std::queue - cppreference.com (original) (raw)

| | | | | ------------------------------------------------------------------------------------------------------------------------------------------------ | | | | 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.

All member functions of std::queue are constexpr: it is possible to create and use std::queue objects in the evaluation of a constant expression.However, std::queue objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression. (since C++26)

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: 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.

[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
the underlying container (protected member object) [edit]

[edit] Member functions

(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

[edit] Helper classes

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges construction and insertion for containers
__cpp_lib_constexpr_queue 202502L (C++26) constexpr std::queue

[edit] Example

#include #include #include   int main() { std::queue 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:

[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 proxyreference 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 with additional support of pop_front(). The resolution of this DR
    added support of std::vector for std::stack and std::priority_queue. The changes involving std::queue
    are for maintaining consistency.

[edit] See also