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std::queue(3) C++ Standard Libary std::queue(3) 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 function- ality 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 SequenceCon- tainer. 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 retroac- tively 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 Con- tainer::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::pri- ority_queue. The changes involving std::queue are for maintaining consistency. See also deque double-ended queue (class template) list doubly-linked list (class template) http://cppreference.com 2024.06.10 std::queue(3)
NAME | Synopsis | Template parameters | Member types | Member objects | Member functions | Element access | Capacity | Modifiers | Non-member functions | Helper classes | Notes | Example | Output: | See also
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