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Difference between revisions of "cpp/container/priority queue"

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{{cpp/title|priority_queue}}
 
{{cpp/title|priority_queue}}
−
{{cpp/container/priority_queue/sidebar}}
+
{{cpp/container/priority_queue/navbar}}
−
{{ddcl | header=queue | 1=
+
{{ddcl|header=queue|1=
 
template<
 
template<
 
     class T,
 
     class T,
−
     class Container = std::deque<T>,
+
     class Container = std::vector<T>,
 
     class Compare = std::less<typename Container::value_type>
 
     class Compare = std::less<typename Container::value_type>
 
> class priority_queue;
 
> class priority_queue;
−
}}  
+
}}
  
−
C++ Priority Queues are like queues, but the elements inside the queue are ordered by some predicate.  
+
The {{enwiki|Queue (abstract data type)|priority queue}} is a [[cpp/container#Container adaptors|container adaptor]] that provides constant time lookup of the largest (by default) element, at the expense of logarithmic insertion and extraction.
  
−
{{todo}}
+
A user-provided {{tt|Compare}} can be supplied to change the ordering, e.g. using {{c|std::greater<T>}} would cause the smallest element to appear as the {{lc|top()}}.
  
−
{{tdcl list begin}}
+
Working with a {{tt|priority_queue}} is similar to managing a [[cpp/algorithm/make_heap|heap]] in some random access container, with the benefit of not being able to accidentally invalidate the heap.
−
{{tdcl list h1 | Member types}}
+
−
{{tdcl list hitem | Member type | Definition}}
+
−
{{tdcl list template | cpp/container/tdcl list container_type | priority_queue}}
+
−
{{tdcl list template | cpp/container/tdcl list value_type | priority_queue}}
+
−
{{tdcl list template | cpp/container/tdcl list size_type | priority_queue}}
+
−
{{tdcl list template | cpp/container/tdcl list reference | priority_queue}}
+
−
{{tdcl list template | cpp/container/tdcl list const_reference | priority_queue}}
+
−
{{tdcl list end}}
+
  
−
{{dcl list begin}}
+
===Template parameters===
−
{{dcl list h1 | Member functions}}
+
{{par begin}}
−
{{dcl list template | cpp/container/dcl list constructor | priority_queue}}
+
{{par|T|The type of the stored elements. The program is ill-formed if {{tt|T}} is not the same type as {{tt|Container::value_type}}.}}
−
{{dcl list template | cpp/container/dcl list destructor | priority_queue}}
+
{{par|Container|The type of the underlying container to use to store the elements. The container must satisfy the requirements of {{named req|SequenceContainer}}, and its iterators must satisfy the requirements of {{named req|RandomAccessIterator}}. Additionally, it must provide the following functions with the [[cpp/named req/SequenceContainer#Optional operations|usual semantics]]:
−
{{dcl list template | cpp/container/dcl list operator{{=}} | priority_queue}}
+
* {{lc|front()}}, e.g., {{lc|std::vector::front()}},
 +
* {{lc|push_back()}}, e.g., {{lc|std::deque::push_back()}},
 +
* {{lc|pop_back()}}, e.g., {{lc|std::vector::pop_back()}}.
  
−
{{dcl list h2 | Element access}}
+
The standard containers {{lc|std::vector}} (including {{rlpt|vector bool|std::vector<bool>}}) and {{lc|std::deque}} satisfy these requirements.}}
−
{{dcl list template | cpp/container/dcl list top | priority_queue}}
+
{{par|Compare|A {{named req|Compare}} type providing a strict weak ordering.
  
−
{{dcl list h2 | Capacity}}
+
Note that the {{named req|Compare}} parameter is defined such that it returns {{c|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 {{named req|Compare}}.}}
−
{{dcl list template | cpp/container/dcl list empty | priority_queue}}
+
{{par end}}
−
{{dcl list template | cpp/container/dcl list size | priority_queue}}
+
  
