NAME¶

std::queue - std::queue

Synopsis¶

Defined in header <queue>
template<

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

> class queue;

The std::queue class 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.
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(),
Container - * 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 inserts a range of elements at the end
(C++23) (public member function)
emplace constructs element in-place at the end
(C++11) (public member function)
pop removes the first element
(public member function)
swap swaps the contents
(C++11) (public member function)

Non-member functions¶

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

Helper classes¶

std::uses_allocator<std::queue> specializes the std::uses_allocator type trait
(C++11) (class template specialization)
std::formatter<std::queue> formatting support for std::queue
(C++23) (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
std::queue did not support
LWG 307 C++98 containers using proxy supported
reference types^[1] in place of
(const) value_type&
LWG 2566 C++98 Missing the requirement for ill-formed if T is not the same
Container::value_type 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.

See also¶

deque double-ended queue
(class template)
list doubly-linked list
(class template)