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std::priority_queue

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Revision as of 01:01, 10 January 2023 by Xmcgcg (Talk | contribs)

 
C++
 
Containers library
 
std::priority_queue
 
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.

Contents

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 (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[edit]
value_compare Compare
value_type Container::value_type[edit]
size_type Container::size_type[edit]
reference Container::reference[edit]
const_reference Container::const_reference[edit]

Member functions

 constructs the priority_queue
(public member function) [edit]
destructs the priority_queue
(public member function) [edit]
assigns values to the container adaptor
(public member function) [edit]
Element access
accesses the top element
(public member function) [edit]
Capacity
checks whether the container adaptor is empty
(public member function) [edit]
returns the number of elements
(public member function) [edit]
Modifiers
inserts element and sorts the underlying container
(public member function) [edit]
(C++11)
 constructs element in-place and sorts the underlying container
(public member function) [edit]
removes the top element
(public member function) [edit]
(C++11)
 swaps the contents
(public member function) [edit]

Member objects

Container c
 the underlying container
(protected member object) [edit]
Compare comp
 the comparison function object
(protected member object)

Non-member functions

(C++11)
 specializes the std::swap algorithm
(function template) [edit]

Helper classes

(C++11)
 specializes the std::uses_allocator type trait
(class template specialization) [edit]

Deduction guides (since C++17)

Example

Run this code
#include <functional>
#include <queue>
#include <vector>
#include <iostream>
#include <string_view>
 
template<typename T>
void print(std::string_view name, T const& q)
{
    std::cout << name << ": \t";
    for (auto const& n : q)
        std::cout << n << ' ';
    std::cout << '\n';
}
 
template<typename Q>
void print_queue(std::string_view name, Q q)
{
    // NB: q is passed by value because there is no way to traverse
    // priority_queue's content without erasing the queue.
    for (std::cout << name << ": \t"; !q.empty(); q.pop())
        std::cout << q.top() << ' ';
    std::cout << '\n';
}
 
int main()
{
    const auto data = {1, 8, 5, 6, 3, 4, 0, 9, 7, 2};
    print("data", data);
 
    std::priority_queue<int> q1; // Max priority queue
    for (int n : data)
        q1.push(n);
 
    print_queue("q1", q1);
 
    // Min 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>>
        minq1(data.begin(), data.end());
 
    print_queue("minq1", minq1);
 
    // Second way to define a min priority queue
    std::priority_queue minq2(data.begin(), data.end(), std::greater<int>());
 
    print_queue("minq2", minq2);
 
    // 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 minq3(data.begin(), data.end(), customLess);
 
    print_queue("minq3", minq3);
 
    // 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)> q5(cmp);
 
    for (int n : data)
        q5.push(n);
 
    print_queue("q5", q5);
}

Output: 

data: 	1 8 5 6 3 4 0 9 7 2 
q1: 	9 8 7 6 5 4 3 2 1 0 
minq1: 	0 1 2 3 4 5 6 7 8 9 
minq2: 	0 1 2 3 4 5 6 7 8 9 
minq3: 	0 1 2 3 4 5 6 7 8 9 
q5: 	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 2684 C++98 priority_queue takes a comparator but lacked member typedef for it added

See also

 dynamic contiguous array
(class template) [edit]
space-efficient dynamic bitset
(class template specialization) [edit]
double-ended queue
(class template) [edit]
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