23 Containers library [containers]

23.6 Container adaptors [container.adaptors]

23.6.3 Class template queue [queue]

23.6.3.1 Definition [queue.defn]

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Any sequence container supporting operations front(), back(), push_back() and pop_front() can be used to instantiate queue.
In particular, list and deque can be used.
namespace std { template<class T, class Container = deque<T>> class queue { public: using value_type = typename Container::value_type; using reference = typename Container::reference; using const_reference = typename Container::const_reference; using size_type = typename Container::size_type; using container_type = Container; protected: Container c; public: constexpr queue() : queue(Container()) {} constexpr explicit queue(const Container&); constexpr explicit queue(Container&&); template<class InputIterator> constexpr queue(InputIterator first, InputIterator last); template<container-compatible-range<T> R> constexpr queue(from_range_t, R&& rg); template<class Alloc> constexpr explicit queue(const Alloc&); template<class Alloc> constexpr queue(const Container&, const Alloc&); template<class Alloc> constexpr queue(Container&&, const Alloc&); template<class Alloc> constexpr queue(const queue&, const Alloc&); template<class Alloc> constexpr queue(queue&&, const Alloc&); template<class InputIterator, class Alloc> constexpr queue(InputIterator first, InputIterator last, const Alloc&); template<container-compatible-range<T> R, class Alloc> constexpr queue(from_range_t, R&& rg, const Alloc&); constexpr bool empty() const { return c.empty(); } constexpr size_type size() const { return c.size(); } constexpr reference front() { return c.front(); } constexpr const_reference front() const { return c.front(); } constexpr reference back() { return c.back(); } constexpr const_reference back() const { return c.back(); } constexpr void push(const value_type& x) { c.push_back(x); } constexpr void push(value_type&& x) { c.push_back(std::move(x)); } template<container-compatible-range<T> R> constexpr void push_range(R&& rg); template<class... Args> constexpr decltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); } constexpr void pop() { c.pop_front(); } constexpr void swap(queue& q) noexcept(is_nothrow_swappable_v<Container>) { using std::swap; swap(c, q.c); } }; template<class Container> queue(Container) -> queue<typename Container::value_type, Container>; template<class InputIterator> queue(InputIterator, InputIterator) -> queue<iter-value-type<InputIterator>>; template<ranges::input_range R> queue(from_range_t, R&&) -> queue<ranges::range_value_t<R>>; template<class Container, class Allocator> queue(Container, Allocator) -> queue<typename Container::value_type, Container>; template<class InputIterator, class Allocator> queue(InputIterator, InputIterator, Allocator) -> queue<iter-value-type<InputIterator>, deque<iter-value-type<InputIterator>, Allocator>>; template<ranges::input_range R, class Allocator> queue(from_range_t, R&&, Allocator) -> queue<ranges::range_value_t<R>, deque<ranges::range_value_t<R>, Allocator>>; template<class T, class Container, class Alloc> struct uses_allocator<queue<T, Container>, Alloc> : uses_allocator<Container, Alloc>::type { }; }

23.6.3.2 Constructors [queue.cons]

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constexpr explicit queue(const Container& cont);
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Effects: Initializes c with cont.
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constexpr explicit queue(Container&& cont);
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Effects: Initializes c with std​::​move(cont).
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template<class InputIterator> constexpr queue(InputIterator first, InputIterator last);
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Effects: Initializes c with first as the first argument and last as the second argument.
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template<container-compatible-range<T> R> constexpr queue(from_range_t, R&& rg);
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Effects: Initializes c with ranges​::​to<Container>(std​::​forward<R>(rg)).

23.6.3.3 Constructors with allocators [queue.cons.alloc]

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If uses_allocator_v<container_type, Alloc> is false the constructors in this subclause shall not participate in overload resolution.
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template<class Alloc> constexpr explicit queue(const Alloc& a);
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Effects: Initializes c with a.
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template<class Alloc> constexpr queue(const container_type& cont, const Alloc& a);
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Effects: Initializes c with cont as the first argument and a as the second argument.
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template<class Alloc> constexpr queue(container_type&& cont, const Alloc& a);
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Effects: Initializes c with std​::​move(cont) as the first argument and a as the second argument.
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template<class Alloc> constexpr queue(const queue& q, const Alloc& a);
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Effects: Initializes c with q.c as the first argument and a as the second argument.
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template<class Alloc> constexpr queue(queue&& q, const Alloc& a);
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Effects: Initializes c with std​::​move(q.c) as the first argument and a as the second argument.
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template<class InputIterator, class Alloc> constexpr queue(InputIterator first, InputIterator last, const Alloc& alloc);
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Effects: Initializes c with first as the first argument, last as the second argument, and alloc as the third argument.
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template<container-compatible-range<T> R, class Alloc> constexpr queue(from_range_t, R&& rg, const Alloc& a);
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Effects: Initializes c with ranges​::​to<Container>(std​::​forward<R>(rg), a).

23.6.3.4 Modifiers [queue.mod]

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template<container-compatible-range<T> R> constexpr void push_range(R&& rg);
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Effects: Equivalent to c.append_range(std​::​forward<R>(rg)) if that is a valid expression, otherwise ranges​::​copy(rg, back_inserter(c)).

23.6.3.5 Operators [queue.ops]

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template<class T, class Container> constexpr bool operator==(const queue<T, Container>& x, const queue<T, Container>& y);
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Returns: x.c == y.c.
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template<class T, class Container> constexpr bool operator!=(const queue<T, Container>& x, const queue<T, Container>& y);
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Returns: x.c != y.c.
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template<class T, class Container> constexpr bool operator< (const queue<T, Container>& x, const queue<T, Container>& y);
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Returns: x.c < y.c.
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template<class T, class Container> constexpr bool operator> (const queue<T, Container>& x, const queue<T, Container>& y);
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Returns: x.c > y.c.
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template<class T, class Container> constexpr bool operator<=(const queue<T, Container>& x, const queue<T, Container>& y);
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Returns: x.c <= y.c.
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template<class T, class Container> constexpr bool operator>=(const queue<T, Container>& x, const queue<T, Container>& y);
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Returns: x.c >= y.c.
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template<class T, three_way_comparable Container> constexpr compare_three_way_result_t<Container> operator<=>(const queue<T, Container>& x, const queue<T, Container>& y);
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Returns: x.c <=> y.c.

23.6.3.6 Specialized algorithms [queue.special]

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template<class T, class Container> constexpr void swap(queue<T, Container>& x, queue<T, Container>& y) noexcept(noexcept(x.swap(y)));
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Constraints: is_swappable_v<Container> is true.
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Effects: As if by x.swap(y).