NAME¶

std::priority_queue - std::priority_queue

Synopsis¶

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¶

The type of the stored elements.
T - The behavior is undefined if T is not the same type as
Container::value_type.
(since C++17)
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:
Container -
* front()
* push_back()
* pop_back()

The standard containers std::vector and std::deque satisfy these
requirements.
A Compare type providing a strict weak ordering.

Note that the Compare parameter is defined such that it returns true if
Compare - 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 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 constructs element in-place and sorts the underlying container
(C++11) (public member function)
pop removes the top element
(public member function)
swap swaps the contents
(C++11) (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
(C++11) (function template)

Helper classes¶

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

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) { // NB: pass by value so the print uses a copy
while(!q.empty()) {
std::cout << q.top() << ' ';
q.pop();
}
std::cout << '\n';
}

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

const auto data = {1,8,5,6,3,4,0,9,7,2};

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

print_queue(q);

std::priority_queue<int, std::vector<int>, std::greater<int>>
q2(data.begin(), data.end());

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 : data)
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

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 2684 C++98 priority_queue takes a comparator but lacked added
member typedef for it