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 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)