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Difference between revisions of "cpp/container/queue"

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The {{tt|std::queue}} class is a [[cpp/container#Container adaptors|container adaptor]] that gives the functionality of a {{enwiki|Queue (abstract data type)|queue}} - specifically, a FIFO (first-in, first-out) data structure.
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The {{tt|std::queue}} class template is a [[cpp/container#Container adaptors|container adaptor]] that gives the functionality of a {{enwiki|Queue (abstract data type)|queue}} - specifically, a FIFO (first-in, first-out) data structure.
  
 
The class template acts as a wrapper to the underlying container - only a specific set of functions is provided. The queue pushes the elements on the back of the underlying container and pops them from the front.
 
The class template acts as a wrapper to the underlying container - only a specific set of functions is provided. The queue pushes the elements on the back of the underlying container and pops them from the front.
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===Template parameters===
 
===Template parameters===
 
{{par begin}}
 
{{par begin}}
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{{par|T|The type of the stored elements. The behavior is undefined if {{tt|T}} is not the same type as {{tt|Container::value_type}}.<!-- LWG2566 -->}}
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{{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}}.}}
 
{{par|Container|The type of the underlying container to use to store the elements. The container must satisfy the requirements of {{named req|SequenceContainer}}. Additionally, it must provide the following functions with the [[cpp/named req/SequenceContainer#Optional operations|usual semantics]]:
 
{{par|Container|The type of the underlying container to use to store the elements. The container must satisfy the requirements of {{named req|SequenceContainer}}. Additionally, it must provide the following functions with the [[cpp/named req/SequenceContainer#Optional operations|usual semantics]]:
 
* {{tt|back()}}, e.g., {{lc|std::deque::back()}},
 
* {{tt|back()}}, e.g., {{lc|std::deque::back()}},
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===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{dr list begin}}
−
{{dr list item|wg=lwg|dr=307|std=C++98|before={{tt|std::queue}} did not support containers using proxy<br>reference types<ref>Such as containers similar to {{ltt|cpp/container/vector bool|std::vector<bool>}} with additional support of {{tt|pop_front()}}. The resolution of this DR added support of {{ltt|cpp/container/vector bool|std::vector<bool>}} for {{lc|std::stack}} and {{lc|std::priority_queue}}. The changes involving {{tt|std::queue}} are for maintaining consistency.</ref> in place of ({{tt|const}}) {{tt|value_type&}}|after=supported}}
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{{dr list item|wg=lwg|dr=307|std=C++98|before={{tt|std::queue}} did not support containers using proxy<br>reference types<ref>Such as containers similar to {{ltt|cpp/container/vector bool|std::vector<bool>}} with additional support of {{tt|pop_front()}}. The resolution of this DR<br>added support of {{ltt|cpp/container/vector bool|std::vector<bool>}} for {{lc|std::stack}} and {{lc|std::priority_queue}}. The changes involving {{tt|std::queue}}<br>are for maintaining consistency.</ref> in place of ({{tt|const}}) {{tt|value_type&}}|after=supported}}
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{{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 end}}
 
{{dr list end}}
 
<references/>
 
<references/>
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===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
 +
{{dsc inc|cpp/container/dsc priority_queue}}
 
{{dsc inc|cpp/container/dsc deque}}
 
{{dsc inc|cpp/container/dsc deque}}
 
{{dsc inc|cpp/container/dsc list}}
 
{{dsc inc|cpp/container/dsc list}}

Latest revision as of 23:51, 2 August 2024

 
C++
 
Containers library
 
std::queue
 
Defined in header <queue>
template<

    class T,
    class Container = std::deque<T>

> class queue;

The std::queue class template is a container adaptor that gives the functionality of a queue - specifically, a FIFO (first-in, first-out) data structure.

The class template acts as a wrapper to the underlying container - only a specific set of functions is provided. The queue pushes the elements on the back of the underlying container and pops them from the front.

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. Additionally, it must provide the following functions with the usual semantics:

The standard containers std::deque and std::list satisfy these requirements.

[edit] Member types

Member type Definition
container_type Container[edit]
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]

[edit] Member functions

 constructs the queue
(public member function) [edit]
destructs the queue
(public member function) [edit]
assigns values to the container adaptor
(public member function) [edit]
Element access
access the first element
(public member function) [edit]
access the last 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 at the end
(public member function) [edit]
(C++23)
 inserts a range of elements at the end
(public member function) [edit]
(C++11)
 constructs element in-place at the end
(public member function) [edit]
removes the first element
(public member function) [edit]
(C++11)
 swaps the contents
(public member function) [edit]

[edit] Non-member functions

(C++20)
 lexicographically compares the values of two queues
(function template) [edit]
(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::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 construction and insertion for containers

[edit] Example

Run this code
#include <cassert>
#include <iostream>
#include <queue>
 
int main()
{
    std::queue<int> q;
 
    q.push(0); // back pushes 0
    q.push(1); // q = 0 1
    q.push(2); // q = 0 1 2
    q.push(3); // q = 0 1 2 3
 
    assert(q.front() == 0);
    assert(q.back() == 3);
    assert(q.size() == 4);
 
    q.pop(); // removes the front element, 0
    assert(q.size() == 3);
 
    // Print and remove all elements. Note that std::queue does not
    // support begin()/end(), so a range-for-loop cannot be used.
    std::cout << "q: ";
    for (; !q.empty(); q.pop())
        std::cout << q.front() << ' ';
    std::cout << '\n';
    assert(q.size() == 0);
}

Output: 

q: 1 2 3

[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 std::queue did not support containers using proxy
reference types[1] in place of (const) value_type& 		supported
LWG 2566 C++98 Missing the requirement for Container::value_type ill-formed if T is not the same type as Container::value_type
  1. ↑ Such as containers similar to std::vector<bool> with additional support of pop_front(). The resolution of this DR
    added support of std::vector<bool> for std::stack and std::priority_queue. The changes involving std::queue
    are for maintaining consistency.

[edit] See also

 adapts a container to provide priority queue
(class template) [edit]
double-ended queue
(class template) [edit]
doubly-linked list
(class template) [edit]
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