# std::priority_queue

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< cpp‎ | container

Defined in header `<queue>` |  |
template< class T, class Container = std::vector <T>, class Compare = std::less <typename Container::value_type> > class priority_queue; |  |

The priority queue is a container adaptor that provides constant time lookup of the largest (by default) element, at the expense of logarithmic insertion and extraction.

A user-provided `Compare` can be supplied to change the ordering, e.g. using std::greater <T> would cause the smallest element to appear as the top().

Working with a `priority_queue` is similar to managing a heap in some random access container, with the benefit of not being able to accidentally invalidate the heap.

### Template parameters

T | - | The type of the stored elements. The program is ill-formed if `T` is not the same type as `Container::value_type`.

Container

-

The type of the underlying container to use to store the elements. The container must satisfy the requirements of SequenceContainer, and its iterators must satisfy the requirements of LegacyRandomAccessIterator. Additionally, it must provide the following functions with the usual semantics:

- front(), e.g., std::vector::front(),
- push_back(), e.g., std::deque::push_back(),
- pop_back(), e.g., std::vector::pop_back().

The standard containers std::vector (including `std::vector<bool>`) and std::deque satisfy these requirements.

Compare

-

A Compare type providing a strict weak ordering.

Note that the Compare parameter is defined such that it returns true if its first argument comes before its second argument in a weak ordering. But because the priority queue outputs largest elements first, the elements that "come before" are actually output last. That is, the front of the queue contains the "last" element according to the weak ordering imposed by Compare.

### Member types

Member type | Definition
`container_type` | `Container`
`value_compare` | `Compare`
`value_type` | `Container::value_type`
`size_type` | Container::size_type
`reference` | `Container::reference`
`const_reference` | `Container::const_reference`

### Member objects

Member name | Definition
Container c | the underlying container (protected member object)
Compare comp | the comparison function object (protected member object)

### Member functions

(constructor) | constructs the `priority_queue` (public member function)
(destructor) | destructs the `priority_queue` (public member function)
operator= | assigns values to the container adaptor (public member function)

##### Element access

top | accesses the top element (public member function)

##### Capacity

empty | checks whether the container adaptor is empty (public member function)
size | returns the number of elements (public member function)

##### Modifiers

push | inserts element and sorts the underlying container (public member function)
push_range (C++23) | inserts a range of elements and sorts the underlying container (public member function)
emplace (C++11) | constructs element in-place and sorts the underlying container (public member function)
pop | removes the top element (public member function)
swap (C++11) | swaps the contents (public member function)

### Non-member functions

std::swap(std::priority_queue) (C++11) | specializes the std::swap algorithm (function template)

### Helper classes

std::uses_allocator<std::priority_queue> (C++11) | specializes the std::uses_allocator type trait (class template specialization)
std::formatter<std::priority_queue> (C++23) | formatting support for `std::priority_queue` (class template specialization)

### Deduction guides

(since C++17)

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | `202202L` | (C++23) | Ranges-aware construction and insertion for containers

### Example

Run this code

    #include <functional>
    #include <iostream>
    #include <queue>
    #include <string_view>
    #include <vector>

    template<typename T>
    void pop_println(std::string_view rem, T& pq)
    {
        std::cout << rem << ": ";
        for (; !pq.empty(); pq.pop())
            std::cout << pq.top() << ' ';
        std::cout << '\n';
    }

    template<typename T>
    void println(std::string_view rem, const T& v)
    {
        std::cout << rem << ": ";
        for (const auto& e : v)
            std::cout << e << ' ';
        std::cout << '\n';
    }

    int main()
    {
        const auto data = {1, 8, 5, 6, 3, 4, 0, 9, 7, 2};
        println("data", data);

        std::priority_queue<int> max_priority_queue;

        // Fill the priority queue.
        for (int n : data)
            max_priority_queue.push(n);

        pop_println("max_priority_queue", max_priority_queue);

        // std::greater<int> makes the max priority queue act as a min priority queue.
        std::priority_queue<int, std::vector<int>, std::greater<int>>
            min_priority_queue1(data.begin(), data.end());

        pop_println("min_priority_queue1", min_priority_queue1);

        // Second way to define a min priority queue.
        std::priority_queue min_priority_queue2(data.begin(), data.end(), std::greater<int>());

        pop_println("min_priority_queue2", min_priority_queue2);

        // Using a custom function object to compare elements.
        struct
        {
            bool operator()(const int l, const int r) const { return l > r; }
        } customLess;

        std::priority_queue custom_priority_queue(data.begin(), data.end(), customLess);

        pop_println("custom_priority_queue", custom_priority_queue);

        // Using lambda to compare elements.
        auto cmp = [](int left, int right) { return (left ^ 1) < (right ^ 1); };
        std::priority_queue<int, std::vector<int>, decltype(cmp)> lambda_priority_queue(cmp);

        for (int n : data)
            lambda_priority_queue.push(n);

        pop_println("lambda_priority_queue", lambda_priority_queue);
    }

Output:

    data: 1 8 5 6 3 4 0 9 7 2
    max_priority_queue: 9 8 7 6 5 4 3 2 1 0
    min_priority_queue1: 0 1 2 3 4 5 6 7 8 9
    min_priority_queue2: 0 1 2 3 4 5 6 7 8 9
    custom_priority_queue: 0 1 2 3 4 5 6 7 8 9
    lambda_priority_queue: 8 9 6 7 4 5 2 3 0 1

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 307 | C++98 | `Container` could not be `std::vector<bool>` | allowed
LWG 2566 | C++98 | Missing the requirement for `Container::value_type` | ill-formed if `T` is not the same type as `Container::value_type`
LWG 2684 | C++98 | `priority_queue` takes a comparator but lacked member typedef for it | added

### See also

vector | dynamic contiguous array (class template)
vector<bool> | space-efficient dynamic bitset (class template specialization)
deque | double-ended queue (class template)

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