std::queue

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

vector<bool>

inplace_vector (C++26)

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

Views

span (C++20)

mdspan (C++23)

Tables

Iterator invalidation

Member function table

Non-member function table

 

std::queue

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)

 

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.

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: back(), e.g., std::deque::back(), front(), e.g. std::list::front(), push_back(), e.g., std::deque::push_back(), pop_front(), e.g., std::list::pop_front(). The standard containers std::deque and std::list satisfy these requirements.

Member types

Member type	Definition
container_type	Container
value_type	Container::value_type
size_type	Container::size_type
reference	Container::reference
const_reference	Container::const_reference

Member objects

Member name	Definition
Container c	the underlying container (protected member object)

Member functions

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

Non-member functions

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

Helper classes

std::uses_allocator<std::queue> (C++11)	specializes the std::uses_allocator type trait (class template specialization)
std::formatter<std::queue> (C++23)	formatting support for std::queue (class template specialization)

Deduction guides

(since C++17)

Notes

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

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

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 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

See also

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