template<class T, class Container, class Alloc>
        struct uses_allocator<queue<T, Container>, Alloc>;

      // class template priority_queue
      template<class T, class Container = vector<T>,
               class Compare = less<typename Container::value_type>>
        class priority_queue;

      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>;
    }

#### Class template std::queue

    namespace std {
      template<class T, class Container = deque<T>>
      class 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;

      protected:
        Container c;

      public:
        queue() : queue(Container()) {}
        explicit queue(const Container&);
        explicit queue(Container&&);
        template<class InputIt> queue(InputIt first, InputIt last);
        template<__container_compatible_range<T> R> queue(from_range_t, R&& rg);
        template<class Alloc> explicit queue(const Alloc&);
        template<class Alloc> queue(const Container&, const Alloc&);
        template<class Alloc> queue(Container&&, const Alloc&);
        template<class Alloc> queue(const queue&, const Alloc&);
        template<class Alloc> queue(queue&&, const Alloc&);
        template<class InputIt, class Alloc>
          queue(InputIt first, InputIt last, const Alloc&);
        template<__container_compatible_range<T> R, class Alloc>
          queue(from_range_t, R&& rg, const Alloc&);

        [[nodiscard]] bool empty() const    { return c.empty(); }
        size_type         size()  const     { return c.size(); }
        reference         front()           { return c.front(); }
        const_reference   front() const     { return c.front(); }
        reference         back()            { return c.back(); }
        const_reference   back() const      { return c.back(); }
        void push(const value_type& x)      { c.push_back(x); }
        void push(value_type&& x)           { c.push_back(std::move(x)); }
        template<__container_compatible_range<T> R> void push_range(R&& rg);
        template<class... Args>
          decltype(auto) emplace(Args&&... args)
            { return c.emplace_back(std::forward<Args>(args)...); }
        void pop()                          { c.pop_front(); }
        void swap(queue& q) noexcept(is_nothrow_swappable_v<Container>)
          { using std::swap; swap(c, q.c); }
      };

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

      template<class InputIt>
        queue(InputIt, InputIt) -> queue<__iter_value_type<InputIt>>;

      template<ranges::input_range R>
        queue(from_range_t, R&&) -> queue<ranges::range_value_t<R>>;

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

      template<class InputIt, class Allocator>
        queue(InputIt, InputIt, Allocator)
          -> queue<__iter_value_type<InputIt>, deque<__iter_value_type<InputIt>,
                   Allocator>>;

      template<ranges::input_range R, class Allocator>
        queue(from_range_t, R&&, Allocator)
          -> queue<ranges::range_value_t<R>, deque<ranges::range_value_t<R>, Allocator>>;

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

#### Class template std::priority_queue

    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 InputIt>
          priority_queue(InputIt first, InputIt last, const Compare& x = Compare());
        template<class InputIt>
          priority_queue(InputIt first, InputIt last, const Compare& x,
                         const Container&);
        template<class InputIt>
          priority_queue(InputIt first, InputIt last, const Compare& x,
                         Container&&);
        template<__container_compatible_range<T> R>
          priority_queue(from_range_t, R&& rg, const Compare& x = Compare());
        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&);
        template<class InputIt, class Alloc>
          priority_queue(InputIt, InputIt, const Alloc&);
        template<class InputIt, class Alloc>
          priority_queue(InputIt, InputIt, const Compare&, const Alloc&);
        template<class InputIt, class Alloc>
          priority_queue(InputIt, InputIt, const Compare&, const Container&,
                         const Alloc&);
        template<class InputIt, class Alloc>
          priority_queue(InputIt, InputIt, const Compare&, Container&&, const Alloc&);
        template<__container_compatible_range<T> R, class Alloc>
          priority_queue(from_range_t, R&& rg, const Compare&, const Alloc&);
        template<__container_compatible_range<T> R, class Alloc>
          priority_queue(from_range_t, R&& rg, 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<__container_compatible_range<T> R>
          void push_range(R&& rg);
        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 InputIt,
               class Compare = less<__iter_value_type<InputIt>>,
               class Container = vector<__iter_value_type<InputIt>>>
        priority_queue(InputIt, InputIt, Compare = Compare(), Container = Container())
          -> priority_queue<__iter_value_type<InputIt>, Container, Compare>;

      template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>>
        priority_queue(from_range_t, R&&, Compare = Compare())
          -> priority_queue<ranges::range_value_t<R>, vector<ranges::range_value_t<R>>,
                            Compare>;

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

      template<class InputIt, class Allocator>
        priority_queue(InputIt, InputIt, Allocator)
          -> priority_queue<__iter_value_type<InputIt>,
                            vector<__iter_value_type<InputIt>, Allocator>,
                            less<__iter_value_type<InputIt>>>;

      template<class InputIt, class Compare, class Allocator>
        priority_queue(InputIt, InputIt, Compare, Allocator)
          -> priority_queue<__iter_value_type<InputIt>,
                            vector<__iter_value_type<InputIt>, Allocator>, Compare>;

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

      template<ranges::input_range R, class Compare, class Allocator>
        priority_queue(from_range_t, R&&, Compare, Allocator)
          -> priority_queue<ranges::range_value_t<R>,
                            vector<ranges::range_value_t<R>, Allocator>,
                            Compare>;

      template<ranges::input_range R, class Allocator>
        priority_queue(from_range_t, R&&, Allocator)
          -> priority_queue<ranges::range_value_t<R>,
                            vector<ranges::range_value_t<R>, Allocator>>;

      // no equality is provided

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

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