Difference between revisions of "cpp/container/queue"

Latest revision as of 23:51, 2 August 2024

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.

(constructor)	constructs the `queue` (public member function) [edit]
(destructor)	destructs the `queue` (public member function) [edit]
operator=	assigns values to the container adaptor (public member function) [edit]
Element access
front	access the first element (public member function) [edit]
back	access the last element (public member function) [edit]
Capacity
empty	checks whether the container adaptor is empty (public member function) [edit]
size	returns the number of elements (public member function) [edit]
Modifiers
push	inserts element at the end (public member function) [edit]
push_range (C++23)	inserts a range of elements at the end (public member function) [edit]
emplace (C++11)	constructs element in-place at the end (public member function) [edit]
pop	removes the first element (public member function) [edit]
swap (C++11)	swaps the contents (public member function) [edit]

[edit] Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)	lexicographically compares the values of two `queue`s (function template) [edit]
std::swap(std::queue) (C++11)	specializes the std::swap algorithm (function template) [edit]

[edit] Helper classes

std::uses_allocator<std::queue> (C++11)	specializes the std::uses_allocator type trait (class template specialization) [edit]
std::formatter<std::queue> (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`

↑ 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

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

[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.

[1]

@@ Line 1: / Line 1: @@
 {{cpp/title|queue}}
-{{cpp/container/queue/sidebar}}
+{{cpp/container/queue/navbar}}
-{{ddcl | header=queue |
+{{ddcl|header=queue|1=
 template<
      class T,
-     class Container {{=}} std::deque<T>
+     class Container = std::deque<T>
 > class queue;
 }}
-The {{tt|std::queue}} class is a container adapter that gives the programmer the functionality of a queue - specifically, a FIFO (first-in, first-out) data structure.
+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.
+===Template parameters===
+{{par begin}}
+{{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]]:
+* {{tt|back()}}, e.g., {{lc|std::deque::back()}},
+* {{tt|front()}}, e.g. {{lc|std::list::front()}},
+* {{tt|push_back()}}, e.g., {{lc|std::deque::push_back()}},
+* {{tt|pop_front()}}, e.g., {{lc|std::list::pop_front()}}.
+The standard containers {{lc|std::deque}} and {{lc|std::list}} satisfy these requirements.
+}}{{par end}}
 ===Member types===
-{{tdcl list begin}}
+{{dsc begin}}
-{{tdcl list hitem | Member type | Definition}}
+{{dsc hitem|Member type|Definition}}
-{{tdcl list template | cpp/container/tdcl list container_type | queue}}
+{{dsc inc|cpp/container/dsc container_type|queue}}
-{{tdcl list template | cpp/container/tdcl list value_type | queue}}
+{{dsc inc|cpp/container/dsc value_type|queue}}
-{{tdcl list template | cpp/container/tdcl list size_type | queue}}
+{{dsc inc|cpp/container/dsc size_type|queue}}
-{{tdcl list template | cpp/container/tdcl list reference | queue}}
+{{dsc inc|cpp/container/dsc reference|queue}}
-{{tdcl list template | cpp/container/tdcl list const_reference | queue}}
+{{dsc inc|cpp/container/dsc const_reference|queue}}
-{{tdcl list end}}
+{{dsc end}}
+===Member objects===
+{{dsc begin}}
+{{dsc hitem|Member name|Definition}}
+{{dsc inc|cpp/container/dsc c|queue}}
+{{dsc end}}
 ===Member functions===
-{{dcl list begin}}
+{{dsc begin}}
-{{dcl list template | cpp/container/dcl list constructor | queue}}
+{{dsc inc|cpp/container/dsc constructor|queue}}
-{{dcl list template | cpp/container/dcl list destructor | queue}}
+{{dsc inc|cpp/container/dsc destructor|queue}}
-{{dcl list template | cpp/container/dcl list operator{{=}} | queue}}
+{{dsc inc|cpp/container/dsc operator{{=}}|queue}}
+{{dsc h2|Element access}}
+{{dsc inc|cpp/container/dsc front|queue}}
+{{dsc inc|cpp/container/dsc back|queue}}
+{{dsc h2|Capacity}}
+{{dsc inc|cpp/container/dsc empty|queue}}
+{{dsc inc|cpp/container/dsc size|queue}}
+{{dsc h2|Modifiers}}
+{{dsc inc|cpp/container/dsc push|queue}}
+{{dsc inc|cpp/container/dsc push_range|queue}}
+{{dsc inc|cpp/container/dsc emplace|queue}}
+{{dsc inc|cpp/container/dsc pop|queue}}
+{{dsc inc|cpp/container/dsc swap|queue}}
+{{dsc end}}
+===Non-member functions===
+{{dsc begin}}
+{{dsc inc|cpp/container/dsc operator_cmp|queue}}
+{{dsc inc|cpp/container/dsc swap2|queue}}
+{{dsc end}}
+===Helper classes===
+{{dsc begin}}
+{{dsc inc|cpp/container/dsc uses_allocator|queue}}
+{{dsc inc|cpp/container/dsc adaptor_formatter|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|Ranges construction and insertion for containers}}
+{{ftm end}}
+===Example===
+{{example
+|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);
-{{dcl list h2 | Element access}}
+    // Print and remove all elements. Note that std::queue does not
-{{dcl list template | cpp/container/dcl list front | queue}}
+    // support begin()/end(), so a range-for-loop cannot be used.
-{{dcl list template | cpp/container/dcl list back | queue}}
+    std::cout << "q: ";
+    for (; !q.empty(); q.pop())
+        std::cout << q.front() << ' ';
+    std::cout << '\n';
+    assert(q.size() == 0);
+}
+|output=
+q: 1 2 3
+}}
-{{dcl list h2 | Capacity}}
+===Defect reports===
-{{dcl list template | cpp/container/dcl list empty | queue}}
+{{dr list begin}}
-{{dcl list template | cpp/container/dcl list size | queue}}
+{{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}}
+{{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}}
+<references/>
-{{dcl list h2 | Modifiers}}
+===See also===
-{{dcl list template | cpp/container/dcl list push | queue}}
+{{dsc begin}}
-{{dcl list template | cpp/container/dcl list emplace | queue}}
+{{dsc inc|cpp/container/dsc priority_queue}}
-{{dcl list template | cpp/container/dcl list pop | queue}}
+{{dsc inc|cpp/container/dsc deque}}
-{{dcl list template | cpp/container/dcl list swap | queue}}
+{{dsc inc|cpp/container/dsc list}}
+{{dsc end}}
-{{dcl list h1 | Member objects}}
+{{langlinks|de|es|fr|it|ja|pl|pt|ru|zh}}
-{{dcl list template | cpp/container/dcl list c | queue}}
-{{dcl list end}}

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Member functions
queue::queue
queue::~queue
queue::operator=
Element access
queue::front
queue::back
Capacity
queue::empty
queue::size
Modifiers
queue::push
queue::push_range (C++23)
queue::emplace (C++11)
queue::pop
queue::swap (C++11)
Non-member functions
swap(std::queue) (C++11)
operator==operator!=operator<operator>operator<=operator>=operator<=> (C++20)
Helper classes
uses_allocator<std::queue> (C++11)
formatter<std::queue> (C++23)
Deduction guides(C++17)

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]

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Difference between revisions of "cpp/container/queue"

Latest revision as of 23:51, 2 August 2024

Contents

[edit] Template parameters

[edit] Member types

[edit] Member objects

[edit] Member functions

Element access

Capacity

Modifiers