−
{{dcl list h2 | Modifiers}}
+
===Member types===
−
{{dcl list template | cpp/container/dcl list push | priority_queue}}
+
{{dsc begin}}
−
{{dcl list template | cpp/container/dcl list emplace | priority_queue}}
+
{{dsc hitem|Member type|Definition}}
−
{{dcl list template | cpp/container/dcl list pop | priority_queue}}
+
{{dsc inc|cpp/container/dsc container_type|priority_queue}}
−
{{dcl list template | cpp/container/dcl list swap | priority_queue}}
+
{{dsc|{{tt|value_compare}}|{{tt|Compare}}}}
 +
{{dsc inc|cpp/container/dsc value_type|priority_queue}}
 +
{{dsc inc|cpp/container/dsc size_type|priority_queue}}
 +
{{dsc inc|cpp/container/dsc reference|priority_queue}}
 +
{{dsc inc|cpp/container/dsc const_reference|priority_queue}}
 +
{{dsc end}}
  
−
{{dcl list h1 | Member objects}}
+
===Member objects===
−
{{dcl list template | cpp/container/dcl list c | priority_queue}}
+
{{dsc begin}}
 +
{{dsc hitem|Member name|Definition}}
 +
{{dsc inc|cpp/container/dsc c|priority_queue}}
 +
{{dsc prot mem obj|nolink=true|{{dsc small|Compare}} comp|the comparison function object}}
 +
{{dsc end}}
  
−
{{dcl list end}}
+
===Member functions===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/container/dsc constructor|priority_queue}}
 +
{{dsc inc|cpp/container/dsc destructor|priority_queue}}
 +
{{dsc inc|cpp/container/dsc operator{{=}}|priority_queue}}
 +
 
 +
{{dsc h2|Element access}}
 +
{{dsc inc|cpp/container/dsc top|priority_queue}}
 +
 
 +
{{dsc h2|Capacity}}
 +
{{dsc inc|cpp/container/dsc empty|priority_queue}}
 +
{{dsc inc|cpp/container/dsc size|priority_queue}}
 +
 
 +
{{dsc h2|Modifiers}}
 +
{{dsc inc|cpp/container/dsc push|priority_queue}}
 +
{{dsc inc|cpp/container/dsc push_range|priority_queue}}
 +
{{dsc inc|cpp/container/dsc emplace|priority_queue}}
 +
{{dsc inc|cpp/container/dsc pop|priority_queue}}
 +
{{dsc inc|cpp/container/dsc swap|priority_queue}}
 +
{{dsc end}}
 +
 
 +
===Non-member functions===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/container/dsc swap2|priority_queue}}
 +
{{dsc end}}
 +
 
 +
===Helper classes===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/container/dsc uses_allocator|priority_queue}}
 +
{{dsc inc|cpp/container/dsc adaptor_formatter|priority_queue}}
 +
{{dsc end}}
 +
 
 +
{{rrev|since=c++17|
 +
==={{rl|deduction guides|Deduction guides}}===
 +
}}
 +
 
 +
===Notes===
 +
{{ftm begin|std=1|comment=1}}
 +
{{ftm|__cpp_lib_containers_ranges|value=202202L|std=C++23|[[cpp/ranges/to#container compatible range|Ranges-aware]] construction and insertion for containers}}
 +
{{ftm end}}
 +
 
 +
===Example===
 +
{{example
 +
|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===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=307|std=C++98|before={{tt|Container}} could not be {{tt|std::vector<bool>}}|after=allowed}}
 +
{{dr list item|wg=lwg|dr=2566|std=C++98|before=Missing the requirement for {{tt|Container::value_type}}|after=ill-formed if {{tt|T}} is not the same type as {{tt|Container::value_type}}}}
 +
{{dr list item|wg=lwg|dr=2684|std=C++98|before={{tt|priority_queue}} takes a comparator<br>but lacked member typedef for it|after=added}}
 +
{{dr list end}}
 +
 
 +
===See also===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/container/dsc vector}}
 +
{{dsc inc|cpp/container/dsc vector bool}}
 +
{{dsc inc|cpp/container/dsc deque}}
 +
{{dsc end}}
 +
 
 +
{{langlinks|cs|de|es|fr|it|ja|pl|pt|ru|tr|zh}}

Latest revision as of 20:39, 20 May 2024

 
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;

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.

Contents

[edit] 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:

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.

[edit] 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]

[edit] Member objects

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

[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++23)
 inserts a range of elements 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]

[edit] Non-member functions

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

[edit] Helper classes

(C++11)
 specializes the std::uses_allocator type trait
(class template specialization) [edit]
(C++23)
 formatting support for std::priority_queue
(class template specialization) [edit]

Deduction guides

 (since C++17)

[edit] Notes

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

[edit] 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

[edit] 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

[edit] 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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