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]

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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.
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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 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; }; }
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Descriptions are provided below only for members that differ from the primary template.
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The following operations perform arithmetic computations.
The correspondence among key, operator, and computation is specified in Table 151.
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constexpr value_type fetch_key(difference_type operand, memory_order order = memory_order::seq_cst) const noexcept;
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Constraints: is_const_v<pointer-type> is false.
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Mandates: remove_pointer_t<pointer-type> is a complete object type.
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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]).
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Returns: Atomically, the value referenced by *ptr immediately before the effects.
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Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.
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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 value_type operator op=(difference_type operand) const noexcept;
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Constraints: is_const_v<pointer-type> is false.
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Effects: Equivalent to: return fetch_key(operand) op operand;