# 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; |  |

A 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 behavior is undefined if `T` is not the same type as `Container::value_type`. (since C++17)

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()`
- `push_back()`
- `pop_back()`

The standard containers std::vector 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` (C++17) | `Compare`
`value_type` | `Container::value_type`
`size_type` | `Container::size_type`
`reference` | `Container::reference`
`const_reference` | `Container::const_reference`

### 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 underlying container 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)
emplace (C++11) | constructs element in-place and sorts the underlying container (public member function)
pop | removes the top element (public member function)
swap | swaps the contents (public member function)

### Member objects

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

### Non-member functions

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

### Helper classes

std::uses_allocator<std::priority_queue> (C++11) | specializes the std::uses_allocator type trait (function template)

### Deduction guides (since C++17)

### Example

Run this code

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

    template<typename T> void print_queue(T& q) {
        while(!q.empty()) {
            std::cout << q.top() << " ";
            q.pop();
        }
        std::cout << '\n';
    }

    int main() {
        std::priority_queue<int> q;

        for(int n : {1,8,5,6,3,4,0,9,7,2})
            q.push(n);

        print_queue(q);

        std::priority_queue<int, std::vector<int>, std::greater<int> > q2;

        for(int n : {1,8,5,6,3,4,0,9,7,2})
            q2.push(n);

        print_queue(q2);

        // 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)> q3(cmp);

        for(int n : {1,8,5,6,3,4,0,9,7,2})
            q3.push(n);

        print_queue(q3);

    }

Output:

    9 8 7 6 5 4 3 2 1 0
    0 1 2 3 4 5 6 7 8 9
    8 9 6 7 4 5 2 3 0 1

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