32 Concurrency support library [thread]

32.5 Atomic operations [atomics]

32.5.7 Class template atomic_ref [atomics.ref.generic]

32.5.7.5 Partial specialization for pointers [atomics.ref.pointer]

There are specializations of the atomic_ref class template for all pointer-to-object types.

For each such type pointer-type, the specialization atomic_ref<pointer-type> provides additional atomic operations appropriate to pointer types.

The program is ill-formed if is_always_lock_free is false and is_volatile_v<T> is true.

namespace std { template<class T> struct atomic_ref<pointer-type> { private: pointer-type* ptr; // exposition only public: using value_type = remove_cv_t<pointer-type>; using difference_type = ptrdiff_t; static constexpr size_t required_alignment = implementation-defined; static constexpr bool is_always_lock_free = implementation-defined; bool is_lock_free() const noexcept; constexpr explicit atomic_ref(pointer-type&); constexpr atomic_ref(const atomic_ref&) noexcept; atomic_ref& operator=(const atomic_ref&) = delete; constexpr void store(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type operator=(value_type) const noexcept; constexpr value_type load(memory_order = memory_order::seq_cst) const noexcept; constexpr operator value_type() const noexcept; constexpr value_type exchange(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_weak(value_type&, value_type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_strong(value_type&, value_type, memory_order, memory_order) const noexcept; constexpr bool compare_exchange_weak(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr bool compare_exchange_strong(value_type&, value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_add(difference_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_sub(difference_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_max(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type fetch_min(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr void store_add(difference_type, memory_order = memory_order::seq_cst) const noexcept; constexpr void store_sub(difference_type, memory_order = memory_order::seq_cst) const noexcept; constexpr void store_max(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr void store_min(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr value_type operator++(int) const noexcept; constexpr value_type operator--(int) const noexcept; constexpr value_type operator++() const noexcept; constexpr value_type operator--() const noexcept; constexpr value_type operator+=(difference_type) const noexcept; constexpr value_type operator-=(difference_type) const noexcept; constexpr void wait(value_type, memory_order = memory_order::seq_cst) const noexcept; constexpr void notify_one() const noexcept; constexpr void notify_all() const noexcept; constexpr pointer-type* address() const noexcept; }; }

Descriptions are provided below only for members that differ from the primary template.

The following operations perform arithmetic computations.

The correspondence among key, operator, and computation is specified in Table 156 .

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constexpr value_type fetch_key(difference_type operand,
                     memory_order order = memory_order::seq_cst) const noexcept;

Constraints: is_const_v<pointer-type> is false.

Mandates: remove_pointer_t<pointer-type> is a complete object type.

Effects: Atomically replaces the value referenced by *ptr with the result of the computation applied to the value referenced by *ptr and the given operand.

Memory is affected according to the value of order.

These operations are atomic read-modify-write operations ([intro.races]).

Returns: Atomically, the value referenced by *ptr immediately before the effects.

Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.

For fetch_max and fetch_min, the maximum and minimum computation is performed as if by max and min algorithms ([alg.min.max]), respectively, with the object value and the first parameter as the arguments.

[Note 1:

If the pointers point to different complete objects (or subobjects thereof), the < operator does not establish a strict weak ordering (Table 29, [expr.rel]).

— end note]

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constexpr void store_key(see above operand,
                     memory_order order = memory_order::seq_cst) const noexcept;

Mandates: T is a complete object type.

Preconditions: order is memory_order::relaxed, memory_order::release, or memory_order::seq_cst.

Effects: Atomically replaces the value referenced by *ptr with the result of the computation applied to the value referenced by *ptr and the given operand.

Memory is affected according to the value of order.

These operations are atomic modify-write operations ([atomics.order]).

Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.

For store_max and store_min, the maximum and minimum computation is performed as if by max and min algorithms ([alg.min.max]), respectively, with *ptr and the first parameter as the arguments.

[Note 2:

If the pointers point to different complete objects (or subobjects thereof), the < operator does not establish a strict weak ordering (Table 29, [expr.rel]).

— end note]

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constexpr value_type operator op=(difference_type operand) const noexcept;

Constraints: is_const_v<pointer-type> is false.

Effects: Equivalent to: return fetch_key(operand) op operand;