# std::queue

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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 `queue`s (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[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<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

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

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